Bael 1496 flyweight (#3598)

* Added flyweight pattern example.

* Refactored VehicleFactory to use computeIfAbsent method.

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;
+	}
+
+}

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!");
+	}
+}

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();
+
+}

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;
+	}
+}

core-java/src/test/java/com/baeldung/designpatterns/flyweight/FlyweightUnitTest.java

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+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);
+	}
+}