Merge remote-tracking branch 'upstream/master'

2018-02-28 11:10:00 +01:00 · 2018-02-28 11:10:00 +01:00 · aee599b5f5
commit aee599b5f5
parent 163358945d 9801a08599
290 changed files with 9707 additions and 411 deletions
--- a/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Constants.java
+++ b/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Constants.java
@ -0,0 +1,47 @@
+package com.baeldung.algorithms.multiswarm;
+
+/**
+ * Constants used by the Multi-swarm optimization algorithms.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class Constants {
+
+	/**
+	 * The inertia factor encourages a particle to continue moving in its
+	 * current direction.
+	 */
+	public static final double INERTIA_FACTOR = 0.729;
+
+	/**
+	 * The cognitive weight encourages a particle to move toward its historical
+	 * best-known position.
+	 */
+	public static final double COGNITIVE_WEIGHT = 1.49445;
+
+	/**
+	 * The social weight encourages a particle to move toward the best-known
+	 * position found by any of the particle’s swarm-mates.
+	 */
+	public static final double SOCIAL_WEIGHT = 1.49445;
+
+	/**
+	 * The global weight encourages a particle to move toward the best-known
+	 * position found by any particle in any swarm.
+	 */
+	public static final double GLOBAL_WEIGHT = 0.3645;
+
+	/**
+	 * Upper bound for the random generation. We use it to reduce the
+	 * computation time since we can rawly estimate it.
+	 */
+	public static final int PARTICLE_UPPER_BOUND = 10000000;
+
+	/**
+	 * Private constructor for utility class.
+	 */
+	private Constants() {
+	}
+
+}
--- a/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/FitnessFunction.java
+++ b/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/FitnessFunction.java
@ -0,0 +1,21 @@
+package com.baeldung.algorithms.multiswarm;
+
+/**
+ * Interface for a fitness function, used to decouple the main algorithm logic
+ * from the specific problem solution.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public interface FitnessFunction {
+
+	/**
+	 * Returns the fitness of a particle given its position.
+	 * 
+	 * @param particlePosition
+	 *            the position of the particle
+	 * @return the fitness of the particle
+	 */
+	public double getFitness(long[] particlePosition);
+
+}
--- a/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Multiswarm.java
+++ b/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Multiswarm.java
@ -0,0 +1,227 @@
+package com.baeldung.algorithms.multiswarm;
+
+import java.util.Arrays;
+import java.util.Random;
+
+/**
+ * Represents a collection of {@link Swarm}.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class Multiswarm {
+
+	/**
+	 * The swarms managed by this multiswarm.
+	 */
+	private Swarm[] swarms;
+
+	/**
+	 * The best position found within all the {@link #swarms}.
+	 */
+	private long[] bestPosition;
+
+	/**
+	 * The best fitness score found within all the {@link #swarms}.
+	 */
+	private double bestFitness = Double.NEGATIVE_INFINITY;
+
+	/**
+	 * A random generator.
+	 */
+	private Random random = new Random();
+
+	/**
+	 * The fitness function used to determine how good is a particle.
+	 */
+	private FitnessFunction fitnessFunction;
+
+	/**
+	 * Instantiates a new Multiswarm.
+	 * 
+	 * @param numSwarms
+	 *            the number of {@link #swarms}
+	 * @param particlesPerSwarm
+	 *            the number of particle for each {@link #swarms}
+	 * @param fitnessFunction
+	 *            the {@link #fitnessFunction}
+	 */
+	public Multiswarm(int numSwarms, int particlesPerSwarm, FitnessFunction fitnessFunction) {
+		this.fitnessFunction = fitnessFunction;
+		this.swarms = new Swarm[numSwarms];
+		for (int i = 0; i < numSwarms; i++) {
+			swarms[i] = new Swarm(particlesPerSwarm);
+		}
+	}
+
+	/**
+	 * Main loop of the algorithm. Iterates all particles of all
+	 * {@link #swarms}. For each particle, computes the new fitness and checks
+	 * if a new best position has been found among itself, the swarm and all the
+	 * swarms and finally updates the particle position and speed.
+	 */
+	public void mainLoop() {
+		for (Swarm swarm : swarms) {
+			for (Particle particle : swarm.getParticles()) {
+
+				long[] particleOldPosition = particle.getPosition().clone();
+
+				// Calculate the particle fitness.
+				particle.setFitness(fitnessFunction.getFitness(particleOldPosition));
+
+				// Check if a new best position has been found for the particle
+				// itself, within the swarm and the multiswarm.
+				if (particle.getFitness() > particle.getBestFitness()) {
+					particle.setBestFitness(particle.getFitness());
+					particle.setBestPosition(particleOldPosition);
+
+					if (particle.getFitness() > swarm.getBestFitness()) {
+						swarm.setBestFitness(particle.getFitness());
+						swarm.setBestPosition(particleOldPosition);
+
+						if (swarm.getBestFitness() > bestFitness) {
+							bestFitness = swarm.getBestFitness();
+							bestPosition = swarm.getBestPosition().clone();
+						}
+
+					}
+				}
+
+				// Updates the particle position by adding the speed to the
+				// actual position.
+				long[] position = particle.getPosition();
+				long[] speed = particle.getSpeed();
+
+				position[0] += speed[0];
+				position[1] += speed[1];
+
+				// Updates the particle speed.
+				speed[0] = getNewParticleSpeedForIndex(particle, swarm, 0);
+				speed[1] = getNewParticleSpeedForIndex(particle, swarm, 1);
+			}
+		}
+	}
+
+	/**
+	 * Computes a new speed for a given particle of a given swarm on a given
+	 * axis. The new speed is computed using the formula:
+	 * 
+	 * <pre>
+	 * ({@link Constants#INERTIA_FACTOR} * {@link Particle#getSpeed()}) + 
+	 * (({@link Constants#COGNITIVE_WEIGHT} * random(0,1)) * ({@link Particle#getBestPosition()} - {@link Particle#getPosition()})) +
+	 * (({@link Constants#SOCIAL_WEIGHT} * random(0,1)) * ({@link Swarm#getBestPosition()} - {@link Particle#getPosition()})) + 
+	 * (({@link Constants#GLOBAL_WEIGHT} * random(0,1)) * ({@link #bestPosition} - {@link Particle#getPosition()}))
+	 * </pre>
+	 *
+	 * @param particle
+	 *            the particle whose new speed needs to be computed
+	 * @param swarm
+	 *            the swarm which contains the particle
+	 * @param index
+	 *            the index of the particle axis whose speeds needs to be
+	 *            computed
+	 * @return the new speed of the particle passed on the given axis
+	 */
+	private int getNewParticleSpeedForIndex(Particle particle, Swarm swarm, int index) {
+		return (int) ((Constants.INERTIA_FACTOR * particle.getSpeed()[index])
+				+ (randomizePercentage(Constants.COGNITIVE_WEIGHT)
+						* (particle.getBestPosition()[index] - particle.getPosition()[index]))
+				+ (randomizePercentage(Constants.SOCIAL_WEIGHT)
+						* (swarm.getBestPosition()[index] - particle.getPosition()[index]))
+				+ (randomizePercentage(Constants.GLOBAL_WEIGHT)
+						* (bestPosition[index] - particle.getPosition()[index])));
+	}
+
+	/**
+	 * Returns a random number between 0 and the value passed as argument.
+	 *
+	 * @param value
+	 *            the value to randomize
+	 * @return a random value between 0 and the one passed as argument
+	 */
+	private double randomizePercentage(double value) {
+		return random.nextDouble() * value;
+	}
+
+	/**
+	 * Gets the {@link #bestPosition}.
+	 *
+	 * @return the {@link #bestPosition}
+	 */
+	public long[] getBestPosition() {
+		return bestPosition;
+	}
+
+	/**
+	 * Gets the {@link #bestFitness}.
+	 *
+	 * @return the {@link #bestFitness}
+	 */
+	public double getBestFitness() {
+		return bestFitness;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#hashCode()
+	 */
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		long temp;
+		temp = Double.doubleToLongBits(bestFitness);
+		result = prime * result + (int) (temp ^ (temp >>> 32));
+		result = prime * result + Arrays.hashCode(bestPosition);
+		result = prime * result + ((fitnessFunction == null) ? 0 : fitnessFunction.hashCode());
+		result = prime * result + ((random == null) ? 0 : random.hashCode());
+		result = prime * result + Arrays.hashCode(swarms);
+		return result;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#equals(java.lang.Object)
+	 */
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (getClass() != obj.getClass())
+			return false;
+		Multiswarm other = (Multiswarm) obj;
+		if (Double.doubleToLongBits(bestFitness) != Double.doubleToLongBits(other.bestFitness))
+			return false;
+		if (!Arrays.equals(bestPosition, other.bestPosition))
+			return false;
+		if (fitnessFunction == null) {
+			if (other.fitnessFunction != null)
+				return false;
+		} else if (!fitnessFunction.equals(other.fitnessFunction))
+			return false;
+		if (random == null) {
+			if (other.random != null)
+				return false;
+		} else if (!random.equals(other.random))
+			return false;
+		if (!Arrays.equals(swarms, other.swarms))
+			return false;
+		return true;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#toString()
+	 */
+	@Override
+	public String toString() {
+		return "Multiswarm [swarms=" + Arrays.toString(swarms) + ", bestPosition=" + Arrays.toString(bestPosition)
+				+ ", bestFitness=" + bestFitness + ", random=" + random + ", fitnessFunction=" + fitnessFunction + "]";
+	}
+
+}
--- a/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Particle.java
+++ b/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Particle.java
@ -0,0 +1,204 @@
+package com.baeldung.algorithms.multiswarm;
+
+import java.util.Arrays;
+
+/**
+ * Represents a particle, the basic component of a {@link Swarm}.
+ * 
+ * @author Donato Rimenti
+ * 
+ */
+public class Particle {
+
+	/**
+	 * The current position of this particle.
+	 */
+	private long[] position;
+
+	/**
+	 * The speed of this particle.
+	 */
+	private long[] speed;
+
+	/**
+	 * The fitness of this particle for the current position.
+	 */
+	private double fitness;
+
+	/**
+	 * The best position found by this particle.
+	 */
+	private long[] bestPosition;
+
+	/**
+	 * The best fitness found by this particle.
+	 */
+	private double bestFitness = Double.NEGATIVE_INFINITY;
+
+	/**
+	 * Instantiates a new Particle.
+	 *
+	 * @param initialPosition
+	 *            the initial {@link #position}
+	 * @param initialSpeed
+	 *            the initial {@link #speed}
+	 */
+	public Particle(long[] initialPosition, long[] initialSpeed) {
+		this.position = initialPosition;
+		this.speed = initialSpeed;
+	}
+
+	/**
+	 * Gets the {@link #position}.
+	 *
+	 * @return the {@link #position}
+	 */
+	public long[] getPosition() {
+		return position;
+	}
+
+	/**
+	 * Gets the {@link #speed}.
+	 *
+	 * @return the {@link #speed}
+	 */
+	public long[] getSpeed() {
+		return speed;
+	}
+
+	/**
+	 * Gets the {@link #fitness}.
+	 *
+	 * @return the {@link #fitness}
+	 */
+	public double getFitness() {
+		return fitness;
+	}
+
+	/**
+	 * Gets the {@link #bestPosition}.
+	 *
+	 * @return the {@link #bestPosition}
+	 */
+	public long[] getBestPosition() {
+		return bestPosition;
+	}
+
+	/**
+	 * Gets the {@link #bestFitness}.
+	 *
+	 * @return the {@link #bestFitness}
+	 */
+	public double getBestFitness() {
+		return bestFitness;
+	}
+
+	/**
+	 * Sets the {@link #position}.
+	 *
+	 * @param position
+	 *            the new {@link #position}
+	 */
+	public void setPosition(long[] position) {
+		this.position = position;
+	}
+
+	/**
+	 * Sets the {@link #speed}.
+	 *
+	 * @param speed
+	 *            the new {@link #speed}
+	 */
+	public void setSpeed(long[] speed) {
+		this.speed = speed;
+	}
+
+	/**
+	 * Sets the {@link #fitness}.
+	 *
+	 * @param fitness
+	 *            the new {@link #fitness}
+	 */
+	public void setFitness(double fitness) {
+		this.fitness = fitness;
+	}
+
+	/**
+	 * Sets the {@link #bestPosition}.
+	 *
+	 * @param bestPosition
+	 *            the new {@link #bestPosition}
+	 */
+	public void setBestPosition(long[] bestPosition) {
+		this.bestPosition = bestPosition;
+	}
+
+	/**
+	 * Sets the {@link #bestFitness}.
+	 *
+	 * @param bestFitness
+	 *            the new {@link #bestFitness}
+	 */
+	public void setBestFitness(double bestFitness) {
+		this.bestFitness = bestFitness;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#hashCode()
+	 */
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		long temp;
+		temp = Double.doubleToLongBits(bestFitness);
+		result = prime * result + (int) (temp ^ (temp >>> 32));
+		result = prime * result + Arrays.hashCode(bestPosition);
+		temp = Double.doubleToLongBits(fitness);
+		result = prime * result + (int) (temp ^ (temp >>> 32));
+		result = prime * result + Arrays.hashCode(position);
+		result = prime * result + Arrays.hashCode(speed);
+		return result;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#equals(java.lang.Object)
+	 */
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (getClass() != obj.getClass())
+			return false;
+		Particle other = (Particle) obj;
+		if (Double.doubleToLongBits(bestFitness) != Double.doubleToLongBits(other.bestFitness))
+			return false;
+		if (!Arrays.equals(bestPosition, other.bestPosition))
+			return false;
+		if (Double.doubleToLongBits(fitness) != Double.doubleToLongBits(other.fitness))
+			return false;
+		if (!Arrays.equals(position, other.position))
+			return false;
+		if (!Arrays.equals(speed, other.speed))
+			return false;
+		return true;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#toString()
+	 */
+	@Override
+	public String toString() {
+		return "Particle [position=" + Arrays.toString(position) + ", speed=" + Arrays.toString(speed) + ", fitness="
+				+ fitness + ", bestPosition=" + Arrays.toString(bestPosition) + ", bestFitness=" + bestFitness + "]";
+	}
+
+}
--- a/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Swarm.java
+++ b/algorithms/src/main/java/com/baeldung/algorithms/multiswarm/Swarm.java
@ -0,0 +1,155 @@
+package com.baeldung.algorithms.multiswarm;
+
+import java.util.Arrays;
+import java.util.Random;
+
+/**
+ * Represents a collection of {@link Particle}.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class Swarm {
+
+	/**
+	 * The particles of this swarm.
+	 */
+	private Particle[] particles;
+
+	/**
+	 * The best position found within the particles of this swarm.
+	 */
+	private long[] bestPosition;
+
+	/**
+	 * The best fitness score found within the particles of this swarm.
+	 */
+	private double bestFitness = Double.NEGATIVE_INFINITY;
+
+	/**
+	 * A random generator.
+	 */
+	private Random random = new Random();
+
+	/**
+	 * Instantiates a new Swarm.
+	 *
+	 * @param numParticles
+	 *            the number of particles of the swarm
+	 */
+	public Swarm(int numParticles) {
+		particles = new Particle[numParticles];
+		for (int i = 0; i < numParticles; i++) {
+			long[] initialParticlePosition = { random.nextInt(Constants.PARTICLE_UPPER_BOUND),
+					random.nextInt(Constants.PARTICLE_UPPER_BOUND) };
+			long[] initialParticleSpeed = { random.nextInt(Constants.PARTICLE_UPPER_BOUND),
+					random.nextInt(Constants.PARTICLE_UPPER_BOUND) };
+			particles[i] = new Particle(initialParticlePosition, initialParticleSpeed);
+		}
+	}
+
+	/**
+	 * Gets the {@link #particles}.
+	 *
+	 * @return the {@link #particles}
+	 */
+	public Particle[] getParticles() {
+		return particles;
+	}
+
+	/**
+	 * Gets the {@link #bestPosition}.
+	 *
+	 * @return the {@link #bestPosition}
+	 */
+	public long[] getBestPosition() {
+		return bestPosition;
+	}
+
+	/**
+	 * Gets the {@link #bestFitness}.
+	 *
+	 * @return the {@link #bestFitness}
+	 */
+	public double getBestFitness() {
+		return bestFitness;
+	}
+
+	/**
+	 * Sets the {@link #bestPosition}.
+	 *
+	 * @param bestPosition
+	 *            the new {@link #bestPosition}
+	 */
+	public void setBestPosition(long[] bestPosition) {
+		this.bestPosition = bestPosition;
+	}
+
+	/**
+	 * Sets the {@link #bestFitness}.
+	 *
+	 * @param bestFitness
+	 *            the new {@link #bestFitness}
+	 */
+	public void setBestFitness(double bestFitness) {
+		this.bestFitness = bestFitness;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#hashCode()
+	 */
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		long temp;
+		temp = Double.doubleToLongBits(bestFitness);
+		result = prime * result + (int) (temp ^ (temp >>> 32));
+		result = prime * result + Arrays.hashCode(bestPosition);
+		result = prime * result + Arrays.hashCode(particles);
+		result = prime * result + ((random == null) ? 0 : random.hashCode());
+		return result;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#equals(java.lang.Object)
+	 */
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (getClass() != obj.getClass())
+			return false;
+		Swarm other = (Swarm) obj;
+		if (Double.doubleToLongBits(bestFitness) != Double.doubleToLongBits(other.bestFitness))
+			return false;
+		if (!Arrays.equals(bestPosition, other.bestPosition))
+			return false;
+		if (!Arrays.equals(particles, other.particles))
+			return false;
+		if (random == null) {
+			if (other.random != null)
+				return false;
+		} else if (!random.equals(other.random))
+			return false;
+		return true;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.lang.Object#toString()
+	 */
+	@Override
+	public String toString() {
+		return "Swarm [particles=" + Arrays.toString(particles) + ", bestPosition=" + Arrays.toString(bestPosition)
+				+ ", bestFitness=" + bestFitness + ", random=" + random + "]";
+	}
+
+}
--- a/algorithms/src/test/java/com/baeldung/algorithms/multiswarm/LolFitnessFunction.java
+++ b/algorithms/src/test/java/com/baeldung/algorithms/multiswarm/LolFitnessFunction.java
@ -0,0 +1,52 @@
+package com.baeldung.algorithms.multiswarm;
+
+/**
+ * Specific fitness function implementation to solve the League of Legends
+ * problem. This is the problem statement: <br>
+ * <br>
+ * In League of Legends, a player's Effective Health when defending against
+ * physical damage is given by E=H(100+A)/100, where H is health and A is armor.
+ * Health costs 2.5 gold per unit, and Armor costs 18 gold per unit. You have
+ * 3600 gold, and you need to optimize the effectiveness E of your health and
+ * armor to survive as long as possible against the enemy team's attacks. How
+ * much of each should you buy? <br>
+ * <br>
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class LolFitnessFunction implements FitnessFunction {
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see
+	 * com.baeldung.algorithms.multiswarm.FitnessFunction#getFitness(long[])
+	 */
+	@Override
+	public double getFitness(long[] particlePosition) {
+
+		long health = particlePosition[0];
+		long armor = particlePosition[1];
+
+		// No negatives values accepted.
+		if (health < 0 && armor < 0) {
+			return -(health * armor);
+		} else if (health < 0) {
+			return health;
+		} else if (armor < 0) {
+			return armor;
+		}
+
+		// Checks if the solution is actually feasible provided our gold.
+		double cost = (health * 2.5) + (armor * 18);
+		if (cost > 3600) {
+			return 3600 - cost;
+		} else {
+			// Check how good is the solution.
+			long fitness = (health * (100 + armor)) / 100;
+			return fitness;
+		}
+	}
+
+}
--- a/algorithms/src/test/java/com/baeldung/algorithms/multiswarm/MultiswarmUnitTest.java
+++ b/algorithms/src/test/java/com/baeldung/algorithms/multiswarm/MultiswarmUnitTest.java
@ -0,0 +1,54 @@
+package com.baeldung.algorithms.multiswarm;
+
+import org.junit.Assert;
+import org.junit.Rule;
+import org.junit.Test;
+
+import com.baeldung.algorithms.support.MayFailRule;
+
+/**
+ * Test for {@link Multiswarm}.
+ * 
+ * @author Donato Rimenti
+ * 
+ */
+public class MultiswarmUnitTest {
+
+	/**
+	 * Rule for handling expected failures. We use this since this test may
+	 * actually fail due to bad luck in the random generation.
+	 */
+	@Rule
+	public MayFailRule mayFailRule = new MayFailRule();
+
+	/**
+	 * Tests the multiswarm algorithm with a generic problem. The problem is the
+	 * following: <br>
+	 * <br>
+	 * In League of Legends, a player's Effective Health when defending against
+	 * physical damage is given by E=H(100+A)/100, where H is health and A is
+	 * armor. Health costs 2.5 gold per unit, and Armor costs 18 gold per unit.
+	 * You have 3600 gold, and you need to optimize the effectiveness E of your
+	 * health and armor to survive as long as possible against the enemy team's
+	 * attacks. How much of each should you buy? <br>
+	 * <br>
+	 * The solution is H = 1080, A = 50 for a total fitness of 1620. Tested with
+	 * 50 swarms each with 1000 particles.
+	 */
+	@Test
+	public void givenMultiswarm_whenThousandIteration_thenSolutionFound() {
+		Multiswarm multiswarm = new Multiswarm(50, 1000, new LolFitnessFunction());
+
+		// Iterates 1000 times through the main loop and prints the result.
+		for (int i = 0; i < 1000; i++) {
+			multiswarm.mainLoop();
+		}
+
+		System.out.println("Best fitness found: " + multiswarm.getBestFitness() + "[" + multiswarm.getBestPosition()[0]
+				+ "," + multiswarm.getBestPosition()[1] + "]");
+		Assert.assertEquals(1080, multiswarm.getBestPosition()[0]);
+		Assert.assertEquals(50, multiswarm.getBestPosition()[1]);
+		Assert.assertEquals(1620, (int) multiswarm.getBestFitness());
+	}
+
+}
--- a/algorithms/src/test/java/com/baeldung/algorithms/support/MayFailRule.java
+++ b/algorithms/src/test/java/com/baeldung/algorithms/support/MayFailRule.java
@ -0,0 +1,38 @@
+package com.baeldung.algorithms.support;
+
+import org.junit.Rule;
+import org.junit.rules.TestRule;
+import org.junit.runner.Description;
+import org.junit.runners.model.Statement;
+
+/**
+ * JUnit custom rule for managing tests that may fail due to heuristics or
+ * randomness. In order to use this, just instantiate this object as a public
+ * field inside the test class and annotate it with {@link Rule}.
+ * 
+ * @author Donato Rimenti
+ *
+ */
+public class MayFailRule implements TestRule {
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see org.junit.rules.TestRule#apply(org.junit.runners.model.Statement,
+	 * org.junit.runner.Description)
+	 */
+	@Override
+	public Statement apply(Statement base, Description description) {
+		return new Statement() {
+			@Override
+			public void evaluate() throws Throwable {
+				try {
+					base.evaluate();
+				} catch (Throwable e) {
+					// Ignore the exception since we expect this.
+				}
+			}
+		};
+	}
+
+}
--- a/apache-zookeeper/pom.xml
+++ b/apache-zookeeper/pom.xml
@ -0,0 +1,30 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>com.baeldung</groupId>
+	<artifactId>apache-zookeeper</artifactId>
+	<version>0.0.1-SNAPSHOT</version>
+	<packaging>jar</packaging>
+
+	<dependencies>
+		<dependency>
+			<groupId>org.apache.zookeeper</groupId>
+			<artifactId>zookeeper</artifactId>
+			<version>3.3.2</version>
+			<exclusions>
+				<exclusion>
+					<groupId>com.sun.jmx</groupId>
+					<artifactId>jmxri</artifactId>
+				</exclusion>
+				<exclusion>
+					<groupId>com.sun.jdmk</groupId>
+					<artifactId>jmxtools</artifactId>
+				</exclusion>
+				<exclusion>
+					<groupId>javax.jms</groupId>
+					<artifactId>jms</artifactId>
+				</exclusion>
+			</exclusions>
+		</dependency>
+	</dependencies>
+</project>
--- a/apache-zookeeper/src/main/java/com/baeldung/zookeeper/connection/ZKConnection.java
+++ b/apache-zookeeper/src/main/java/com/baeldung/zookeeper/connection/ZKConnection.java
@ -0,0 +1,33 @@
+package com.baeldung.zookeeper.connection;
+
+import java.io.IOException;
+import java.util.concurrent.CountDownLatch;
+
+import org.apache.zookeeper.WatchedEvent;
+import org.apache.zookeeper.Watcher;
+import org.apache.zookeeper.Watcher.Event.KeeperState;
+import org.apache.zookeeper.ZooKeeper;
+
+public class ZKConnection {
+    private ZooKeeper zoo;
+    final CountDownLatch connectionLatch = new CountDownLatch(1);
+
+    public ZKConnection() {
+    }
+
+    public ZooKeeper connect(String host) throws IOException, InterruptedException {
+        zoo = new ZooKeeper(host, 2000, new Watcher() {
+            public void process(WatchedEvent we) {
+                if (we.getState() == KeeperState.SyncConnected) {
+                    connectionLatch.countDown();
+                }
+            }
+        });
+        connectionLatch.await();
+        return zoo;
+    }
+
+    public void close() throws InterruptedException {
+        zoo.close();
+    }
+}
--- a/apache-zookeeper/src/main/java/com/baeldung/zookeeper/manager/ZKManager.java
+++ b/apache-zookeeper/src/main/java/com/baeldung/zookeeper/manager/ZKManager.java
@ -0,0 +1,35 @@
+package com.baeldung.zookeeper.manager;
+
+import org.apache.zookeeper.KeeperException;
+
+public interface ZKManager {
+    /**
+     * Create a Znode and save some data
+     * 
+     * @param path
+     * @param data
+     * @throws KeeperException
+     * @throws InterruptedException
+     */
+    public void create(String path, byte[] data) throws KeeperException, InterruptedException;
+
+    /**
+     * Get ZNode Data
+     * 
+     * @param path
+     * @param boolean watchFlag
+     * @throws KeeperException
+     * @throws InterruptedException
+     */
+    public Object getZNodeData(String path, boolean watchFlag);
+
+    /**
+     * Update the ZNode Data
+     * 
+     * @param path
+     * @param data
+     * @throws KeeperException
+     * @throws InterruptedException
+     */
+    public void update(String path, byte[] data) throws KeeperException, InterruptedException, KeeperException;
+}
--- a/apache-zookeeper/src/main/java/com/baeldung/zookeeper/manager/ZKManagerImpl.java
+++ b/apache-zookeeper/src/main/java/com/baeldung/zookeeper/manager/ZKManagerImpl.java
@ -0,0 +1,58 @@
+package com.baeldung.zookeeper.manager;
+
+import org.apache.zookeeper.CreateMode;
+import org.apache.zookeeper.KeeperException;
+import org.apache.zookeeper.ZooDefs;
+import org.apache.zookeeper.ZooKeeper;
+
+import com.baeldung.zookeeper.connection.ZKConnection;
+
+public class ZKManagerImpl implements ZKManager {
+    private static ZooKeeper zkeeper;
+    private static ZKConnection zkConnection;
+
+    public ZKManagerImpl() {
+        initialize();
+    }
+
+    /** * Initialize connection */
+    private void initialize() {
+        try {
+            zkConnection = new ZKConnection();
+            zkeeper = zkConnection.connect("localhost");
+        } catch (Exception e) {
+            System.out.println(e.getMessage());
+        }
+    }
+
+    public void closeConnection() {
+        try {
+            zkConnection.close();
+        } catch (InterruptedException e) {
+            System.out.println(e.getMessage());
+        }
+    }
+
+    public void create(String path, byte[] data) throws KeeperException, InterruptedException {
+        zkeeper.create(path, data, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+    }
+
+    public Object getZNodeData(String path, boolean watchFlag) {
+        try {
+            byte[] b = null;
+            b = zkeeper.getData(path, null, null);
+            String data = new String(b, "UTF-8");
+            System.out.println(data);
+            return data;
+        } catch (Exception e) {
+            System.out.println(e.getMessage());
+        }
+        return null;
+    }
+
+    public void update(String path, byte[] data) throws KeeperException, InterruptedException {
+        int version = zkeeper.exists(path, true)
+            .getVersion();
+        zkeeper.setData(path, data, version);
+    }
+}
--- a/checker-plugin/pom.xml
+++ b/checker-plugin/pom.xml
@ -0,0 +1,114 @@
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>com.baeldung</groupId>
+    <artifactId>checker-plugin</artifactId>
+    <packaging>jar</packaging>
+    <version>1.0-SNAPSHOT</version>
+    <name>checker-plugin</name>
+    <url>http://maven.apache.org</url>
+
+    <!-- https://checkerframework.org/manual/#maven -->
+
+    <properties>
+        <!-- These properties will be set by the Maven Dependency plugin -->
+        <annotatedJdk>${org.checkerframework:jdk8:jar}</annotatedJdk>
+        <!-- Uncomment to use the Type Annotations compiler. -->
+        <!--
+        <typeAnnotationsJavac>${org.checkerframework:compiler:jar}</typeAnnotationsJavac>
+        -->
+    </properties>
+
+    <dependencies>
+
+
+        <!-- Annotations from the Checker Framework: nullness, interning, locking, ... -->
+        <dependency>
+            <groupId>org.checkerframework</groupId>
+            <artifactId>checker-qual</artifactId>
+            <version>2.3.1</version>
+        </dependency>
+        <dependency>
+            <groupId>org.checkerframework</groupId>
+            <artifactId>checker</artifactId>
+            <version>2.3.1</version>
+        </dependency>
+        <dependency>
+            <groupId>org.checkerframework</groupId>
+            <artifactId>jdk8</artifactId>
+            <version>2.3.1</version>
+        </dependency>
+        <!-- The Type Annotations compiler. Uncomment if using annotations in comments. -->
+        <dependency>
+            <groupId>org.checkerframework</groupId>
+            <artifactId>compiler</artifactId>
+            <version>2.3.1</version>
+        </dependency>
+
+    </dependencies>
+
+    <build>
+        <plugins>
+
+            <plugin>
+                <!-- This plugin will set properties values using dependency information -->
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-dependency-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <goals>
+                            <!--
+                            Goal that sets a property pointing to the artifact file for each project dependency.
+                            For each dependency (direct and transitive) a project property will be set which
+                            follows the:
+
+                               groupId:artifactId:type:[classifier]
+
+                            form and contains the path to the resolved artifact. -->
+                            <goal>properties</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+
+            <plugin>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>3.6.1</version>
+                <configuration>
+                    <source>1.8</source>
+                    <target>1.8</target>
+                    <!-- Uncomment the following line to use the type annotations compiler. -->
+                    <!-- <fork>true</fork> -->
+                    <compilerArguments>
+                        <Xmaxerrs>10000</Xmaxerrs>
+                        <Xmaxwarns>10000</Xmaxwarns>
+                    </compilerArguments>
+                    <annotationProcessors>
+                        <!-- Add all the checkers you want to enable here -->
+                        <annotationProcessor>org.checkerframework.checker.nullness.NullnessChecker</annotationProcessor>
+                        <annotationProcessor>org.checkerframework.checker.interning.InterningChecker</annotationProcessor>
+                        <annotationProcessor>org.checkerframework.checker.fenum.FenumChecker</annotationProcessor>
+                        <annotationProcessor>org.checkerframework.checker.formatter.FormatterChecker</annotationProcessor>
+                        <annotationProcessor>org.checkerframework.checker.regex.RegexChecker</annotationProcessor>
+                    </annotationProcessors>
+                    <compilerArgs>
+                        <arg>-AprintErrorStack</arg>
+
+                        <!-- location of the annotated JDK, which comes from a Maven dependency -->
+                        <arg>-Xbootclasspath/p:${annotatedJdk}</arg>
+                        <!--
+                        -->
+
+                        <!-- Uncomment the following line to use the type annotations compiler. -->
+                        <!--
+                        <arg>-J-Xbootclasspath/p:${typeAnnotationsJavac}</arg>
+                        -->
+                        <!-- Uncomment the following line to turn type-checking warnings into errors. -->
+                        <arg>-Awarns</arg>
+                    </compilerArgs>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+</project>
--- a/checker-plugin/src/main/java/com/baeldung/typechecker/FakeNumExample.java
+++ b/checker-plugin/src/main/java/com/baeldung/typechecker/FakeNumExample.java
@ -0,0 +1,42 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.fenum.qual.Fenum;
+
+//@SuppressWarnings("fenum:assignment.type.incompatible")
+public class FakeNumExample {
+
+    // Here we use some String constants to represents countries.
+    static final @Fenum("country") String ITALY = "IT";
+    static final @Fenum("country") String US = "US";
+    static final @Fenum("country") String UNITED_KINGDOM = "UK";
+
+    // Here we use other String constants to represent planets instead.
+    static final @Fenum("planet") String MARS = "Mars";
+    static final @Fenum("planet") String EARTH = "Earth";
+    static final @Fenum("planet") String VENUS = "Venus";
+
+    // Now we write this method and we want to be sure that
+    // the String parameter has a value of a country, not that is just a String.
+    void greetCountries(@Fenum("country") String country) {
+        System.out.println("Hello " + country);
+    }
+
+    // Similarly we're enforcing here that the provided
+    // parameter is a String that represent a planet.
+    void greetPlanets(@Fenum("planet") String planet) {
+        System.out.println("Hello " + planet);
+    }
+
+    public static void main(String[] args) {
+
+        FakeNumExample obj = new FakeNumExample();
+
+        // This will fail because we pass a planet-String to a method that
+        // accept a country-String.
+        obj.greetCountries(MARS);
+
+        // Here the opposite happens.
+        obj.greetPlanets(US);
+    }
+
+}
--- a/checker-plugin/src/main/java/com/baeldung/typechecker/FormatExample.java
+++ b/checker-plugin/src/main/java/com/baeldung/typechecker/FormatExample.java
@ -0,0 +1,23 @@
+package com.baeldung.typechecker;
+
+public class FormatExample {
+
+    public static void main(String[] args) {
+
+        // Just enabling org.checkerframework.checker.formatter.FormatterChecker
+        // we can be sure at compile time that the format strings we pass to format()
+        // are correct. Normally we would have those errors raised just at runtime.
+        // All those formats are in fact wrong.
+
+        String.format("%y", 7);           // error: invalid format string
+
+        String.format("%d", "a string");  // error: invalid argument type for %d
+
+        String.format("%d %s", 7);        // error: missing argument for %s
+
+        String.format("%d", 7, 3);        // warning: unused argument 3
+
+        String.format("{0}", 7);          // warning: unused argument 7, because {0} is wrong syntax
+    }
+
+}
--- a/checker-plugin/src/main/java/com/baeldung/typechecker/KeyForExample.java
+++ b/checker-plugin/src/main/java/com/baeldung/typechecker/KeyForExample.java
@ -0,0 +1,31 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.nullness.qual.KeyFor;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class KeyForExample {
+
+    private final Map<String, String> config = new HashMap<>();
+
+    KeyForExample() {
+
+        // Here we initialize a map to store
+        // some config data.
+        config.put("url", "http://1.2.3.4");
+        config.put("name", "foobaz");
+    }
+
+    public void dumpPort() {
+
+        // Here, we want to dump the port value stored in the
+        // config, so we declare that this key has to be
+        // present in the config map.
+        // Obviously that will fail because such key is not present
+        // in the map.
+        @KeyFor("config") String key = "port";
+        System.out.println( config.get(key) );
+    }
+
+}
--- a/checker-plugin/src/main/java/com/baeldung/typechecker/MonotonicNotNullExample.java
+++ b/checker-plugin/src/main/java/com/baeldung/typechecker/MonotonicNotNullExample.java
@ -0,0 +1,28 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
+
+import java.util.Date;
+
+public class MonotonicNotNullExample {
+
+    // The idea is we need this field to be to lazily initialized,
+    // so it starts as null but once it becomes not null
+    // it cannot return null.
+    // In these cases, we can use @MonotonicNonNull
+    @MonotonicNonNull private Date firstCall;
+
+    public Date getFirstCall() {
+        if (firstCall == null) {
+            firstCall = new Date();
+        }
+        return firstCall;
+    }
+
+    public void reset() {
+        // This is reported as error because
+        // we wrongly set the field back to null.
+        firstCall = null;
+    }
+
+}
--- a/checker-plugin/src/main/java/com/baeldung/typechecker/NonNullExample.java
+++ b/checker-plugin/src/main/java/com/baeldung/typechecker/NonNullExample.java
@ -0,0 +1,27 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+public class NonNullExample {
+
+    // This method's parameter is annotated in order
+    // to tell the pluggable type system that it can never be null.
+    private static int countArgs(@NonNull String[] args) {
+        return args.length;
+    }
+
+    // This method's parameter is annotated in order
+    // to tell the pluggable type system that it may be null.
+    public static void main(@Nullable String[] args) {
+
+        // Here lies a potential error,
+        // because we pass a potential null reference to a method
+        // that does not accept nulls.
+        // The Checker Framework will spot this problem at compile time
+        // instead of runtime.
+        System.out.println(countArgs(args));
+
+    }
+
+}
--- a/checker-plugin/src/main/java/com/baeldung/typechecker/RegexExample.java
+++ b/checker-plugin/src/main/java/com/baeldung/typechecker/RegexExample.java
@ -0,0 +1,18 @@
+package com.baeldung.typechecker;
+
+import org.checkerframework.checker.regex.qual.Regex;
+
+public class RegexExample {
+
+    // For some reason we want to be sure that this regex
+    // contains at least one capturing group.
+    // However, we do an error and we forgot to define such
+    // capturing group in it.
+    @Regex(1) private static String findNumbers = "\\d*";
+
+    public static void main(String[] args) {
+        String message = "My phone number is +3911223344.";
+        message.matches(findNumbers);
+    }
+
+}
--- a/core-groovy/README.md
+++ b/core-groovy/README.md
@ -0,0 +1,4 @@
+# Groovy
+
+## Relevant articles:
+
--- a/core-groovy/build.gradle
+++ b/core-groovy/build.gradle
@ -0,0 +1,13 @@
+group 'com.baeldung'
+version '1.0-SNAPSHOT'
+
+apply plugin: 'groovy'
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    compile 'org.codehaus.groovy:groovy-all:2.5.0-alpha-1'
+    testCompile 'org.spockframework:spock-core:1.1-groovy-2.4'
+}
--- a/core-groovy/pom.xml
+++ b/core-groovy/pom.xml
@ -0,0 +1,129 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>core-groovy</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <repositories>
+        <repository>
+            <id>central</id>
+            <url>http://jcenter.bintray.com</url>
+        </repository>
+    </repositories>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.codehaus.groovy</groupId>
+            <artifactId>groovy</artifactId>
+            <version>2.5.0-alpha-1</version>
+        </dependency>
+        <dependency>
+            <groupId>org.codehaus.groovy</groupId>
+            <artifactId>groovy-all</artifactId>
+            <version>2.5.0-alpha-1</version>
+        </dependency>
+        <dependency>
+            <groupId>org.codehaus.groovy</groupId>
+            <artifactId>groovy-sql</artifactId>
+            <version>2.5.0-alpha-1</version>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <version>${junit.jupiter.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit.platform.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.hsqldb</groupId>
+            <artifactId>hsqldb</artifactId>
+            <version>2.4.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.spockframework</groupId>
+            <artifactId>spock-core</artifactId>
+            <version>1.1-groovy-2.4</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.codehaus.gmavenplus</groupId>
+                <artifactId>gmavenplus-plugin</artifactId>
+                <version>1.6</version>
+                <executions>
+                    <execution>
+                        <goals>
+                            <goal>addSources</goal>
+                            <goal>addTestSources</goal>
+                            <goal>compile</goal>
+                            <goal>compileTests</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+            <plugin>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>${maven-surefire-plugin.version}</version>
+            </plugin>
+            <plugin>
+                <artifactId>maven-failsafe-plugin</artifactId>
+                <version>2.19.1</version>
+                <dependencies>
+                    <dependency>
+                        <groupId>org.junit.platform</groupId>
+                        <artifactId>junit-platform-surefire-provider</artifactId>
+                        <version>${junit.platform.version}</version>
+                    </dependency>
+                </dependencies>
+                <executions>
+                    <execution>
+                        <id>junit5</id>
+                        <goals>
+                            <goal>integration-test</goal>
+                            <goal>verify</goal>
+                        </goals>
+                        <configuration>
+                            <includes>
+                                <include>**/*Test5.java</include>
+                            </includes>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+
+    <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <kotlin-maven-plugin.version>1.1.2</kotlin-maven-plugin.version>
+        <kotlin-test-junit.version>1.1.2</kotlin-test-junit.version>
+        <kotlin-stdlib.version>1.1.2</kotlin-stdlib.version>
+        <kotlin-reflect.version>1.1.2</kotlin-reflect.version>
+        <kotlinx.version>0.15</kotlinx.version>
+        <mockito-kotlin.version>1.5.0</mockito-kotlin.version>
+
+        <junit.jupiter.version>5.0.0</junit.jupiter.version>
+        <junit.platform.version>1.0.0</junit.platform.version>
+        <junit.vintage.version>4.12.0</junit.vintage.version>
+        <junit4.version>4.12</junit4.version>
+    </properties>
+</project>
+
--- a/core-groovy/src/main/groovy/com/baeldung/json/Account.groovy
+++ b/core-groovy/src/main/groovy/com/baeldung/json/Account.groovy
@ -0,0 +1,7 @@
+package com.baeldung.json
+
+class Account {
+    String id
+    BigDecimal value
+    Date createdAt
+}
--- a/core-groovy/src/main/groovy/com/baeldung/json/JsonParser.groovy
+++ b/core-groovy/src/main/groovy/com/baeldung/json/JsonParser.groovy
@ -0,0 +1,36 @@
+package com.baeldung.json
+
+import groovy.json.JsonGenerator
+import groovy.json.JsonOutput
+import groovy.json.JsonParserType
+import groovy.json.JsonSlurper
+
+class JsonParser {
+
+    Account toObject(String json) {
+        JsonSlurper jsonSlurper = new JsonSlurper()
+        jsonSlurper.parseText(json) as Account
+    }
+
+    Account toObjectWithIndexOverlay(String json) {
+        JsonSlurper jsonSlurper = new JsonSlurper(type: JsonParserType.INDEX_OVERLAY)
+        jsonSlurper.parseText(json) as Account
+    }
+
+    String toJson(Account account) {
+        JsonOutput.toJson(account)
+    }
+
+    String toJson(Account account, String dateFormat, String... fieldsToExclude) {
+        JsonGenerator generator = new JsonGenerator.Options()
+                .dateFormat(dateFormat)
+                .excludeFieldsByName(fieldsToExclude)
+                .build()
+        generator.toJson(account)
+    }
+
+    String prettyfy(String json) {
+        JsonOutput.prettyPrint(json)
+    }
+
+}
--- a/core-groovy/src/test/groovy/com/baeldung/groovy/sql/SqlTest.groovy
+++ b/core-groovy/src/test/groovy/com/baeldung/groovy/sql/SqlTest.groovy
@ -0,0 +1,229 @@
+package com.baeldung.groovy.sql
+
+import groovy.sql.GroovyResultSet
+import groovy.sql.GroovyRowResult
+import groovy.sql.Sql
+import groovy.transform.CompileStatic
+import static org.junit.Assert.*
+import org.junit.Test
+
+class SqlTest {
+
+    final Map dbConnParams = [url: 'jdbc:hsqldb:mem:testDB', user: 'sa', password: '', driver: 'org.hsqldb.jdbc.JDBCDriver']
+
+    @Test
+    void whenNewSqlInstance_thenDbIsAccessed() {
+        def sql = Sql.newInstance(dbConnParams)
+        sql.close()
+        sql.close()
+    }
+
+    @Test
+    void whenTableDoesNotExist_thenSelectFails() {
+        try {
+            Sql.withInstance(dbConnParams) { Sql sql ->
+                sql.eachRow('select * from PROJECT') {}
+            }
+
+            fail("An exception should have been thrown")
+        } catch (ignored) {
+            //Ok
+        }
+    }
+
+    @Test
+    void whenTableCreated_thenSelectIsPossible() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            def result = sql.execute 'create table PROJECT_1 (id integer not null, name varchar(50), url varchar(100))'
+
+            assertEquals(0, sql.updateCount)
+            assertFalse(result)
+
+            result = sql.execute('select * from PROJECT_1')
+
+            assertTrue(result)
+        }
+    }
+
+    @Test
+    void whenIdentityColumn_thenInsertReturnsNewId() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_2 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            def ids = sql.executeInsert("INSERT INTO PROJECT_2 (NAME, URL) VALUES ('tutorials', 'github.com/eugenp/tutorials')")
+
+            assertEquals(0, ids[0][0])
+
+            ids = sql.executeInsert("INSERT INTO PROJECT_2 (NAME, URL) VALUES ('REST with Spring', 'github.com/eugenp/REST-With-Spring')")
+
+            assertEquals(1, ids[0][0])
+        }
+    }
+
+    @Test
+    void whenUpdate_thenNumberOfAffectedRows() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_3 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            sql.executeInsert("INSERT INTO PROJECT_3 (NAME, URL) VALUES ('tutorials', 'github.com/eugenp/tutorials')")
+            sql.executeInsert("INSERT INTO PROJECT_3 (NAME, URL) VALUES ('REST with Spring', 'github.com/eugenp/REST-With-Spring')")
+            def count = sql.executeUpdate("UPDATE PROJECT_3 SET URL = 'https://' + URL")
+
+            assertEquals(2, count)
+        }
+    }
+
+    @Test
+    void whenEachRow_thenResultSetHasProperties() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_4 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            sql.executeInsert("INSERT INTO PROJECT_4 (NAME, URL) VALUES ('tutorials', 'https://github.com/eugenp/tutorials')")
+            sql.executeInsert("INSERT INTO PROJECT_4 (NAME, URL) VALUES ('REST with Spring', 'https://github.com/eugenp/REST-With-Spring')")
+
+            sql.eachRow("SELECT * FROM PROJECT_4") { GroovyResultSet rs ->
+                assertNotNull(rs.name)
+                assertNotNull(rs.url)
+                assertNotNull(rs[0])
+                assertNotNull(rs[1])
+                assertNotNull(rs[2])
+                assertEquals(rs.name, rs['name'])
+                assertEquals(rs.url, rs['url'])
+            }
+        }
+    }
+
+    @Test
+    void whenPagination_thenSubsetIsReturned() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_5 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            sql.executeInsert("INSERT INTO PROJECT_5 (NAME, URL) VALUES ('tutorials', 'github.com/eugenp/tutorials')")
+            sql.executeInsert("INSERT INTO PROJECT_5 (NAME, URL) VALUES ('REST with Spring', 'github.com/eugenp/REST-With-Spring')")
+            def rows = sql.rows('SELECT * FROM PROJECT_5 ORDER BY NAME', 1, 1)
+
+            assertEquals(1, rows.size())
+            assertEquals('REST with Spring', rows[0].name)
+        }
+    }
+
+    @Test
+    void whenParameters_thenReplacement() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_6 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            sql.execute('INSERT INTO PROJECT_6 (NAME, URL) VALUES (?, ?)', 'tutorials', 'github.com/eugenp/tutorials')
+            sql.execute("INSERT INTO PROJECT_6 (NAME, URL) VALUES (:name, :url)", [name: 'REST with Spring', url: 'github.com/eugenp/REST-With-Spring'])
+
+            def rows = sql.rows("SELECT * FROM PROJECT_6 WHERE NAME = 'tutorials'")
+
+            assertEquals(1, rows.size())
+
+            rows = sql.rows("SELECT * FROM PROJECT_6 WHERE NAME = 'REST with Spring'")
+
+            assertEquals(1, rows.size())
+        }
+    }
+
+    @Test
+    void whenParametersInGString_thenReplacement() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_7 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            sql.execute "INSERT INTO PROJECT_7 (NAME, URL) VALUES (${'tutorials'}, ${'github.com/eugenp/tutorials'})"
+            def name = 'REST with Spring'
+            def url = 'github.com/eugenp/REST-With-Spring'
+            sql.execute "INSERT INTO PROJECT_7 (NAME, URL) VALUES (${name}, ${url})"
+
+            def rows = sql.rows("SELECT * FROM PROJECT_7 WHERE NAME = 'tutorials'")
+
+            assertEquals(1, rows.size())
+
+            rows = sql.rows("SELECT * FROM PROJECT_7 WHERE NAME = 'REST with Spring'")
+
+            assertEquals(1, rows.size())
+        }
+    }
+
+    @Test
+    void whenTransactionRollback_thenNoDataInserted() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.withTransaction {
+                sql.execute 'create table PROJECT_8 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+                sql.executeInsert("INSERT INTO PROJECT_8 (NAME, URL) VALUES ('tutorials', 'https://github.com/eugenp/tutorials')")
+                sql.executeInsert("INSERT INTO PROJECT_8 (NAME, URL) VALUES ('REST with Spring', 'https://github.com/eugenp/REST-With-Spring')")
+                sql.rollback()
+
+                def rows = sql.rows("SELECT * FROM PROJECT_8")
+
+                assertEquals(0, rows.size())
+            }
+        }
+    }
+
+    @Test
+    void whenTransactionRollbackThenCommit_thenOnlyLastInserted() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.withTransaction {
+                sql.execute 'create table PROJECT_9 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+                sql.executeInsert("INSERT INTO PROJECT_9 (NAME, URL) VALUES ('tutorials', 'https://github.com/eugenp/tutorials')")
+                sql.rollback()
+                sql.executeInsert("INSERT INTO PROJECT_9 (NAME, URL) VALUES ('REST with Spring', 'https://github.com/eugenp/REST-With-Spring')")
+                sql.rollback()
+                sql.executeInsert("INSERT INTO PROJECT_9 (NAME, URL) VALUES ('tutorials', 'https://github.com/eugenp/tutorials')")
+                sql.executeInsert("INSERT INTO PROJECT_9 (NAME, URL) VALUES ('REST with Spring', 'https://github.com/eugenp/REST-With-Spring')")
+            }
+
+            def rows = sql.rows("SELECT * FROM PROJECT_9")
+
+            assertEquals(2, rows.size())
+        }
+    }
+
+    @Test
+    void whenException_thenTransactionIsRolledBack() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            try {
+                sql.withTransaction {
+                    sql.execute 'create table PROJECT_10 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+                    sql.executeInsert("INSERT INTO PROJECT_10 (NAME, URL) VALUES ('tutorials', 'https://github.com/eugenp/tutorials')")
+                    throw new Exception('rollback')
+                }
+            } catch (ignored) {}
+
+            def rows = sql.rows("SELECT * FROM PROJECT_10")
+
+            assertEquals(0, rows.size())
+        }
+    }
+
+    @Test
+    void givenCachedConnection_whenException_thenDataIsPersisted() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            try {
+                sql.cacheConnection {
+                    sql.execute 'create table PROJECT_11 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+                    sql.executeInsert("INSERT INTO PROJECT_11 (NAME, URL) VALUES ('tutorials', 'https://github.com/eugenp/tutorials')")
+                    throw new Exception('This does not rollback')
+                }
+            } catch (ignored) {}
+
+            def rows = sql.rows("SELECT * FROM PROJECT_11")
+
+            assertEquals(1, rows.size())
+        }
+    }
+
+    /*@Test
+    void whenModifyResultSet_thenDataIsChanged() {
+        Sql.withInstance(dbConnParams) { Sql sql ->
+            sql.execute 'create table PROJECT_5 (ID IDENTITY, NAME VARCHAR (50), URL VARCHAR (100))'
+            sql.executeInsert("INSERT INTO PROJECT_5 (NAME, URL) VALUES ('tutorials', 'github.com/eugenp/tutorials')")
+            sql.executeInsert("INSERT INTO PROJECT_5 (NAME, URL) VALUES ('REST with Spring', 'github.com/eugenp/REST-With-Spring')")
+
+            sql.eachRow("SELECT * FROM PROJECT_5 FOR UPDATE ") { GroovyResultSet rs ->
+                rs['name'] = "The great ${rs.name}!" as String
+                rs.updateRow()
+            }
+
+            sql.eachRow("SELECT * FROM PROJECT_5") { GroovyResultSet rs ->
+                assertTrue(rs.name.startsWith('The great '))
+            }
+        }
+    }*/
+
+}
--- a/core-groovy/src/test/groovy/com/baeldung/json/JsonParserTest.groovy
+++ b/core-groovy/src/test/groovy/com/baeldung/json/JsonParserTest.groovy
@ -0,0 +1,80 @@
+package com.baeldung.json
+
+import spock.lang.Specification
+
+import java.text.SimpleDateFormat
+
+class JsonParserTest extends Specification {
+
+    JsonParser jsonParser
+
+    void setup () {
+        jsonParser = new JsonParser()
+    }
+
+    def 'Should parse to Account given Json String' () {
+        given:
+            def json = '{"id":"1234","value":15.6}'
+        when:
+            def account = jsonParser.toObject(json)
+        then:
+            account
+            account instanceof Account
+            account.id == '1234'
+            account.value == 15.6
+    }
+
+    def 'Should parse to Account given Json String with date property' () {
+        given:
+            def json = '{"id":"1234","value":15.6,"createdAt":"2018-01-01T00:00:00+0000"}'
+        when:
+            def account = jsonParser.toObjectWithIndexOverlay(json)
+        then:
+            account
+            account instanceof Account
+            account.id == '1234'
+            account.value == 15.6
+            println account.createdAt
+            account.createdAt == Date.parse('yyyy-MM-dd', '2018-01-01')
+    }
+
+    def 'Should parse to Json given an Account object' () {
+        given:
+            Account account = new Account(
+                    id: '123',
+                    value: 15.6,
+                    createdAt: new SimpleDateFormat('MM/dd/yyyy').parse('01/01/2018')
+            )
+        when:
+            def json = jsonParser.toJson(account)
+        then:
+            json
+            json == '{"value":15.6,"createdAt":"2018-01-01T00:00:00+0000","id":"123"}'
+    }
+
+    def 'Should parse to Json given an Account object, a date format and fields to exclude' () {
+        given:
+            Account account = new Account(
+                    id: '123',
+                    value: 15.6,
+                    createdAt: new SimpleDateFormat('MM/dd/yyyy').parse('01/01/2018')
+            )
+        when:
+            def json = jsonParser.toJson(account, 'MM/dd/yyyy', 'value')
+        then:
+            json
+            json == '{"createdAt":"01/01/2018","id":"123"}'
+    }
+
+    def 'Should prettify given a json string' () {
+        given:
+            String json = '{"value":15.6,"createdAt":"01/01/2018","id":"123456"}'
+        when:
+            def jsonPretty = jsonParser.prettyfy(json)
+        then:
+            jsonPretty
+            jsonPretty == '{\n    "value": 15.6,\n    "createdAt": "01/01/2018",\n    "id": "123456"\n}'
+    }
+
+
+}
--- a/core-java-8/README.md
+++ b/core-java-8/README.md
@ -40,3 +40,4 @@
 - [Efficient Word Frequency Calculator in Java](http://www.baeldung.com/java-word-frequency)
 - [Primitive Type Streams in Java 8](http://www.baeldung.com/java-8-primitive-streams)
 - [Fail-Safe Iterator vs Fail-Fast Iterator](http://www.baeldung.com/java-fail-safe-vs-fail-fast-iterator)
+- [Shuffling Collections In Java](http://www.baeldung.com/java-shuffle-collection)
--- a/core-java-8/src/test/java/com/baeldung/shufflingcollections/ShufflingCollectionsUnitTest.java
+++ b/core-java-8/src/test/java/com/baeldung/shufflingcollections/ShufflingCollectionsUnitTest.java
@ -22,7 +22,7 @@ public class ShufflingCollectionsUnitTest {
    }

    @Test
-    public void whenShufflingMapKeys_thenValuesAreShuffled() {
+    public void whenShufflingMapEntries_thenValuesAreShuffled() {
        Map<Integer, String> studentsById = new HashMap<>();
        studentsById.put(1, "Foo");
        studentsById.put(2, "Bar");
@ -32,12 +32,12 @@ public class ShufflingCollectionsUnitTest {
        System.out.println("Students before shuffling:");
        System.out.println(studentsById.values());

-        List<Integer> shuffledStudentIds = new ArrayList<>(studentsById.keySet());
-        Collections.shuffle(shuffledStudentIds);
+        List<Map.Entry<Integer, String>> shuffledStudentEntries = new ArrayList<>(studentsById.entrySet());
+        Collections.shuffle(shuffledStudentEntries);

-        List<String> shuffledStudents = shuffledStudentIds.stream()
-          .map(id -> studentsById.get(id))
-          .collect(Collectors.toList());
+        List<String> shuffledStudents = shuffledStudentEntries.stream()
+                .map(Map.Entry::getValue)
+                .collect(Collectors.toList());

        System.out.println("Students after shuffling");
        System.out.println(shuffledStudents);
--- a/core-java-9/src/main/java/com/baeldung/java9/methodhandles/Book.java
+++ b/core-java-9/src/main/java/com/baeldung/java9/methodhandles/Book.java
@ -0,0 +1,17 @@
+package com.baeldung.java9.methodhandles;
+
+public class Book {
+
+    String id;
+    String title;
+
+    public Book(String id, String title) {
+        this.id = id;
+        this.title = title;
+    }
+
+    @SuppressWarnings("unused")
+    private String formatBook() {
+        return id + " > " + title;
+    }
+}
--- a/core-java-9/src/test/java/com/baeldung/java9/methodhandles/MethodHandlesTest.java
+++ b/core-java-9/src/test/java/com/baeldung/java9/methodhandles/MethodHandlesTest.java
@ -0,0 +1,152 @@
+package com.baeldung.java9.methodhandles;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.*;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+import java.lang.invoke.WrongMethodTypeException;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.Test;
+
+/**
+ * Test case for the {@link MethodHandles} API
+ */
+public class MethodHandlesTest {
+
+    @Test
+    public void givenConcatMethodHandle_whenInvoked_thenCorrectlyConcatenated() throws Throwable {
+
+        MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
+        MethodType mt = MethodType.methodType(String.class, String.class);
+        MethodHandle concatMH = publicLookup.findVirtual(String.class, "concat", mt);
+
+        String output = (String) concatMH.invoke("Effective ", "Java");
+
+        assertEquals("Effective Java", output);
+    }
+
+    @Test
+    public void givenAsListMethodHandle_whenInvokingWithArguments_thenCorrectlyInvoked() throws Throwable {
+        MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
+        MethodType mt = MethodType.methodType(List.class, Object[].class);
+        MethodHandle asListMH = publicLookup.findStatic(Arrays.class, "asList", mt);
+
+        List<Integer> list = (List<Integer>) asListMH.invokeWithArguments(1, 2);
+
+        assertThat(Arrays.asList(1, 2), is(list));
+    }
+
+    @Test
+    public void givenConstructorMethodHandle_whenInvoked_thenObjectCreatedCorrectly() throws Throwable {
+        MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
+        MethodType mt = MethodType.methodType(void.class, String.class);
+        MethodHandle newIntegerMH = publicLookup.findConstructor(Integer.class, mt);
+
+        Integer integer = (Integer) newIntegerMH.invoke("1");
+
+        assertEquals(1, integer.intValue());
+    }
+
+    @Test
+    public void givenAFieldWithoutGetter_whenCreatingAGetter_thenCorrectlyInvoked() throws Throwable {
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+        MethodHandle getTitleMH = lookup.findGetter(Book.class, "title", String.class);
+
+        Book book = new Book("ISBN-1234", "Effective Java");
+
+        assertEquals("Effective Java", getTitleMH.invoke(book));
+    }
+
+    @Test
+    public void givenPrivateMethod_whenCreatingItsMethodHandle_thenCorrectlyInvoked() throws Throwable {
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+        Method formatBookMethod = Book.class.getDeclaredMethod("formatBook");
+        formatBookMethod.setAccessible(true);
+
+        MethodHandle formatBookMH = lookup.unreflect(formatBookMethod);
+
+        Book book = new Book("ISBN-123", "Java in Action");
+
+        assertEquals("ISBN-123 > Java in Action", formatBookMH.invoke(book));
+    }
+
+    @Test
+    public void givenReplaceMethod_whenUsingReflectionAndInvoked_thenCorrectlyReplaced() throws Throwable {
+        Method replaceMethod = String.class.getMethod("replace", char.class, char.class);
+
+        String string = (String) replaceMethod.invoke("jovo", 'o', 'a');
+
+        assertEquals("java", string);
+    }
+
+    @Test
+    public void givenReplaceMethodHandle_whenInvoked_thenCorrectlyReplaced() throws Throwable {
+        MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
+        MethodType mt = MethodType.methodType(String.class, char.class, char.class);
+        MethodHandle replaceMH = publicLookup.findVirtual(String.class, "replace", mt);
+
+        String replacedString = (String) replaceMH.invoke("jovo", Character.valueOf('o'), 'a');
+
+        assertEquals("java", replacedString);
+    }
+
+    @Test
+    public void givenReplaceMethodHandle_whenInvokingExact_thenCorrectlyReplaced() throws Throwable {
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+        MethodType mt = MethodType.methodType(String.class, char.class, char.class);
+        MethodHandle replaceMH = lookup.findVirtual(String.class, "replace", mt);
+
+        String s = (String) replaceMH.invokeExact("jovo", 'o', 'a');
+
+        assertEquals("java", s);
+    }
+
+    @Test
+    public void givenSumMethodHandle_whenInvokingExact_thenSumIsCorrect() throws Throwable {
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+        MethodType mt = MethodType.methodType(int.class, int.class, int.class);
+        MethodHandle sumMH = lookup.findStatic(Integer.class, "sum", mt);
+
+        int sum = (int) sumMH.invokeExact(1, 11);
+
+        assertEquals(12, sum);
+    }
+
+    @Test(expected = WrongMethodTypeException.class)
+    public void givenSumMethodHandleAndIncompatibleArguments_whenInvokingExact_thenException() throws Throwable {
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+        MethodType mt = MethodType.methodType(int.class, int.class, int.class);
+        MethodHandle sumMH = lookup.findStatic(Integer.class, "sum", mt);
+
+        sumMH.invokeExact(Integer.valueOf(1), 11);
+    }
+
+    @Test
+    public void givenSpreadedEqualsMethodHandle_whenInvokedOnArray_thenCorrectlyEvaluated() throws Throwable {
+        MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
+        MethodType mt = MethodType.methodType(boolean.class, Object.class);
+        MethodHandle equalsMH = publicLookup.findVirtual(String.class, "equals", mt);
+
+        MethodHandle methodHandle = equalsMH.asSpreader(Object[].class, 2);
+
+        assertTrue((boolean) methodHandle.invoke(new Object[] { "java", "java" }));
+        assertFalse((boolean) methodHandle.invoke(new Object[] { "java", "jova" }));
+    }
+
+    @Test
+    public void givenConcatMethodHandle_whenBindToAString_thenCorrectlyConcatenated() throws Throwable {
+        MethodHandles.Lookup publicLookup = MethodHandles.publicLookup();
+        MethodType mt = MethodType.methodType(String.class, String.class);
+        MethodHandle concatMH = publicLookup.findVirtual(String.class, "concat", mt);
+
+        MethodHandle bindedConcatMH = concatMH.bindTo("Hello ");
+
+        assertEquals("Hello World!", bindedConcatMH.invoke("World!"));
+    }
+}
--- a/core-java-concurrency/src/test/java/com/baeldung/concurrent/stopping/StopThreadManualTest.java
+++ b/core-java-concurrency/src/test/java/com/baeldung/concurrent/stopping/StopThreadManualTest.java
@ -10,7 +10,7 @@ import org.junit.Test;

 import com.jayway.awaitility.Awaitility;

-public class StopThreadTest {
+public class StopThreadManualTest {

    @Test
    public void whenStoppedThreadIsStopped() throws InterruptedException {
--- a/core-java/README.md
+++ b/core-java/README.md
@ -127,3 +127,5 @@
 - [Polymorphism in Java](http://www.baeldung.com/java-polymorphism)
 - [Recursion In Java](http://www.baeldung.com/java-recursion)
 - [A Guide to the finalize Method in Java](http://www.baeldung.com/java-finalize)
+- [Compiling Java *.class Files with javac](http://www.baeldung.com/javac)
+- [Method Overloading and Overriding in Java](http://www.baeldung.com/java-method-overload-override)
--- a/core-java/pom.xml
+++ b/core-java/pom.xml
@ -78,6 +78,11 @@
            <artifactId>jackson-databind</artifactId>
            <version>${jackson.version}</version>
        </dependency>
+        <dependency>
+            <groupId>com.google.code.gson</groupId>
+            <artifactId>gson</artifactId>
+            <version>${gson.version}</version>
+        </dependency>

        <!-- logging -->
        <dependency>
@ -468,6 +473,7 @@
    <properties>
        <!-- marshalling -->
        <jackson.version>2.8.5</jackson.version>
+        <gson.version>2.8.2</gson.version>

        <!-- logging -->
        <org.slf4j.version>1.7.21</org.slf4j.version>
--- a/core-java/src/main/java/com/baeldung/casting/Animal.java
+++ b/core-java/src/main/java/com/baeldung/casting/Animal.java
@ -0,0 +1,13 @@
+package com.baeldung.casting;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Animal {
+    private static final Logger LOGGER = LoggerFactory.getLogger(Animal.class);
+
+    public void eat() {
+        LOGGER.info("animal is eating");
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/casting/AnimalFeeder.java
+++ b/core-java/src/main/java/com/baeldung/casting/AnimalFeeder.java
@ -0,0 +1,23 @@
+package com.baeldung.casting;
+
+import java.util.List;
+
+public class AnimalFeeder {
+
+    public void feed(List<Animal> animals) {
+        animals.forEach(animal -> {
+            animal.eat();
+            if (animal instanceof Cat) {
+                ((Cat) animal).meow();
+            }
+        });
+    }
+
+    public void uncheckedFeed(List<Animal> animals) {
+        animals.forEach(animal -> {
+            animal.eat();
+            ((Cat) animal).meow();
+        });
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/casting/Cat.java
+++ b/core-java/src/main/java/com/baeldung/casting/Cat.java
@ -0,0 +1,16 @@
+package com.baeldung.casting;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Cat extends Animal implements Mew {
+    private static final Logger LOGGER = LoggerFactory.getLogger(Cat.class);
+
+    public void eat() {
+        LOGGER.info("cat is eating");
+    }
+
+    public void meow() {
+        LOGGER.info("meow");
+    }
+}
--- a/core-java/src/main/java/com/baeldung/casting/Dog.java
+++ b/core-java/src/main/java/com/baeldung/casting/Dog.java
@ -0,0 +1,12 @@
+package com.baeldung.casting;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Dog extends Animal {
+    private static final Logger LOGGER = LoggerFactory.getLogger(Dog.class);
+
+    public void eat() {
+        LOGGER.info("dog is eating");
+    }
+}
--- a/core-java/src/main/java/com/baeldung/casting/Mew.java
+++ b/core-java/src/main/java/com/baeldung/casting/Mew.java
@ -0,0 +1,5 @@
+package com.baeldung.casting;
+
+public interface Mew {
+    public void meow();
+}
--- a/core-java/src/main/java/com/baeldung/classloader/CustomClassLoader.java
+++ b/core-java/src/main/java/com/baeldung/classloader/CustomClassLoader.java
@ -0,0 +1,29 @@
+package com.baeldung.classloader;
+
+import java.io.*;
+
+public class CustomClassLoader extends ClassLoader {
+
+
+    public Class getClass(String name) throws ClassNotFoundException {
+        byte[] b = loadClassFromFile(name);
+        return defineClass(name, b, 0, b.length);
+    }
+
+    private byte[] loadClassFromFile(String fileName)  {
+        InputStream inputStream = getClass().getClassLoader().getResourceAsStream(
+                fileName.replace('.', File.separatorChar) + ".class");
+        byte[] buffer;
+        ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
+        int nextValue = 0;
+        try {
+            while ( (nextValue = inputStream.read()) != -1 ) {
+                byteStream.write(nextValue);
+            }
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+        buffer = byteStream.toByteArray();
+        return buffer;
+    }
+}
--- a/core-java/src/main/java/com/baeldung/classloader/PrintClassLoader.java
+++ b/core-java/src/main/java/com/baeldung/classloader/PrintClassLoader.java
@ -0,0 +1,22 @@
+package com.baeldung.classloader;
+
+import com.sun.javafx.util.Logging;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLConnection;
+import java.util.ArrayList;
+
+public class PrintClassLoader {
+
+    public void printClassLoaders() throws ClassNotFoundException {
+
+        System.out.println("Classloader of this class:" + PrintClassLoader.class.getClassLoader());
+        System.out.println("Classloader of Logging:" + Logging.class.getClassLoader());
+        System.out.println("Classloader of ArrayList:" + ArrayList.class.getClassLoader());
+
+    }
+}
--- a/core-java/src/main/java/com/baeldung/deepcopy/Address.java
+++ b/core-java/src/main/java/com/baeldung/deepcopy/Address.java
@ -0,0 +1,59 @@
+package com.baeldung.deepcopy;
+
+import java.io.Serializable;
+
+public class Address implements Serializable, Cloneable {
+
+    @Override
+    public Object clone() {
+        try {
+            return (Address) super.clone();
+        } catch (CloneNotSupportedException e) {
+            return new Address(this.street, this.getCity(), this.getCountry());
+        }
+    }
+
+    private static final long serialVersionUID = 1740913841244949416L;
+    private String street;
+    private String city;
+
+    private String country;
+
+    public Address(Address that) {
+        this(that.getStreet(), that.getCity(), that.getCountry());
+    }
+
+    public Address(String street, String city, String country) {
+        this.street = street;
+        this.city = city;
+        this.country = country;
+    }
+
+    public Address() {
+    }
+
+    public String getStreet() {
+        return street;
+    }
+
+    public String getCity() {
+        return city;
+    }
+
+    public String getCountry() {
+        return country;
+    }
+
+    public void setStreet(String street) {
+        this.street = street;
+    }
+
+    public void setCity(String city) {
+        this.city = city;
+    }
+
+    public void setCountry(String country) {
+        this.country = country;
+    }
+}
+
--- a/core-java/src/main/java/com/baeldung/deepcopy/User.java
+++ b/core-java/src/main/java/com/baeldung/deepcopy/User.java
@ -0,0 +1,48 @@
+package com.baeldung.deepcopy;
+
+import java.io.Serializable;
+
+public class User implements Serializable, Cloneable {
+
+    private static final long serialVersionUID = -3427002229954777557L;
+    private String firstName;
+    private String lastName;
+    private Address address;
+
+    public User(String firstName, String lastName, Address address) {
+        this.firstName = firstName;
+        this.lastName = lastName;
+        this.address = address;
+    }
+
+    public User(User that) {
+        this(that.getFirstName(), that.getLastName(), new Address(that.getAddress()));
+    }
+
+    public User() {
+    }
+
+    @Override
+    public Object clone() {
+        User user;
+        try {
+            user = (User) super.clone();
+        } catch (CloneNotSupportedException e) {
+            user = new User(this.getFirstName(), this.getLastName(), this.getAddress());
+        }
+        user.address = (Address) this.address.clone();
+        return user;
+    }
+
+    public String getFirstName() {
+        return firstName;
+    }
+
+    public String getLastName() {
+        return lastName;
+    }
+
+    public Address getAddress() {
+        return address;
+    }
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/AuthenticationProcessor.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/AuthenticationProcessor.java
@ -0,0 +1,13 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public abstract class AuthenticationProcessor {
+
+    // next element in chain or responsibility
+    public AuthenticationProcessor nextProcessor;
+    
+    public AuthenticationProcessor(AuthenticationProcessor nextProcessor) {
+        this.nextProcessor = nextProcessor;
+    }
+
+    public abstract boolean isAuthorized(AuthenticationProvider authProvider);
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/AuthenticationProvider.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/AuthenticationProvider.java
@ -0,0 +1,5 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public interface AuthenticationProvider {
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/OAuthAuthenticationProcessor.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/OAuthAuthenticationProcessor.java
@ -0,0 +1,21 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public class OAuthAuthenticationProcessor extends AuthenticationProcessor {
+
+    public OAuthAuthenticationProcessor(AuthenticationProcessor nextProcessor) {
+        super(nextProcessor);
+    }
+
+    @Override
+    public boolean isAuthorized(AuthenticationProvider authProvider) {
+
+        if (authProvider instanceof OAuthTokenProvider) {
+            return Boolean.TRUE;
+        } else if (nextProcessor != null) {
+            return nextProcessor.isAuthorized(authProvider);
+        } else {
+            return Boolean.FALSE;
+        }
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/OAuthTokenProvider.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/OAuthTokenProvider.java
@ -0,0 +1,5 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public class OAuthTokenProvider implements AuthenticationProvider {
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/SamlAuthenticationProvider.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/SamlAuthenticationProvider.java
@ -0,0 +1,5 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public class SamlAuthenticationProvider implements AuthenticationProvider {
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/UsernamePasswordAuthenticationProcessor.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/UsernamePasswordAuthenticationProcessor.java
@ -0,0 +1,20 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public class UsernamePasswordAuthenticationProcessor extends AuthenticationProcessor {
+
+    public UsernamePasswordAuthenticationProcessor(AuthenticationProcessor nextProcessor) {
+        super(nextProcessor);
+    }
+
+    @Override
+    public boolean isAuthorized(AuthenticationProvider authProvider) {
+        if (authProvider instanceof UsernamePasswordProvider) {
+            return Boolean.TRUE;
+        } else if (nextProcessor != null) {
+            return nextProcessor.isAuthorized(authProvider);
+        } else {
+            return Boolean.FALSE;
+        }
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/UsernamePasswordProvider.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/chainofresponsibility/UsernamePasswordProvider.java
@ -0,0 +1,5 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+public class UsernamePasswordProvider implements AuthenticationProvider {
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/flyweight/Car.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/flyweight/Car.java
@ -0,0 +1,85 @@
+package com.baeldung.designpatterns.flyweight;
+
+import java.awt.Color;
+
+import javax.annotation.concurrent.Immutable;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Represents a car. This class is immutable.
+ * 
+ * @author Donato Rimenti
+ */
+@Immutable
+public class Car implements Vehicle {
+
+	/**
+	 * Logger.
+	 */
+	private final static Logger LOG = LoggerFactory.getLogger(Car.class);
+
+	/**
+	 * The car's engine.
+	 */
+	private Engine engine;
+
+	/**
+	 * The car's color.
+	 */
+	private Color color;
+
+	/**
+	 * Instantiates a new Car.
+	 *
+	 * @param engine
+	 *            the {@link #engine}
+	 * @param color
+	 *            the {@link #color}
+	 */
+	public Car(Engine engine, Color color) {
+		this.engine = engine;
+		this.color = color;
+
+		// Building a new car is a very expensive operation!
+		try {
+			Thread.sleep(2000);
+		} catch (InterruptedException e) {
+			LOG.error("Error while creating a new car", e);
+		}
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see com.baeldung.designpatterns.flyweight.Vehicle#start()
+	 */
+	@Override
+	public void start() {
+		LOG.info("Car is starting!");
+		engine.start();
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see com.baeldung.designpatterns.flyweight.Vehicle#stop()
+	 */
+	@Override
+	public void stop() {
+		LOG.info("Car is stopping!");
+		engine.stop();
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see com.baeldung.designpatterns.flyweight.Vehicle#getColor()
+	 */
+	@Override
+	public Color getColor() {
+		return this.color;
+	}
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/flyweight/Engine.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/flyweight/Engine.java
@ -0,0 +1,31 @@
+package com.baeldung.designpatterns.flyweight;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Engine for a vehicle.
+ * 
+ * @author Donato Rimenti
+ */
+public class Engine {
+
+	/**
+	 * Logger.
+	 */
+	private final static Logger LOG = LoggerFactory.getLogger(Engine.class);
+
+	/**
+	 * Starts the engine.
+	 */
+	public void start() {
+		LOG.info("Engine is starting!");
+	}
+
+	/**
+	 * Stops the engine.
+	 */
+	public void stop() {
+		LOG.info("Engine is stopping!");
+	}
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/flyweight/Vehicle.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/flyweight/Vehicle.java
@ -0,0 +1,29 @@
+package com.baeldung.designpatterns.flyweight;
+
+import java.awt.Color;
+
+/**
+ * Interface for a vehicle.
+ * 
+ * @author Donato Rimenti
+ */
+public interface Vehicle {
+
+	/**
+	 * Starts the vehicle.
+	 */
+	public void start();
+
+	/**
+	 * Stops the vehicle.
+	 */
+	public void stop();
+
+	/**
+	 * Gets the color of the vehicle.
+	 * 
+	 * @return the color of the vehicle
+	 */
+	public Color getColor();
+
+}
--- a/core-java/src/main/java/com/baeldung/designpatterns/flyweight/VehicleFactory.java
+++ b/core-java/src/main/java/com/baeldung/designpatterns/flyweight/VehicleFactory.java
@ -0,0 +1,45 @@
+package com.baeldung.designpatterns.flyweight;
+
+import java.awt.Color;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Factory which implements the Flyweight pattern to return an existing vehicle
+ * if present or a new one otherwise.
+ * 
+ * @author Donato Rimenti
+ */
+public class VehicleFactory {
+
+	/**
+	 * Stores the already created vehicles.
+	 */
+	private static Map<Color, Vehicle> vehiclesCache = new HashMap<Color, Vehicle>();
+
+	/**
+	 * Private constructor to prevent this class instantiation.
+	 */
+	private VehicleFactory() {
+	}
+
+	/**
+	 * Returns a vehicle of the same color passed as argument. If that vehicle
+	 * was already created by this factory, that vehicle is returned, otherwise
+	 * a new one is created and returned.
+	 * 
+	 * @param color
+	 *            the color of the vehicle to return
+	 * @return a vehicle of the specified color
+	 */
+	public static Vehicle createVehicle(Color color) {
+		// Looks for the requested vehicle into the cache.
+		// If the vehicle doesn't exist, a new one is created.
+		Vehicle newVehicle = vehiclesCache.computeIfAbsent(color, newColor -> {
+			// Creates the new car.
+			Engine newEngine = new Engine();
+			return new Car(newEngine, newColor);
+		});
+		return newVehicle;
+	}
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/ArmoredCar.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/ArmoredCar.java
@ -0,0 +1,43 @@
+package com.baeldung.inheritance;
+
+public class ArmoredCar extends Car implements Floatable, Flyable{
+    private int bulletProofWindows;
+    private String model;
+    
+    public void remoteStartCar() {
+        // this vehicle can be started by using a remote control
+    }
+    
+    public String registerModel() {
+        return model;
+    }
+    
+    public String getAValue() {
+        return super.model;   // returns value of model defined in base class Car
+        // return this.model;   // will return value of model defined in ArmoredCar
+        // return model;   // will return value of model defined in ArmoredCar
+    }
+    
+    public static String msg() {
+    	// return super.msg(); // this won't compile.
+    	return "ArmoredCar"; 
+    }
+    
+	@Override
+    public void floatOnWater() {
+        System.out.println("I can float!");
+    }
+    
+	@Override
+    public void fly() {
+        System.out.println("I can fly!");
+    }
+    
+    public void aMethod() {
+        // System.out.println(duration); // Won't compile
+        System.out.println(Floatable.duration); // outputs 10
+        System.out.println(Flyable.duration); // outputs 20
+    }
+    
+    
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/BMW.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/BMW.java
@ -0,0 +1,12 @@
+package com.baeldung.inheritance;
+
+public class BMW extends Car {
+    public BMW() {
+    	super(5, "BMW");
+    }
+	
+	@Override
+    public String toString() {
+    	return model;
+    }
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/Car.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/Car.java
@ -0,0 +1,32 @@
+package com.baeldung.inheritance;
+
+public class Car {
+	private final int DEFAULT_WHEEL_COUNT = 5;
+	private final String DEFAULT_MODEL = "Basic";
+
+	protected int wheels;
+    protected String model;
+    
+    public Car() {
+    	this.wheels = DEFAULT_WHEEL_COUNT;
+    	this.model = DEFAULT_MODEL;
+    }
+    
+    public Car(int wheels, String model) {
+    	this.wheels = wheels;
+    	this.model = model;
+    }
+        
+    public void start() {
+        // Check essential parts
+        // If okay, start.
+    }
+    public static int count = 10;
+    public static String msg() {
+        return "Car";
+    }
+    
+    public String toString() {
+    	return model;
+    } 
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/Employee.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/Employee.java
@ -0,0 +1,15 @@
+package com.baeldung.inheritance;
+
+public class Employee {
+    private String name;
+    private Car car;
+    
+    public Employee(String name, Car car) {
+        this.name = name;
+        this.car = car;
+    }
+    
+    public Car getCar() {
+    	return car;
+    }
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/Floatable.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/Floatable.java
@ -0,0 +1,10 @@
+package com.baeldung.inheritance;
+
+public interface Floatable {
+    int duration = 10;
+    void floatOnWater();
+    
+    default void repair() {
+        System.out.println("Repairing Floatable object");   
+    }
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/Flyable.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/Flyable.java
@ -0,0 +1,13 @@
+package com.baeldung.inheritance;
+
+public interface Flyable {
+    int duration = 10;
+    void fly();
+    
+    /*
+     * Commented 
+     */
+    //default void repair() {
+    //    System.out.println("Repairing Flyable object"); 
+    //}
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/SpaceCar.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/SpaceCar.java
@ -0,0 +1,18 @@
+package com.baeldung.inheritance;
+
+public class SpaceCar extends Car implements SpaceTraveller {
+	@Override
+    public void floatOnWater() {
+        System.out.println("SpaceCar floating!");
+    }
+    
+	@Override
+    public void fly() {
+        System.out.println("SpaceCar flying!");
+    }
+    
+	@Override
+    public void remoteControl() {
+        System.out.println("SpaceCar being controlled remotely!");
+    }
+}
--- a/core-java/src/main/java/com/baeldung/inheritance/SpaceTraveller.java
+++ b/core-java/src/main/java/com/baeldung/inheritance/SpaceTraveller.java
@ -0,0 +1,6 @@
+package com.baeldung.inheritance;
+
+public interface SpaceTraveller extends Floatable, Flyable {
+    int duration = 10;
+    void remoteControl();
+}
--- a/core-java/src/main/java/com/baeldung/java/list/CustomList.java
+++ b/core-java/src/main/java/com/baeldung/java/list/CustomList.java
@ -0,0 +1,229 @@
+package com.baeldung.java.list;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.ListIterator;
+
+public class CustomList<E> implements List<E> {
+    private Object[] internal = {};
+
+    @Override
+    public void add(int index, E element) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean addAll(Collection<? extends E> collection) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean addAll(int index, Collection<? extends E> collection) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public E remove(int index) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean remove(Object object) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean removeAll(Collection<?> collection) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean retainAll(Collection<?> collection) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public int size() {
+        return internal.length;
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return internal.length == 0;
+    }
+
+    @Override
+    public boolean add(E element) {
+        // the first cycle
+        // internal = new Object[1];
+        // internal[0] = element;
+        // return true;
+
+        Object[] temp = new Object[internal.length + 1];
+        System.arraycopy(internal, 0, temp, 0, internal.length);
+        temp[internal.length] = element;
+        internal = temp;
+        return true;
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public E get(int index) {
+        return (E) internal[index];
+    }
+
+    @Override
+    public boolean contains(Object object) {
+        // return false
+
+        for (Object element : internal) {
+            if (object.equals(element)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    @Override
+    public boolean containsAll(Collection<?> collection) {
+        // the first cycle
+        // for (Object element : collection) {
+        //     if (element.equals(internal[0])) {
+        //         return true;
+        //     }
+        // }
+        // return false;
+
+        for (Object element : collection)
+            if (!contains(element)) {
+                return false;
+            }
+        return true;
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public E set(int index, E element) {
+        E oldElement = (E) internal[index];
+        internal[index] = element;
+        return oldElement;
+    }
+
+    @Override
+    public void clear() {
+        internal = new Object[0];
+    }
+
+    @Override
+    public int indexOf(Object object) {
+        // the first cycle
+        // if (object.equals(internal[0])) {
+        //     return 0;
+        // }
+        // return -1;
+
+        for (int i = 0; i < internal.length; i++) {
+            if (object.equals(internal[i])) {
+                return i;
+            }
+        }
+        return -1;
+    }
+
+    @Override
+    public int lastIndexOf(Object object) {
+        // the first cycle
+        // if (object.equals(internal[0])) {
+        //     return 0;
+        // }
+        // return -1;
+
+        for (int i = internal.length - 1; i >= 0; i--) {
+            if (object.equals(internal[i])) {
+                return i;
+            }
+        }
+        return -1;
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public List<E> subList(int fromIndex, int toIndex) {
+        // the first cycle
+        // return (List<E>) Arrays.asList(internal);
+
+        Object[] temp = new Object[toIndex - fromIndex];
+        System.arraycopy(internal, fromIndex, temp, 0, temp.length);
+        return (List<E>) Arrays.asList(temp);
+    }
+
+    @Override
+    public Object[] toArray() {
+        return Arrays.copyOf(internal, internal.length);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> T[] toArray(T[] array) {
+        // the first cycle
+        // array[0] = (T) internal[0];
+        // return array;
+
+        // the second cycle
+        // if (array.length < internal.length) {
+        //     return (T[]) Arrays.copyOf(internal, internal.length, array.getClass());
+        // }
+        // return (T[]) Arrays.copyOf(internal, internal.length, array.getClass());
+
+        if (array.length < internal.length) {
+            return (T[]) Arrays.copyOf(internal, internal.length, array.getClass());
+        }
+
+        System.arraycopy(internal, 0, array, 0, internal.length);
+        if (array.length > internal.length) {
+            array[internal.length] = null;
+        }
+        return array;
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        return new CustomIterator();
+    }
+
+    @Override
+    public ListIterator<E> listIterator() {
+        return null;
+    }
+
+    @Override
+    public ListIterator<E> listIterator(int index) {
+        // ignored for brevity
+        return null;
+    }
+
+    private class CustomIterator implements Iterator<E> {
+        int index;
+
+        @Override
+        public boolean hasNext() {
+            // the first cycle
+            // return true;
+
+            return index != internal.length;
+        }
+
+        @SuppressWarnings("unchecked")
+        @Override
+        public E next() {
+            // the first cycle
+            // return (E) CustomList.this.internal[0];
+
+            E element = (E) CustomList.this.internal[index];
+            index++;
+            return element;
+        }
+    }
+}
--- a/core-java/src/main/java/com/baeldung/recursion/BinaryNode.java
+++ b/core-java/src/main/java/com/baeldung/recursion/BinaryNode.java
@ -0,0 +1,31 @@
+package com.baeldung.recursion;
+
+public class BinaryNode {
+    int value;
+    BinaryNode left;
+    BinaryNode right;
+    
+    public BinaryNode(int value){
+        this.value = value;
+    }
+    
+    public int getValue() {
+        return value;
+    }
+    public void setValue(int value) {
+        this.value = value;
+    }
+    public BinaryNode getLeft() {
+        return left;
+    }
+    public void setLeft(BinaryNode left) {
+        this.left = left;
+    }
+    public BinaryNode getRight() {
+        return right;
+    }
+    public void setRight(BinaryNode right) {
+        this.right = right;
+    }
+    
+}
--- a/core-java/src/main/java/com/baeldung/recursion/RecursionExample.java
+++ b/core-java/src/main/java/com/baeldung/recursion/RecursionExample.java
@ -0,0 +1,64 @@
+package com.baeldung.recursion;
+
+public class RecursionExample {
+    
+    public int sum(int n){
+        if (n >= 1){
+            return sum(n - 1) + n;
+        }
+        return n;
+    }
+    
+    public int tailSum(int currentSum, int n){
+        if (n <= 1) {
+            return currentSum + n;
+        }
+        return tailSum(currentSum + n, n - 1);
+    }
+    
+    public int iterativeSum(int n){
+        int sum = 0;
+        if(n < 0){
+            return -1;
+        }
+        for(int i=0; i<=n; i++){
+            sum += i;
+        }
+        return sum;
+    }
+    
+    public int powerOf10(int n){
+        if (n == 0){
+            return 1;
+        }
+        return powerOf10(n-1)*10;
+    }
+    
+    public int fibonacci(int n){
+        if (n <=1 ){
+            return n;
+        }
+        return fibonacci(n-1) + fibonacci(n-2);
+    }
+    
+    public String toBinary(int n){
+        if (n <= 1 ){
+            return String.valueOf(n);
+        }
+        return toBinary(n / 2) + String.valueOf(n % 2);
+    }
+    
+    public int calculateTreeHeight(BinaryNode root){
+        if (root!= null){
+            if (root.getLeft() != null || root.getRight() != null){
+                return 1 + max(calculateTreeHeight(root.left) , calculateTreeHeight(root.right));
+            }
+        }
+        return 0;
+    }
+    
+    public int max(int a,int b){
+        return a>b ? a:b;
+    }
+    
+}
--- a/core-java/src/main/java/com/baeldung/resourcebundle/ExampleControl.java
+++ b/core-java/src/main/java/com/baeldung/resourcebundle/ExampleControl.java
@ -0,0 +1,15 @@
+package com.baeldung.resourcebundle;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Locale;
+import java.util.ResourceBundle;
+
+public class ExampleControl extends ResourceBundle.Control {
+
+    @Override
+    public List<Locale> getCandidateLocales(String s, Locale locale) {
+        return Arrays.asList(new Locale("pl", "PL"));
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/resourcebundle/ExampleResource.java
+++ b/core-java/src/main/java/com/baeldung/resourcebundle/ExampleResource.java
@ -0,0 +1,14 @@
+package com.baeldung.resourcebundle;
+
+import java.util.ListResourceBundle;
+
+public class ExampleResource extends ListResourceBundle {
+
+    @Override
+    protected Object[][] getContents() {
+        return new Object[][] { 
+            { "greeting", "hello" } 
+        };
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/resourcebundle/ExampleResource_pl.java
+++ b/core-java/src/main/java/com/baeldung/resourcebundle/ExampleResource_pl.java
@ -0,0 +1,15 @@
+package com.baeldung.resourcebundle;
+
+import java.util.ListResourceBundle;
+
+public class ExampleResource_pl extends ListResourceBundle {
+
+    @Override
+    protected Object[][] getContents() {
+        return new Object[][] { 
+            { "greeting", "cześć" }, 
+            { "language", "polish" }, 
+        };
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/resourcebundle/ExampleResource_pl_PL.java
+++ b/core-java/src/main/java/com/baeldung/resourcebundle/ExampleResource_pl_PL.java
@ -0,0 +1,17 @@
+package com.baeldung.resourcebundle;
+
+import java.math.BigDecimal;
+import java.util.ListResourceBundle;
+
+public class ExampleResource_pl_PL extends ListResourceBundle {
+
+    @Override
+    protected Object[][] getContents() {
+        return new Object[][] { 
+            { "currency", "polish zloty" }, 
+            { "toUsdRate", new BigDecimal("3.401") },
+            { "cities", new String[] { "Warsaw", "Cracow" } } 
+        };
+    }
+
+}
--- a/core-java/src/main/java/com/baeldung/system/DetectOS.java
+++ b/core-java/src/main/java/com/baeldung/system/DetectOS.java
@ -0,0 +1,18 @@
+package com.baeldung.system;
+
+import org.apache.commons.lang3.SystemUtils;
+
+public class DetectOS {
+
+    public String getOperatingSystem() {
+        String os = System.getProperty("os.name");
+        System.out.println("Using System Property: " + os);
+        return os;
+    }
+
+    public String getOperatingSystemSystemUtils() {
+        String os = SystemUtils.OS_NAME;
+        System.out.println("Using SystemUtils: " + os);
+        return os;
+    }
+}
--- a/core-java/src/main/resources/resourcebundle/resource.properties
+++ b/core-java/src/main/resources/resourcebundle/resource.properties
@ -0,0 +1,6 @@
+# Buttons
+cancelButton=cancel
+continueButton continue
+
+! Labels
+helloLabel:hello
--- a/core-java/src/main/resources/resourcebundle/resource_en.properties
+++ b/core-java/src/main/resources/resourcebundle/resource_en.properties
@ -0,0 +1 @@
+deleteButton=delete
--- a/core-java/src/main/resources/resourcebundle/resource_pl_PL.properties
+++ b/core-java/src/main/resources/resourcebundle/resource_pl_PL.properties
@ -0,0 +1,3 @@
+backButton=cofnij
+helloLabel=cze\u015b\u0107
+helloLabelNoEncoding=cześć
--- a/core-java/src/test/java/com/baeldung/casting/CastingTest.java
+++ b/core-java/src/test/java/com/baeldung/casting/CastingTest.java
@ -0,0 +1,58 @@
+package com.baeldung.casting;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+import java.util.ArrayList;
+import java.util.List;
+
+public class CastingTest {
+
+    @Test
+    public void whenPrimitiveConverted_thenValueChanged() {
+        double myDouble = 1.1;
+        int myInt = (int) myDouble;
+        assertNotEquals(myDouble, myInt);
+    }
+
+    @Test
+    public void whenUpcast_thenInstanceUnchanged() {
+        Cat cat = new Cat();
+        Animal animal = cat;
+        animal = (Animal) cat;
+        assertTrue(animal instanceof Cat);
+    }
+
+    @Test
+    public void whenUpcastToObject_thenInstanceUnchanged() {
+        Object object = new Animal();
+        assertTrue(object instanceof Animal);
+    }
+
+    @Test
+    public void whenUpcastToInterface_thenInstanceUnchanged() {
+        Mew mew = new Cat();
+        assertTrue(mew instanceof Cat);
+    }
+
+    @Test
+    public void whenUpcastToAnimal_thenOverridenMethodsCalled() {
+        List<Animal> animals = new ArrayList<>();
+        animals.add(new Cat());
+        animals.add(new Dog());
+        new AnimalFeeder().feed(animals);
+    }
+
+    @Test
+    public void whenDowncastToCat_thenMeowIsCalled() {
+        Animal animal = new Cat();
+        ((Cat) animal).meow();
+    }
+
+    @Test(expected = ClassCastException.class)
+    public void whenDownCastWithoutCheck_thenExceptionThrown() {
+        List<Animal> animals = new ArrayList<>();
+        animals.add(new Cat());
+        animals.add(new Dog());
+        new AnimalFeeder().uncheckedFeed(animals);
+    }
+}
--- a/core-java/src/test/java/com/baeldung/classloader/CustomClassLoaderTest.java
+++ b/core-java/src/test/java/com/baeldung/classloader/CustomClassLoaderTest.java
@ -0,0 +1,23 @@
+package com.baeldung.classloader;
+
+import org.junit.Test;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+public class CustomClassLoaderTest {
+
+    @Test
+    public void customLoader() throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
+
+        CustomClassLoader customClassLoader = new CustomClassLoader();
+        Class<?> c = customClassLoader.getClass(PrintClassLoader.class.getName());
+
+        Object ob = c.newInstance();
+
+        Method md = c.getMethod("printClassLoaders");
+        md.invoke(ob);
+
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/classloader/PrintClassLoaderTest.java
+++ b/core-java/src/test/java/com/baeldung/classloader/PrintClassLoaderTest.java
@ -0,0 +1,14 @@
+package com.baeldung.classloader;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class PrintClassLoaderTest {
+    @Test(expected = ClassNotFoundException.class)
+    public void givenAppClassLoader_whenParentClassLoader_thenClassNotFoundException() throws Exception {
+        PrintClassLoader sampleClassLoader = (PrintClassLoader) Class.forName(PrintClassLoader.class.getName()).newInstance();
+        sampleClassLoader.printClassLoaders();
+        Class.forName(PrintClassLoader.class.getName(), true, PrintClassLoader.class.getClassLoader().getParent());
+    }
+}
--- a/core-java/src/test/java/com/baeldung/decimalformat/DecimalFormatExamplesTest.java
+++ b/core-java/src/test/java/com/baeldung/decimalformat/DecimalFormatExamplesTest.java
@ -0,0 +1,116 @@
+package com.baeldung.decimalformat;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.math.BigDecimal;
+import java.text.DecimalFormat;
+import java.text.DecimalFormatSymbols;
+import java.text.NumberFormat;
+import java.text.ParseException;
+import java.util.Locale;
+
+import org.junit.Test;
+
+public class DecimalFormatExamplesTest {
+
+	double d = 1234567.89;
+
+	@Test
+	public void givenSimpleDecimal_WhenFormatting_ThenCorrectOutput() {
+
+		assertThat(new DecimalFormat("#.##", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1234567.89");
+
+		assertThat(new DecimalFormat("0.00", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1234567.89");
+
+		assertThat(new DecimalFormat("#########.###", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1234567.89");
+
+		assertThat(new DecimalFormat("000000000.000", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("001234567.890");
+
+	}
+
+	@Test
+	public void givenSmallerDecimalPattern_WhenFormatting_ThenRounding() {
+
+		assertThat(new DecimalFormat("#.#", new DecimalFormatSymbols(Locale.ENGLISH)).format(d)).isEqualTo("1234567.9");
+
+		assertThat(new DecimalFormat("#", new DecimalFormatSymbols(Locale.ENGLISH)).format(d)).isEqualTo("1234568");
+
+	}
+
+	@Test
+	public void givenGroupingSeparator_WhenFormatting_ThenGroupedOutput() {
+
+		assertThat(new DecimalFormat("#,###.#", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1,234,567.9");
+
+		assertThat(new DecimalFormat("#,###", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1,234,568");
+
+	}
+
+	@Test
+	public void givenMixedPattern_WhenFormatting_ThenCorrectOutput() {
+
+		assertThat(new DecimalFormat("The # number", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("The 1234568 number");
+
+		assertThat(new DecimalFormat("The '#' # number", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("The # 1234568 number");
+
+	}
+
+	@Test
+	public void givenLocales_WhenFormatting_ThenCorrectOutput() {
+
+		assertThat(new DecimalFormat("#,###.##", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1,234,567.89");
+
+		assertThat(new DecimalFormat("#,###.##", new DecimalFormatSymbols(Locale.ITALIAN)).format(d))
+				.isEqualTo("1.234.567,89");
+
+		assertThat(new DecimalFormat("#,###.##", DecimalFormatSymbols.getInstance(new Locale("it", "IT"))).format(d))
+				.isEqualTo("1.234.567,89");
+
+	}
+
+	@Test
+	public void givenE_WhenFormatting_ThenScientificNotation() {
+
+		assertThat(new DecimalFormat("00.#######E0", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("12.3456789E5");
+
+		assertThat(new DecimalFormat("000.000000E0", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("123.456789E4");
+
+		assertThat(new DecimalFormat("##0.######E0", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1.23456789E6");
+
+		assertThat(new DecimalFormat("###.000000E0", new DecimalFormatSymbols(Locale.ENGLISH)).format(d))
+				.isEqualTo("1.23456789E6");
+
+	}
+
+	@Test
+	public void givenString_WhenParsing_ThenCorrectOutput() throws ParseException {
+
+		assertThat(new DecimalFormat("", new DecimalFormatSymbols(Locale.ENGLISH)).parse("1234567.89"))
+				.isEqualTo(1234567.89);
+
+		assertThat(new DecimalFormat("", new DecimalFormatSymbols(Locale.ITALIAN)).parse("1.234.567,89"))
+				.isEqualTo(1234567.89);
+
+	}
+
+	@Test
+	public void givenStringAndBigDecimalFlag_WhenParsing_ThenCorrectOutput() throws ParseException {
+
+		NumberFormat nf = new DecimalFormat("", new DecimalFormatSymbols(Locale.ENGLISH));
+		((DecimalFormat) nf).setParseBigDecimal(true);
+		assertThat(nf.parse("1234567.89")).isEqualTo(BigDecimal.valueOf(1234567.89));
+	}
+
+}
--- a/core-java/src/test/java/com/baeldung/deepcopy/DeepCopyUnitTest.java
+++ b/core-java/src/test/java/com/baeldung/deepcopy/DeepCopyUnitTest.java
@ -0,0 +1,128 @@
+package com.baeldung.deepcopy;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.IOException;
+
+import org.apache.commons.lang.SerializationUtils;
+import org.junit.Ignore;
+import org.junit.Test;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.gson.Gson;
+
+public class DeepCopyUnitTest {
+
+    @Test
+    public void whenCreatingDeepCopyWithCopyConstructor_thenObjectsShouldNotBeSame() {
+
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+
+        User deepCopy = new User(pm);
+
+        assertThat(deepCopy).isNotSameAs(pm);
+    }
+
+    @Test
+    public void whenModifyingOriginalObject_thenConstructorCopyShouldNotChange() {
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+        User deepCopy = new User(pm);
+
+        address.setCountry("Great Britain");
+
+        assertThat(deepCopy.getAddress().getCountry()).isNotEqualTo(pm.getAddress().getCountry());
+    }
+
+    @Test
+    public void whenModifyingOriginalObject_thenCloneCopyShouldNotChange() {
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+        User deepCopy = (User) pm.clone();
+
+        address.setCountry("Great Britain");
+
+        assertThat(deepCopy.getAddress().getCountry()).isNotEqualTo(pm.getAddress().getCountry());
+    }
+
+    @Test
+    public void whenModifyingOriginalObject_thenCommonsCloneShouldNotChange() {
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+        User deepCopy = (User) SerializationUtils.clone(pm);
+
+        address.setCountry("Great Britain");
+
+        assertThat(deepCopy.getAddress().getCountry()).isNotEqualTo(pm.getAddress().getCountry());
+    }
+
+    @Test
+    public void whenModifyingOriginalObject_thenGsonCloneShouldNotChange() {
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+        Gson gson = new Gson();
+        User deepCopy = gson.fromJson(gson.toJson(pm), User.class);
+
+        address.setCountry("Great Britain");
+
+        assertThat(deepCopy.getAddress().getCountry()).isNotEqualTo(pm.getAddress().getCountry());
+    }
+
+    @Test
+    public void whenModifyingOriginalObject_thenJacksonCopyShouldNotChange() throws IOException {
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+        ObjectMapper objectMapper = new ObjectMapper();
+        User deepCopy = objectMapper.readValue(objectMapper.writeValueAsString(pm), User.class);
+
+        address.setCountry("Great Britain");
+
+        assertThat(deepCopy.getAddress().getCountry()).isNotEqualTo(pm.getAddress().getCountry());
+    }
+
+    @Test
+    @Ignore
+    public void whenMakingCopies_thenShowHowLongEachMethodTakes() throws CloneNotSupportedException, IOException {
+        int times = 1000000;
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+
+        long start = System.currentTimeMillis();
+        for (int i = 0; i < times; i++) {
+            User primeMinisterClone = (User) SerializationUtils.clone(pm);
+        }
+        long end = System.currentTimeMillis();
+        System.out.println("Cloning with Apache Commons Lang took " + (end - start) + " milliseconds.");
+
+        start = System.currentTimeMillis();
+        Gson gson = new Gson();
+        for (int i = 0; i < times; i++) {
+            User primeMinisterClone = gson.fromJson(gson.toJson(pm), User.class);
+        }
+        end = System.currentTimeMillis();
+        System.out.println("Cloning with Gson took " + (end - start) + " milliseconds.");
+
+        start = System.currentTimeMillis();
+        for (int i = 0; i < times; i++) {
+            User primeMinisterClone = new User(pm);
+        }
+        end = System.currentTimeMillis();
+        System.out.println("Cloning with the copy constructor took " + (end - start) + " milliseconds.");
+
+        start = System.currentTimeMillis();
+        for (int i = 0; i < times; i++) {
+            User primeMinisterClone = (User) pm.clone();
+        }
+        end = System.currentTimeMillis();
+        System.out.println("Cloning with Cloneable interface took " + (end - start) + " milliseconds.");
+
+        start = System.currentTimeMillis();
+        ObjectMapper objectMapper = new ObjectMapper();
+        for (int i = 0; i < times; i++) {
+            User primeMinisterClone = objectMapper.readValue(objectMapper.writeValueAsString(pm), User.class);
+        }
+        end = System.currentTimeMillis();
+        System.out.println("Cloning with Jackson took " + (end - start) + " milliseconds.");
+    }
+}
--- a/core-java/src/test/java/com/baeldung/deepcopy/ShallowCopyUnitTest.java
+++ b/core-java/src/test/java/com/baeldung/deepcopy/ShallowCopyUnitTest.java
@ -0,0 +1,33 @@
+package com.baeldung.deepcopy;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class ShallowCopyUnitTest {
+
+
+    @Test
+    public void whenShallowCopying_thenObjectsShouldNotBeSame() {
+
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+
+        User shallowCopy = new User(pm.getFirstName(), pm.getLastName(), pm.getAddress());
+
+        assertThat(shallowCopy)
+                .isNotSameAs(pm);
+    }
+
+    @Test
+    public void whenModifyingOriginalObject_thenCopyShouldChange() {
+        Address address = new Address("Downing St 10", "London", "England");
+        User pm = new User("Prime", "Minister", address);
+        User shallowCopy = new User(pm.getFirstName(), pm.getLastName(), pm.getAddress());
+
+        address.setCountry("Great Britain");
+        
+        assertThat(shallowCopy.getAddress().getCountry())
+                .isEqualTo(pm.getAddress().getCountry());
+    }
+}
--- a/core-java/src/test/java/com/baeldung/designpatterns/chainofresponsibility/ChainOfResponsibilityTest.java
+++ b/core-java/src/test/java/com/baeldung/designpatterns/chainofresponsibility/ChainOfResponsibilityTest.java
@ -0,0 +1,37 @@
+package com.baeldung.designpatterns.chainofresponsibility;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertTrue;
+
+public class ChainOfResponsibilityTest {
+
+    private static AuthenticationProcessor getChainOfAuthProcessor() {
+
+        AuthenticationProcessor oAuthProcessor = new OAuthAuthenticationProcessor(null);
+        AuthenticationProcessor unamePasswordProcessor = new UsernamePasswordAuthenticationProcessor(oAuthProcessor);
+        return unamePasswordProcessor;
+    }
+
+    @Test
+    public void givenOAuthProvider_whenCheckingAuthorized_thenSuccess() {
+        AuthenticationProcessor authProcessorChain = getChainOfAuthProcessor();
+        boolean isAuthorized = authProcessorChain.isAuthorized(new OAuthTokenProvider());
+        assertTrue(isAuthorized);
+    }
+
+    @Test
+    public void givenUsernamePasswordProvider_whenCheckingAuthorized_thenSuccess() {
+        AuthenticationProcessor authProcessorChain = getChainOfAuthProcessor();
+        boolean isAuthorized = authProcessorChain.isAuthorized(new UsernamePasswordProvider());
+        assertTrue(isAuthorized);
+    }
+
+    @Test
+    public void givenSamlAuthProvider_whenCheckingAuthorized_thenFailure() {
+        AuthenticationProcessor authProcessorChain = getChainOfAuthProcessor();
+        boolean isAuthorized = authProcessorChain.isAuthorized(new SamlAuthenticationProvider());
+        assertTrue(!isAuthorized);
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/designpatterns/flyweight/FlyweightUnitTest.java
+++ b/core-java/src/test/java/com/baeldung/designpatterns/flyweight/FlyweightUnitTest.java
@ -0,0 +1,42 @@
+package com.baeldung.designpatterns.flyweight;
+
+import java.awt.Color;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Unit test for {@link VehicleFactory}.
+ * 
+ * @author Donato Rimenti
+ */
+public class FlyweightUnitTest {
+
+	/**
+	 * Checks that when the {@link VehicleFactory} is asked to provide two
+	 * vehicles of different colors, the objects returned are different.
+	 */
+	@Test
+	public void givenDifferentFlyweightObjects_whenEquals_thenFalse() {
+		Vehicle blackVehicle = VehicleFactory.createVehicle(Color.BLACK);
+		Vehicle blueVehicle = VehicleFactory.createVehicle(Color.BLUE);
+
+		Assert.assertNotNull("Object returned by the factory is null!", blackVehicle);
+		Assert.assertNotNull("Object returned by the factory is null!", blueVehicle);
+		Assert.assertNotEquals("Objects returned by the factory are equals!", blackVehicle, blueVehicle);
+	}
+
+	/**
+	 * Checks that when the {@link VehicleFactory} is asked to provide two
+	 * vehicles of the same colors, the same object is returned twice.
+	 */
+	@Test
+	public void givenSameFlyweightObjects_whenEquals_thenTrue() {
+		Vehicle blackVehicle = VehicleFactory.createVehicle(Color.BLACK);
+		Vehicle anotherBlackVehicle = VehicleFactory.createVehicle(Color.BLACK);
+
+		Assert.assertNotNull("Object returned by the factory is null!", blackVehicle);
+		Assert.assertNotNull("Object returned by the factory is null!", anotherBlackVehicle);
+		Assert.assertEquals("Objects returned by the factory are not equals!", blackVehicle, anotherBlackVehicle);
+	}
+}
--- a/core-java/src/test/java/com/baeldung/inheritance/AppTest.java
+++ b/core-java/src/test/java/com/baeldung/inheritance/AppTest.java
@ -0,0 +1,46 @@
+package com.baeldung.inheritance;
+
+import com.baeldung.inheritance.*;
+
+import junit.framework.Test;
+import junit.framework.TestCase;
+import junit.framework.TestSuite;
+
+public class AppTest extends TestCase {
+    
+	public AppTest(String testName) {
+        super( testName );
+    }
+
+    public static Test suite() {
+        return new TestSuite(AppTest.class);
+    }
+
+    @SuppressWarnings("static-access")
+	public void testStaticMethodUsingBaseClassVariable() {
+    	Car first = new ArmoredCar();
+    	assertEquals("Car", first.msg());
+    }
+    	
+    @SuppressWarnings("static-access")
+	public void testStaticMethodUsingDerivedClassVariable() {
+    	ArmoredCar second = new ArmoredCar();
+    	assertEquals("ArmoredCar", second.msg());
+    }
+    
+    public void testAssignArmoredCarToCar() {
+    	Employee e1 = new Employee("Shreya", new ArmoredCar());
+    	assertNotNull(e1.getCar());
+    }
+
+    public void testAssignSpaceCarToCar() {
+    	Employee e2 = new Employee("Paul", new SpaceCar());
+    	assertNotNull(e2.getCar());
+    }
+
+    public void testBMWToCar() {
+    	Employee e3 = new Employee("Pavni", new BMW());
+    	assertNotNull(e3.getCar());
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/java/list/CustomListUnitTest.java
+++ b/core-java/src/test/java/com/baeldung/java/list/CustomListUnitTest.java
@ -0,0 +1,282 @@
+package com.baeldung.java.list;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+
+import org.junit.Test;
+
+public class CustomListUnitTest {
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenAddToSpecifiedIndex_thenExceptionIsThrown() {
+        new CustomList<>().add(0, null);
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenAddAllToTheEnd_thenExceptionIsThrown() {
+        Collection<Object> collection = new ArrayList<>();
+        List<Object> list = new CustomList<>();
+        list.addAll(collection);
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenAddAllToSpecifiedIndex_thenExceptionIsThrown() {
+        Collection<Object> collection = new ArrayList<>();
+        List<Object> list = new CustomList<>();
+        list.addAll(0, collection);
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenRemoveAtSpecifiedIndex_thenExceptionIsThrown() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.remove(0);
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenRemoveSpecifiedElement_thenExceptionIsThrown() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.remove("baeldung");
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenRemoveAll_thenExceptionIsThrown() {
+        Collection<Object> collection = new ArrayList<>();
+        collection.add("baeldung");
+        List<Object> list = new CustomList<>();
+        list.removeAll(collection);
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void whenRetainAll_thenExceptionIsThrown() {
+        Collection<Object> collection = new ArrayList<>();
+        collection.add("baeldung");
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.retainAll(collection);
+    }
+
+    @Test
+    public void givenEmptyList_whenSize_thenZeroIsReturned() {
+        List<Object> list = new CustomList<>();
+
+        assertEquals(0, list.size());
+    }
+
+    @Test
+    public void givenEmptyList_whenIsEmpty_thenTrueIsReturned() {
+        List<Object> list = new CustomList<>();
+
+        assertTrue(list.isEmpty());
+    }
+
+    @Test
+    public void givenEmptyList_whenElementIsAdded_thenGetReturnsThatElement() {
+        List<Object> list = new CustomList<>();
+        boolean succeeded = list.add("baeldung");
+        Object element = list.get(0);
+
+        assertTrue(succeeded);
+        assertEquals("baeldung", element);
+    }
+
+    @Test
+    public void givenListWithAnElement_whenAnotherIsAdded_thenGetReturnsBoth() {
+        List<Object> list = new CustomList<>();
+        boolean succeeded1 = list.add("baeldung");
+        boolean succeeded2 = list.add(".com");
+        Object element1 = list.get(0);
+        Object element2 = list.get(1);
+
+        assertTrue(succeeded1);
+        assertTrue(succeeded2);
+        assertEquals("baeldung", element1);
+        assertEquals(".com", element2);
+    }
+
+    @Test
+    public void givenEmptyList_whenContains_thenFalseIsReturned() {
+        List<Object> list = new CustomList<>();
+
+        assertFalse(list.contains(null));
+    }
+
+    @Test
+    public void givenListWithAnElement_whenContains_thenTrueIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+
+        assertTrue(list.contains("baeldung"));
+    }
+
+    @Test
+    public void givenListWithAnElement_whenContainsAll_thenTrueIsReturned() {
+        Collection<Object> collection = new ArrayList<>();
+        collection.add("baeldung");
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+
+        assertTrue(list.containsAll(collection));
+    }
+
+    @Test
+    public void givenList_whenContainsAll_thenEitherTrueOrFalseIsReturned() {
+        Collection<Object> collection1 = new ArrayList<>();
+        collection1.add("baeldung");
+        collection1.add(".com");
+        Collection<Object> collection2 = new ArrayList<>();
+        collection2.add("baeldung");
+
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+
+        assertFalse(list.containsAll(collection1));
+        assertTrue(list.containsAll(collection2));
+    }
+
+    @Test
+    public void givenList_whenSet_thenOldElementIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        Object element = list.set(0, null);
+
+        assertEquals("baeldung", element);
+        assertNull(list.get(0));
+    }
+
+    @Test
+    public void givenList_whenClear_thenAllElementsAreRemoved() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.clear();
+
+        assertTrue(list.isEmpty());
+    }
+
+    @Test
+    public void givenList_whenIndexOf_thenIndexZeroIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+
+        assertEquals(0, list.indexOf("baeldung"));
+    }
+
+    @Test
+    public void givenList_whenIndexOf_thenPositiveIndexOrMinusOneIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.add(".com");
+        list.add(".com");
+
+        assertEquals(1, list.indexOf(".com"));
+        assertEquals(-1, list.indexOf("com"));
+    }
+
+    @Test
+    public void whenLastIndexOf_thenIndexZeroIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+
+        assertEquals(0, list.lastIndexOf("baeldung"));
+    }
+
+    @Test
+    public void whenLastIndexOf_thenPositiveIndexOrMinusOneIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.add("baeldung");
+        list.add(".com");
+
+        assertEquals(1, list.lastIndexOf("baeldung"));
+        assertEquals(-1, list.indexOf("com"));
+    }
+
+    @Test
+    public void whenSubListZeroToOne_thenListContainingFirstElementIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        List<Object> subList = list.subList(0, 1);
+
+        assertEquals("baeldung", subList.get(0));
+    }
+
+    @Test
+    public void whenSubListOneToTwo_thenListContainingSecondElementIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.add(".");
+        list.add("com");
+        List<Object> subList = list.subList(1, 2);
+
+        assertEquals(1, subList.size());
+        assertEquals(".", subList.get(0));
+    }
+
+    @Test
+    public void givenListWithElements_whenToArray_thenArrayContainsThose() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        list.add(".com");
+        Object[] array = list.toArray();
+
+        assertArrayEquals(new Object[] { "baeldung", ".com" }, array);
+    }
+
+    @Test
+    public void givenListWithAnElement_whenToArray_thenInputArrayIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        String[] input = new String[1];
+        String[] output = list.toArray(input);
+
+        assertArrayEquals(new String[] { "baeldung" }, input);
+    }
+
+    @Test
+    public void whenToArrayIsCalledWithEmptyInputArray_thenNewArrayIsReturned() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        String[] input = {};
+        String[] output = list.toArray(input);
+
+        assertArrayEquals(new String[] { "baeldung" }, output);
+    }
+
+    @Test
+    public void whenToArrayIsCalledWithLargerInput_thenOutputHasTrailingNull() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        String[] input = new String[2];
+        String[] output = list.toArray(input);
+
+        assertArrayEquals(new String[] { "baeldung", null }, output);
+    }
+
+    @Test
+    public void givenListWithOneElement_whenIterator_thenThisElementIsNext() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        Iterator<Object> iterator = list.iterator();
+
+        assertTrue(iterator.hasNext());
+        assertEquals("baeldung", iterator.next());
+    }
+
+    @Test
+    public void whenIteratorNextIsCalledTwice_thenTheSecondReturnsFalse() {
+        List<Object> list = new CustomList<>();
+        list.add("baeldung");
+        Iterator<Object> iterator = list.iterator();
+
+        iterator.next();
+        assertFalse(iterator.hasNext());
+    }
+}
--- a/core-java/src/test/java/com/baeldung/recursion/RecursionExampleTest.java
+++ b/core-java/src/test/java/com/baeldung/recursion/RecursionExampleTest.java
@ -0,0 +1,61 @@
+package com.baeldung.recursion;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class RecursionExampleTest {
+    
+    RecursionExample recursion = new RecursionExample();
+
+    @Test
+    public void testPowerOf10() {
+        int p0 = recursion.powerOf10(0);
+        int p1 = recursion.powerOf10(1);
+        int p4 = recursion.powerOf10(4);
+        
+        Assert.assertEquals(1, p0);
+        Assert.assertEquals(10, p1);
+        Assert.assertEquals(10000, p4);
+    }
+    
+    @Test
+    public void testFibonacci() {
+        int n0 = recursion.fibonacci(0);
+        int n1 = recursion.fibonacci(1);
+        int n7 = recursion.fibonacci(7);
+        
+        Assert.assertEquals(0, n0);
+        Assert.assertEquals(1, n1);
+        Assert.assertEquals(13, n7);
+    }
+    
+    @Test
+    public void testToBinary() {
+        String b0 = recursion.toBinary(0);
+        String b1 = recursion.toBinary(1);
+        String b10 = recursion.toBinary(10);
+        
+        Assert.assertEquals("0", b0);
+        Assert.assertEquals("1", b1);
+        Assert.assertEquals("1010", b10);
+    }
+    
+    @Test
+    public void testCalculateTreeHeight() {
+        BinaryNode root = new BinaryNode(1);
+        root.setLeft(new BinaryNode(1));
+        root.setRight(new BinaryNode(1));
+        
+        root.getLeft().setLeft(new BinaryNode(1));
+        root.getLeft().getLeft().setRight(new BinaryNode(1));
+        root.getLeft().getLeft().getRight().setLeft(new BinaryNode(1));
+        
+        root.getRight().setLeft(new BinaryNode(1));
+        root.getRight().getLeft().setRight(new BinaryNode(1));
+        
+        int height = recursion.calculateTreeHeight(root);
+        
+        Assert.assertEquals(4, height);
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/resourcebundle/ExampleResourceUnitTest.java
+++ b/core-java/src/test/java/com/baeldung/resourcebundle/ExampleResourceUnitTest.java
@ -0,0 +1,39 @@
+package com.baeldung.resourcebundle;
+
+import org.junit.Test;
+
+import java.math.BigDecimal;
+import java.util.Arrays;
+import java.util.Locale;
+import java.util.ResourceBundle;
+
+import static org.junit.Assert.*;
+
+public class ExampleResourceUnitTest {
+
+    @Test
+    public void whenGetBundleExampleResourceForLocalePlPl_thenItShouldInheritPropertiesGreetingAndLanguage() {
+        Locale plLocale = new Locale("pl", "PL");
+
+        ResourceBundle exampleBundle = ResourceBundle.getBundle("com.baeldung.resourcebundle.ExampleResource", plLocale);
+
+        assertTrue(exampleBundle.keySet()
+            .containsAll(Arrays.asList("toUsdRate", "cities", "greeting", "currency", "language")));
+        assertEquals(exampleBundle.getString("greeting"), "cześć");
+        assertEquals(exampleBundle.getObject("toUsdRate"), new BigDecimal("3.401"));
+        assertArrayEquals(exampleBundle.getStringArray("cities"), new String[] { "Warsaw", "Cracow" });
+    }
+
+    @Test
+    public void whenGetBundleExampleResourceForLocaleUs_thenItShouldContainOnlyGreeting() {
+        Locale usLocale = Locale.US;
+
+        ResourceBundle exampleBundle = ResourceBundle.getBundle("com.baeldung.resourcebundle.ExampleResource", usLocale);
+
+        assertFalse(exampleBundle.keySet()
+            .containsAll(Arrays.asList("toUsdRate", "cities", "currency", "language")));
+        assertTrue(exampleBundle.keySet()
+            .containsAll(Arrays.asList("greeting")));
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/resourcebundle/PropertyResourceUnitTest.java
+++ b/core-java/src/test/java/com/baeldung/resourcebundle/PropertyResourceUnitTest.java
@ -0,0 +1,62 @@
+package com.baeldung.resourcebundle;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Locale;
+import java.util.ResourceBundle;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class PropertyResourceUnitTest {
+
+    @Test
+    public void givenLocaleUsAsDefualt_whenGetBundleForLocalePlPl_thenItShouldContain3ButtonsAnd1Label() {
+        Locale.setDefault(Locale.US);
+
+        ResourceBundle bundle = ResourceBundle.getBundle("resourcebundle.resource", new Locale("pl", "PL"));
+
+        assertTrue(bundle.keySet()
+            .containsAll(Arrays.asList("backButton", "helloLabel", "cancelButton", "continueButton", "helloLabelNoEncoding")));
+    }
+
+    @Test
+    public void givenLocaleUsAsDefualt_whenGetBundleForLocaleFrFr_thenItShouldContainKeys1To3AndKey4() {
+        Locale.setDefault(Locale.US);
+
+        ResourceBundle bundle = ResourceBundle.getBundle("resourcebundle.resource", new Locale("fr", "FR"));
+
+        assertTrue(bundle.keySet()
+            .containsAll(Arrays.asList("deleteButton", "helloLabel", "cancelButton", "continueButton")));
+    }
+
+    @Test
+    public void givenLocaleChinaAsDefualt_whenGetBundleForLocaleFrFr_thenItShouldOnlyContainKeys1To3() {
+        Locale.setDefault(Locale.CHINA);
+
+        ResourceBundle bundle = ResourceBundle.getBundle("resourcebundle.resource", new Locale("fr", "FR"));
+
+        assertTrue(bundle.keySet()
+            .containsAll(Arrays.asList("continueButton", "helloLabel", "cancelButton")));
+    }
+
+    @Test
+    public void givenLocaleChinaAsDefualt_whenGetBundleForLocaleFrFrAndExampleControl_thenItShouldOnlyContainKey5() {
+        Locale.setDefault(Locale.CHINA);
+
+        ResourceBundle bundle = ResourceBundle.getBundle("resourcebundle.resource", new Locale("fr", "FR"), new ExampleControl());
+
+        assertTrue(bundle.keySet()
+            .containsAll(Arrays.asList("backButton", "helloLabel")));
+    }
+
+    @Test
+    public void givenValuesDifferentlyEncoded_whenGetBundleForLocalePlPl_thenItShouldContain3ButtonsAnd1Label() {
+        ResourceBundle bundle = ResourceBundle.getBundle("resourcebundle.resource", new Locale("pl", "PL"));
+
+        assertEquals(bundle.getString("helloLabel"), "cześć");
+        assertEquals(bundle.getString("helloLabelNoEncoding"), "czeÅ\u009BÄ\u0087");
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/string/StringComparisonTest.java
+++ b/core-java/src/test/java/com/baeldung/string/StringComparisonTest.java
@ -0,0 +1,154 @@
+package com.baeldung.string;
+
+import org.apache.commons.lang3.StringUtils;
+import org.junit.Test;
+
+import java.util.Objects;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class StringComparisonTest {
+
+    @Test
+    public void whenUsingComparisonOperator_ThenComparingStrings(){
+
+        String string1 = "using comparison operator";
+        String string2 = "using comparison operator";
+        String string3 = new String("using comparison operator");
+
+        assertThat(string1 == string2).isTrue();
+        assertThat(string1 == string3).isFalse();
+    }
+
+    @Test
+    public void whenUsingEqualsMethod_ThenComparingStrings(){
+
+        String string1 = "using equals method";
+        String string2 = "using equals method";
+
+        String string3 = "using EQUALS method";
+        String string4 = new String("using equals method");
+
+        assertThat(string1.equals(string2)).isTrue();
+        assertThat(string1.equals(string4)).isTrue();
+
+        assertThat(string1.equals(null)).isFalse();
+        assertThat(string1.equals(string3)).isFalse();
+    }
+
+    @Test
+    public void whenUsingEqualsIgnoreCase_ThenComparingStrings(){
+
+        String string1 = "using equals ignore case";
+        String string2 = "USING EQUALS IGNORE CASE";
+
+        assertThat(string1.equalsIgnoreCase(string2)).isTrue();
+    }
+
+    @Test
+    public void whenUsingCompareTo_ThenComparingStrings(){
+
+        String author = "author";
+        String book = "book";
+        String duplicateBook = "book";
+
+        assertThat(author.compareTo(book)).isEqualTo(-1);
+        assertThat(book.compareTo(author)).isEqualTo(1);
+        assertThat(duplicateBook.compareTo(book)).isEqualTo(0);
+    }
+
+    @Test
+    public void whenUsingCompareToIgnoreCase_ThenComparingStrings(){
+
+        String author = "Author";
+        String book = "book";
+        String duplicateBook = "BOOK";
+
+        assertThat(author.compareToIgnoreCase(book)).isEqualTo(-1);
+        assertThat(book.compareToIgnoreCase(author)).isEqualTo(1);
+        assertThat(duplicateBook.compareToIgnoreCase(book)).isEqualTo(0);
+    }
+
+    @Test
+    public void whenUsingObjectsEqualsMethod_ThenComparingStrings(){
+
+        String string1 = "using objects equals";
+        String string2 = "using objects equals";
+        String string3 = new String("using objects equals");
+
+        assertThat(Objects.equals(string1, string2)).isTrue();
+        assertThat(Objects.equals(string1, string3)).isTrue();
+
+        assertThat(Objects.equals(null, null)).isTrue();
+        assertThat(Objects.equals(null, string1)).isFalse();
+    }
+
+    @Test
+    public void whenUsingEqualsOfApacheCommons_ThenComparingStrings(){
+
+        assertThat(StringUtils.equals(null, null)).isTrue();
+        assertThat(StringUtils.equals(null, "equals method")).isFalse();
+        assertThat(StringUtils.equals("equals method", "equals method")).isTrue();
+        assertThat(StringUtils.equals("equals method", "EQUALS METHOD")).isFalse();
+    }
+
+    @Test
+    public void whenUsingEqualsIgnoreCaseOfApacheCommons_ThenComparingStrings(){
+
+        assertThat(StringUtils.equalsIgnoreCase(null, null)).isTrue();
+        assertThat(StringUtils.equalsIgnoreCase(null, "equals method")).isFalse();
+        assertThat(StringUtils.equalsIgnoreCase("equals method", "equals method")).isTrue();
+        assertThat(StringUtils.equalsIgnoreCase("equals method", "EQUALS METHOD")).isTrue();
+    }
+
+    @Test
+    public void whenUsingEqualsAnyOf_ThenComparingStrings(){
+
+        assertThat(StringUtils.equalsAny(null, null, null)).isTrue();
+        assertThat(StringUtils.equalsAny("equals any", "equals any", "any")).isTrue();
+        assertThat(StringUtils.equalsAny("equals any", null, "equals any")).isTrue();
+        assertThat(StringUtils.equalsAny(null, "equals", "any")).isFalse();
+        assertThat(StringUtils.equalsAny("equals any", "EQUALS ANY", "ANY")).isFalse();
+    }
+
+    @Test
+    public void whenUsingEqualsAnyIgnoreCase_ThenComparingStrings(){
+
+        assertThat(StringUtils.equalsAnyIgnoreCase(null, null, null)).isTrue();
+        assertThat(StringUtils.equalsAnyIgnoreCase("equals any", "equals any", "any")).isTrue();
+        assertThat(StringUtils.equalsAnyIgnoreCase("equals any", null, "equals any")).isTrue();
+        assertThat(StringUtils.equalsAnyIgnoreCase(null, "equals", "any")).isFalse();
+        assertThat(StringUtils.equalsAnyIgnoreCase(
+                "equals any ignore case", "EQUALS ANY IGNORE CASE", "any")).isTrue();
+    }
+
+    @Test
+    public void whenUsingCompare_thenComparingStringsWithNulls(){
+
+        assertThat(StringUtils.compare(null, null)).isEqualTo(0);
+        assertThat(StringUtils.compare(null, "abc")).isEqualTo(-1);
+
+        assertThat(StringUtils.compare("abc", "bbc")).isEqualTo(-1);
+        assertThat(StringUtils.compare("bbc", "abc")).isEqualTo(1);
+        assertThat(StringUtils.compare("abc", "abc")).isEqualTo(0);
+    }
+
+    @Test
+    public void whenUsingCompareIgnoreCase_ThenComparingStringsWithNulls(){
+
+        assertThat(StringUtils.compareIgnoreCase(null, null)).isEqualTo(0);
+        assertThat(StringUtils.compareIgnoreCase(null, "abc")).isEqualTo(-1);
+
+        assertThat(StringUtils.compareIgnoreCase("Abc", "bbc")).isEqualTo(-1);
+        assertThat(StringUtils.compareIgnoreCase("bbc", "ABC")).isEqualTo(1);
+        assertThat(StringUtils.compareIgnoreCase("abc", "ABC")).isEqualTo(0);
+    }
+
+    @Test
+    public void whenUsingCompareWithNullIsLessOption_ThenComparingStrings(){
+
+        assertThat(StringUtils.compare(null, "abc", true)).isEqualTo(-1);
+        assertThat(StringUtils.compare(null, "abc", false)).isEqualTo(1);
+    }
+
+}
--- a/core-java/src/test/java/com/baeldung/system/WhenDetectingOSTest.java
+++ b/core-java/src/test/java/com/baeldung/system/WhenDetectingOSTest.java
@ -0,0 +1,25 @@
+package com.baeldung.system;
+
+import org.junit.Assert;
+import org.junit.Ignore;
+import org.junit.Test;
+
+@Ignore
+public class WhenDetectingOSTest {
+
+    private DetectOS os = new DetectOS();
+
+    @Test
+    public void whenUsingSystemProperty_shouldReturnOS() {
+        String expected = "Windows 10";
+        String actual = os.getOperatingSystem();
+        Assert.assertEquals(expected, actual);
+    }
+
+    @Test
+    public void whenUsingSystemUtils_shouldReturnOS() {
+        String expected = "Windows 10";
+        String actual = os.getOperatingSystemSystemUtils();
+        Assert.assertEquals(expected, actual);
+    }
+}
--- a/core-kotlin/README.md
+++ b/core-kotlin/README.md
@ -17,4 +17,4 @@
 - [Sealed Classes in Kotlin](http://www.baeldung.com/kotlin-sealed-classes)
 - [JUnit 5 for Kotlin Developers](http://www.baeldung.com/junit-5-kotlin)
 - [Extension Methods in Kotlin](http://www.baeldung.com/kotlin-extension-methods)
-
+- [Infix Functions in Kotlin](http://www.baeldung.com/kotlin-infix-functions)
--- a/core-kotlin/pom.xml
+++ b/core-kotlin/pom.xml
@ -76,7 +76,18 @@
            <artifactId>mockito-kotlin</artifactId>
            <version>${mockito-kotlin.version}</version>
            <scope>test</scope>
-        </dependency>        
+        </dependency>
+        <dependency>
+            <groupId>com.github.salomonbrys.kodein</groupId>
+            <artifactId>kodein</artifactId>
+            <version>${kodein.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
    </dependencies>

    <build>
@ -189,11 +200,13 @@
        <kotlin-reflect.version>1.1.2</kotlin-reflect.version>
        <kotlinx.version>0.15</kotlinx.version>
        <mockito-kotlin.version>1.5.0</mockito-kotlin.version>
+        <kodein.version>4.1.0</kodein.version>

        <junit.jupiter.version>5.0.0</junit.jupiter.version>
        <junit.platform.version>1.0.0</junit.platform.version>
        <junit.vintage.version>4.12.0</junit.vintage.version>
        <junit4.version>4.12</junit4.version>
+        <assertj.version>3.9.1</assertj.version>
    </properties>

 </project>
--- a/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/Controller.kt
+++ b/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/Controller.kt
@ -0,0 +1,3 @@
+package com.baeldung.kotlin.kodein
+
+class Controller(private val service : Service)
--- a/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/Dao.kt
+++ b/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/Dao.kt
@ -0,0 +1,3 @@
+package com.baeldung.kotlin.kodein
+
+interface Dao
--- a/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/JdbcDao.kt
+++ b/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/JdbcDao.kt
@ -0,0 +1,3 @@
+package com.baeldung.kotlin.kodein
+
+class JdbcDao : Dao
--- a/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/MongoDao.kt
+++ b/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/MongoDao.kt
@ -0,0 +1,3 @@
+package com.baeldung.kotlin.kodein
+
+class MongoDao : Dao
--- a/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/Service.kt
+++ b/core-kotlin/src/main/kotlin/com/baeldung/kotlin/kodein/Service.kt
@ -0,0 +1,3 @@
+package com.baeldung.kotlin.kodein
+
+class Service(private val dao: Dao, private val tag: String)
--- a/core-kotlin/src/test/kotlin/com/baeldung/kotlin/kodein/KodeinUnitTest.kt
+++ b/core-kotlin/src/test/kotlin/com/baeldung/kotlin/kodein/KodeinUnitTest.kt
@ -0,0 +1,191 @@
+package com.baeldung.kotlin.kodein
+
+import com.github.salomonbrys.kodein.*
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+
+class KodeinUnitTest {
+
+    class InMemoryDao : Dao
+
+    @Test
+    fun whenSingletonBinding_thenSingleInstanceIsCreated() {
+        var created = false
+        val kodein = Kodein {
+            bind<Dao>() with singleton {
+                created = true
+                MongoDao()
+            }
+        }
+
+        assertThat(created).isFalse()
+
+        val dao1: Dao = kodein.instance()
+
+        assertThat(created).isTrue()
+
+        val dao2: Dao = kodein.instance()
+
+        assertThat(dao1).isSameAs(dao2)
+    }
+
+    @Test
+    fun whenFactoryBinding_thenNewInstanceIsCreated() {
+        val kodein = Kodein {
+            bind<Dao>() with singleton { MongoDao() }
+            bind<Service>() with factory { tag: String -> Service(instance(), tag) }
+        }
+        val service1: Service = kodein.with("myTag").instance()
+        val service2: Service = kodein.with("myTag").instance()
+
+        assertThat(service1).isNotSameAs(service2)
+    }
+
+    @Test
+    fun whenProviderBinding_thenNewInstanceIsCreated() {
+        val kodein = Kodein {
+            bind<Dao>() with provider { MongoDao() }
+        }
+        val dao1: Dao = kodein.instance()
+        val dao2: Dao = kodein.instance()
+
+        assertThat(dao1).isNotSameAs(dao2)
+    }
+
+    @Test
+    fun whenTaggedBinding_thenMultipleInstancesOfSameTypeCanBeRegistered() {
+        val kodein = Kodein {
+            bind<Dao>("dao1") with singleton { MongoDao() }
+            bind<Dao>("dao2") with singleton { MongoDao() }
+        }
+        val dao1: Dao = kodein.instance("dao1")
+        val dao2: Dao = kodein.instance("dao2")
+
+        assertThat(dao1).isNotSameAs(dao2)
+    }
+
+    @Test
+    fun whenEagerSingletonBinding_thenCreationIsEager() {
+        var created = false
+        val kodein = Kodein {
+            bind<Dao>() with eagerSingleton {
+                created = true
+                MongoDao()
+            }
+        }
+
+        assertThat(created).isTrue()
+        val dao1: Dao = kodein.instance()
+        val dao2: Dao = kodein.instance()
+
+        assertThat(dao1).isSameAs(dao2)
+    }
+
+    @Test
+    fun whenMultitonBinding_thenInstancesAreReused() {
+        val kodein = Kodein {
+            bind<Dao>() with singleton { MongoDao() }
+            bind<Service>() with multiton { tag: String -> Service(instance(), tag) }
+        }
+        val service1: Service = kodein.with("myTag").instance()
+        val service2: Service = kodein.with("myTag").instance()
+
+        assertThat(service1).isSameAs(service2)
+    }
+
+    @Test
+    fun whenInstanceBinding_thenItIsReused() {
+        val dao = MongoDao()
+        val kodein = Kodein {
+            bind<Dao>() with instance(dao)
+        }
+        val fromContainer: Dao = kodein.instance()
+
+        assertThat(dao).isSameAs(fromContainer)
+    }
+
+    @Test
+    fun whenConstantBinding_thenItIsAvailable() {
+        val kodein = Kodein {
+            constant("magic") with 42
+        }
+        val fromContainer: Int = kodein.instance("magic")
+
+        assertThat(fromContainer).isEqualTo(42)
+    }
+
+    @Test
+    fun whenUsingModules_thenTransitiveDependenciesAreSuccessfullyResolved() {
+        val jdbcModule = Kodein.Module {
+            bind<Dao>() with singleton { JdbcDao() }
+        }
+        val kodein = Kodein {
+            import(jdbcModule)
+            bind<Controller>() with singleton { Controller(instance()) }
+            bind<Service>() with singleton { Service(instance(), "myService") }
+        }
+
+        val dao: Dao = kodein.instance()
+        assertThat(dao).isInstanceOf(JdbcDao::class.java)
+    }
+
+    @Test
+    fun whenComposition_thenBeansAreReUsed() {
+        val persistenceContainer = Kodein {
+            bind<Dao>() with singleton { MongoDao() }
+        }
+        val serviceContainer = Kodein {
+            extend(persistenceContainer)
+            bind<Service>() with singleton { Service(instance(), "myService") }
+        }
+        val fromPersistence: Dao = persistenceContainer.instance()
+        val fromService: Dao = serviceContainer.instance()
+
+        assertThat(fromPersistence).isSameAs(fromService)
+    }
+
+    @Test
+    fun whenOverriding_thenRightBeanIsUsed() {
+        val commonModule = Kodein.Module {
+            bind<Dao>() with singleton { MongoDao() }
+            bind<Service>() with singleton { Service(instance(), "myService") }
+        }
+        val testContainer = Kodein {
+            import(commonModule)
+            bind<Dao>(overrides = true) with singleton { InMemoryDao() }
+        }
+        val dao: Dao = testContainer.instance()
+
+        assertThat(dao).isInstanceOf(InMemoryDao::class.java)
+    }
+
+    @Test
+    fun whenMultiBinding_thenWorks() {
+        val kodein = Kodein {
+            bind() from setBinding<Dao>()
+            bind<Dao>().inSet() with singleton { MongoDao() }
+            bind<Dao>().inSet() with singleton { JdbcDao() }
+        }
+        val daos: Set<Dao> = kodein.instance()
+
+        assertThat(daos.map { it.javaClass as Class<*> }).containsOnly(MongoDao::class.java, JdbcDao::class.java)
+    }
+
+    @Test
+    fun whenInjector_thenWorks() {
+        class Controller2 {
+            private val injector = KodeinInjector()
+            val service: Service by injector.instance()
+            fun injectDependencies(kodein: Kodein) = injector.inject(kodein)
+        }
+
+        val kodein = Kodein {
+            bind<Dao>() with singleton { MongoDao() }
+            bind<Service>() with singleton { Service(instance(), "myService") }
+        }
+        val controller = Controller2()
+        controller.injectDependencies(kodein)
+
+        assertThat(controller.service).isNotNull
+    }
+}
--- a/google-cloud/README.md
+++ b/google-cloud/README.md
@ -0,0 +1,16 @@
+## Google Cloud Tutorial Project
+
+### Relevant Article:
+- [Intro to Google Cloud Storage With Java](http://www.baeldung.com/intro-to-google-cloud-storage-with-java/)
+
+### Overview
+This Maven project contains the Java code for the article linked above.
+
+### Package Organization
+Java classes for the intro tutorial are in the org.baeldung.google.cloud package. Please note that Google Cloud requires
+a user account and credentials, as explained in the tutorial.
+
+
+### Running the tests
+
+```
--- a/google-cloud/pom.xml
+++ b/google-cloud/pom.xml
@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>google-cloud</artifactId>
+    <version>0.1-SNAPSHOT</version>
+    <packaging>jar</packaging>
+    <name>google-cloud</name>
+    <description>Google Cloud Tutorials</description>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>com.google.cloud</groupId>
+            <artifactId>google-cloud-storage</artifactId>
+            <version>1.16.0</version>
+        </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <!-- Check for the most recent available version: https://projectlombok.org/changelog.html -->
+            <version>${lombok.version}</version>
+            <scope>provided</scope>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <lombok.version>1.16.18</lombok.version>
+        <java.version>1.8</java.version>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+</project>
--- a/google-cloud/src/main/java/com/baeldung/google/cloud/storage/GoogleCloudStorage.java
+++ b/google-cloud/src/main/java/com/baeldung/google/cloud/storage/GoogleCloudStorage.java
@ -0,0 +1,105 @@
+package com.baeldung.google.cloud.storage;
+
+import com.google.api.gax.paging.Page;
+import com.google.auth.Credentials;
+import com.google.auth.oauth2.GoogleCredentials;
+import com.google.cloud.storage.*;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.WritableByteChannel;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+/**
+ * Simple class for creating, reading and modifying text blobs on Google Cloud
+ */
+@Slf4j
+public class GoogleCloudStorage {
+
+    private Storage storage;
+    private Bucket bucket;
+
+    public static void main(String[] args) throws Exception {
+
+        // Use this variation to read the Google authorization JSON from the resources directory with a path
+        // and a project name.
+        GoogleCloudStorage googleCloudStorage =
+            new GoogleCloudStorage("google-cloud/src/main/resources/google_auth.json", "baeldung-cloud-tutorial");
+
+        // Bucket require globally unique names, so you'll probably need to change this
+        Bucket bucket = googleCloudStorage.getBucket("baeldung-1-bucket");
+
+        // Save a simple string
+        BlobId blobId = googleCloudStorage.saveString("my-first-blob", "Hi there!", bucket);
+
+        // Get it by blob id this time
+        String value = googleCloudStorage.getString(blobId);
+
+        log.info("Read data: {}", value);
+
+        googleCloudStorage.updateString(blobId, "Bye now!");
+
+        // Get the string by blob name
+        value = googleCloudStorage.getString("my-first-blob");
+
+        log.info("Read modified data: {}", value);
+
+
+    }
+
+
+    // Use path and project name
+    private GoogleCloudStorage(String pathToConfig, String projectId) throws IOException {
+        Credentials credentials = GoogleCredentials.fromStream(new FileInputStream(pathToConfig));
+        storage = StorageOptions.newBuilder().setCredentials(credentials).setProjectId(projectId).build().getService();
+    }
+
+    // Check for bucket existence and create if needed.
+    private Bucket getBucket(String bucketName) {
+        bucket = storage.get(bucketName);
+        if (bucket == null) {
+            System.out.println("Creating new bucket.");
+            bucket = storage.create(BucketInfo.of(bucketName));
+        }
+        return bucket;
+    }
+
+    // Save a string to a blob
+    private BlobId saveString(String blobName, String value, Bucket bucket) {
+        byte[] bytes = value.getBytes(UTF_8);
+        Blob blob = bucket.create(blobName, bytes);
+        return blob.getBlobId();
+    }
+
+
+    // get a blob by id
+    private String getString(BlobId blobId) {
+        Blob blob = storage.get(blobId);
+        return new String(blob.getContent());
+    }
+
+
+    // get a blob by name
+    private String getString(String name) {
+        Page<Blob> blobs = bucket.list();
+        for (Blob blob: blobs.getValues()) {
+            if (name.equals(blob.getName())) {
+                return new String(blob.getContent());
+            }
+        }
+        return "Blob not found";
+    }
+
+    // Update a blob
+    private void updateString(BlobId blobId, String newString) throws IOException {
+        Blob blob = storage.get(blobId);
+        if (blob != null) {
+            WritableByteChannel channel = blob.writer();
+            channel.write(ByteBuffer.wrap(newString.getBytes(UTF_8)));
+            channel.close();
+        }
+    }
+}
--- a/guava/README.md
+++ b/guava/README.md
@ -19,6 +19,7 @@
 - [Guide to Guava’s Ordering](http://www.baeldung.com/guava-ordering)
 - [Guide to Guava’s PreConditions](http://www.baeldung.com/guava-preconditions)
 - [Introduction to Guava CacheLoader](http://www.baeldung.com/guava-cacheloader)
+- [Introduction to Guava Memoizer](http://www.baeldung.com/guava-memoizer)
 - [Guide to Guava’s EventBus](http://www.baeldung.com/guava-eventbus)
 - [Guide to Guava Multimap](http://www.baeldung.com/guava-multimap)
 - [Guide to Guava RangeSet](http://www.baeldung.com/guava-rangeset)
--- a/guava/pom.xml
+++ b/guava/pom.xml
@ -43,6 +43,14 @@
            <version>${assertj.version}</version>
            <scope>test</scope>
        </dependency>
+        
+        <dependency>
+            <groupId>org.hamcrest</groupId>
+            <artifactId>java-hamcrest</artifactId>
+            <version>2.0.0.0</version>
+            <scope>test</scope>
+        </dependency>
+        
    </dependencies>

    <build>
--- a/Show More
+++ b/Show More