Merge pull request #57 from eugenp/master

update

algorithms-miscellaneous-2/src/main/java/com/baeldung/algorithms/play2048/Board.java

@@ -0,0 +1,208 @@
+package com.baeldung.algorithms.play2048;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Board {
+    private static final Logger LOG = LoggerFactory.getLogger(Board.class);
+
+    private final int[][] board;
+
+    private final int score;
+
+    public Board(int size) {
+        assert(size > 0);
+
+        this.board = new int[size][];
+        this.score = 0;
+
+        for (int x = 0; x < size; ++x) {
+            this.board[x] = new int[size];
+            for (int y = 0; y < size; ++y) {
+                board[x][y] = 0;
+            }
+        }
+    }
+
+    private Board(int[][] board, int score) {
+        this.score = score;
+        this.board = new int[board.length][];
+
+        for (int x = 0; x < board.length; ++x) {
+            this.board[x] = Arrays.copyOf(board[x], board[x].length);
+        }
+    }
+
+    public int getSize() {
+        return board.length;
+    }
+
+    public int getScore() {
+        return score;
+    }
+
+    public int getCell(Cell cell) {
+        int x = cell.getX();
+        int y = cell.getY();
+        assert(x >= 0 && x < board.length);
+        assert(y >= 0 && y < board.length);
+
+        return board[x][y];
+    }
+
+    public boolean isEmpty(Cell cell) {
+        return getCell(cell) == 0;
+    }
+
+    public List<Cell> emptyCells() {
+        List<Cell> result = new ArrayList<>();
+        for (int x = 0; x < board.length; ++x) {
+            for (int y = 0; y < board[x].length; ++y) {
+                Cell cell = new Cell(x, y);
+                if (isEmpty(cell)) {
+                    result.add(cell);
+                }
+            }
+        }
+        return result;
+    }
+
+    public Board placeTile(Cell cell, int number) {
+        if (!isEmpty(cell)) {
+            throw new IllegalArgumentException("That cell is not empty");
+        }
+
+        Board result = new Board(this.board, this.score);
+        result.board[cell.getX()][cell.getY()] = number;
+        return result;
+    }
+
+    public Board move(Move move) {
+        // Clone the board
+        int[][] tiles = new int[this.board.length][];
+        for (int x = 0; x < this.board.length; ++x) {
+            tiles[x] = Arrays.copyOf(this.board[x], this.board[x].length);
+        }
+
+        LOG.debug("Before move: {}", Arrays.deepToString(tiles));
+        // If we're doing an Left/Right move then transpose the board to make it a Up/Down move
+        if (move == Move.LEFT || move == Move.RIGHT) {
+            tiles = transpose(tiles);
+            LOG.debug("After transpose: {}", Arrays.deepToString(tiles));
+        }
+        // If we're doing a Right/Down move then reverse the board.
+        // With the above we're now always doing an Up move
+        if (move == Move.DOWN || move == Move.RIGHT) {
+            tiles = reverse(tiles);
+            LOG.debug("After reverse: {}", Arrays.deepToString(tiles));
+        }
+        LOG.debug("Ready to move: {}", Arrays.deepToString(tiles));
+
+        // Shift everything up
+        int[][] result = new int[tiles.length][];
+        int newScore = 0;
+        for (int x = 0; x < tiles.length; ++x) {
+            LinkedList<Integer> thisRow = new LinkedList<>();
+            for (int y = 0; y < tiles[0].length; ++y) {
+                if (tiles[x][y] > 0) {
+                    thisRow.add(tiles[x][y]);
+                }
+            }
+
+            LOG.debug("Unmerged row: {}", thisRow);
+            LinkedList<Integer> newRow = new LinkedList<>();
+            while (thisRow.size() >= 2) {
+                int first = thisRow.pop();
+                int second = thisRow.peek();
+                LOG.debug("Looking at numbers {} and {}", first, second);
+                if (second == first) {
+                    LOG.debug("Numbers match, combining");
+                    int newNumber = first * 2;
+                    newRow.add(newNumber);
+                    newScore += newNumber;
+                    thisRow.pop();
+                } else {
+                    LOG.debug("Numbers don't match");
+                    newRow.add(first);
+                }
+            }
+            newRow.addAll(thisRow);
+            LOG.debug("Merged row: {}", newRow);
+
+            result[x] = new int[tiles[0].length];
+            for (int y = 0; y < tiles[0].length; ++y) {
+                if (newRow.isEmpty()) {
+                    result[x][y] = 0;
+                } else {
+                    result[x][y] = newRow.pop();
+                }
+            }
+        }
+        LOG.debug("After moves: {}", Arrays.deepToString(result));
+
+        // Un-reverse the board
+        if (move == Move.DOWN || move == Move.RIGHT) {
+            result = reverse(result);
+            LOG.debug("After reverse: {}", Arrays.deepToString(result));
+        }
+        // Un-transpose the board
+        if (move == Move.LEFT || move == Move.RIGHT) {
+            result = transpose(result);
+            LOG.debug("After transpose: {}", Arrays.deepToString(result));
+        }
+        return new Board(result, this.score + newScore);
+    }
+
+    private static int[][] transpose(int[][] input) {
+        int[][] result = new int[input.length][];
+
+        for (int x = 0; x < input.length; ++x) {
+            result[x] = new int[input[0].length];
+            for (int y = 0; y < input[0].length; ++y) {
+                result[x][y] = input[y][x];
+            }
+        }
+
+        return result;
+    }
+
+    private static int[][] reverse(int[][] input) {
+        int[][] result = new int[input.length][];
+
+        for (int x = 0; x < input.length; ++x) {
+            result[x] = new int[input[0].length];
+            for (int y = 0; y < input[0].length; ++y) {
+                result[x][y] = input[x][input.length - y - 1];
+            }
+        }
+
+        return result;
+    }
+
+    @Override
+    public String toString() {
+        return Arrays.deepToString(board);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        Board board1 = (Board) o;
+        return Arrays.deepEquals(board, board1.board);
+    }
+
+    @Override
+    public int hashCode() {
+        return Arrays.deepHashCode(board);
+    }
+}

algorithms-miscellaneous-2/src/main/java/com/baeldung/algorithms/play2048/Cell.java

@@ -0,0 +1,26 @@
+package com.baeldung.algorithms.play2048;
+
+import java.util.StringJoiner;
+
+public class Cell {
+    private int x;
+    private int y;
+
+    public Cell(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    public int getX() {
+        return x;
+    }
+
+    public int getY() {
+        return y;
+    }
+
+    @Override
+    public String toString() {
+        return new StringJoiner(", ", Cell.class.getSimpleName() + "[", "]").add("x=" + x).add("y=" + y).toString();
+    }
+}

algorithms-miscellaneous-2/src/main/java/com/baeldung/algorithms/play2048/Computer.java

@@ -0,0 +1,27 @@
+package com.baeldung.algorithms.play2048;
+
+import java.security.SecureRandom;
+import java.util.List;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Computer {
+    private static final Logger LOG = LoggerFactory.getLogger(Computer.class);
+
+    private final SecureRandom rng = new SecureRandom();
+
+    public Board makeMove(Board input) {
+        List<Cell> emptyCells = input.emptyCells();
+        LOG.info("Number of empty cells: {}", emptyCells.size());
+
+        double numberToPlace = rng.nextDouble();
+        LOG.info("New number probability: {}", numberToPlace);
+
+        int indexToPlace = rng.nextInt(emptyCells.size());
+        Cell cellToPlace = emptyCells.get(indexToPlace);
+        LOG.info("Placing number into empty cell: {}", cellToPlace);
+
+        return input.placeTile(cellToPlace, numberToPlace >= 0.9 ? 4 : 2);
+    }
+}

algorithms-miscellaneous-2/src/main/java/com/baeldung/algorithms/play2048/Human.java

@@ -0,0 +1,126 @@
+package com.baeldung.algorithms.play2048;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.apache.commons.math3.util.Pair;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Human {
+    private static final Logger LOG = LoggerFactory.getLogger(Human.class);
+
+    public Board makeMove(Board input) {
+        // For each move in MOVE
+        //   Generate board from move
+        //   Generate Score for Board
+        // Return board with the best score
+        //
+        // Generate Score
+        //   If Depth Limit
+        //     Return Final Score
+        //   Total Score = 0
+        //   For every empty square in new board
+        //     Generate board with "2" in square
+        //       Calculate Score
+        //       Total Score += (Score * 0.9)
+        //     Generate board with "4" in square
+        //       Calculate Score
+        //       Total Score += (Score * 0.1)
+        //
+        // Calculate Score
+        //   For each move in MOVE
+        //     Generate board from move
+        //     Generate score for board
+        //   Return the best generated score
+
+        return Arrays.stream(Move.values())
+            .parallel()
+            .map(input::move)
+            .filter(board -> !board.equals(input))
+            .max(Comparator.comparingInt(board -> generateScore(board, 0)))
+            .orElse(input);
+    }
+
+    private int generateScore(Board board, int depth) {
+        if (depth >= 3) {
+            int finalScore = calculateFinalScore(board);
+            LOG.debug("Final score for board {}: {}", board,finalScore);
+            return finalScore;
+        }
+
+        return board.emptyCells().stream()
+            .parallel()
+            .flatMap(cell -> Stream.of(new Pair<>(cell, 2), new Pair<>(cell, 4)))
+            .mapToInt(move -> {
+                LOG.debug("Simulating move {} at depth {}", move, depth);
+                Board newBoard = board.placeTile(move.getFirst(), move.getSecond());
+                int boardScore = calculateScore(newBoard, depth + 1);
+                int calculatedScore = (int) (boardScore * (move.getSecond() == 2 ? 0.9 : 0.1));
+                LOG.debug("Calculated score for board {} and move {} at depth {}: {}", newBoard, move, depth, calculatedScore);
+                return calculatedScore;
+            })
+            .sum();
+    }
+
+    private int calculateScore(Board board, int depth) {
+        return Arrays.stream(Move.values())
+            .parallel()
+            .map(board::move)
+            .filter(moved -> !moved.equals(board))
+            .mapToInt(newBoard -> generateScore(newBoard, depth))
+            .max()
+            .orElse(0);
+    }
+
+    private int calculateFinalScore(Board board) {
+        List<List<Integer>> rowsToScore = new ArrayList<>();
+        for (int i = 0; i < board.getSize(); ++i) {
+            List<Integer> row = new ArrayList<>();
+            List<Integer> col = new ArrayList<>();
+
+            for (int j = 0; j < board.getSize(); ++j) {
+                row.add(board.getCell(new Cell(i, j)));
+                col.add(board.getCell(new Cell(j, i)));
+            }
+
+            rowsToScore.add(row);
+            rowsToScore.add(col);
+        }
+
+        return rowsToScore.stream()
+            .parallel()
+            .mapToInt(row -> {
+                List<Integer> preMerged = row.stream()
+                    .filter(value -> value != 0)
+                    .collect(Collectors.toList());
+
+                int numMerges = 0;
+                int monotonicityLeft = 0;
+                int monotonicityRight = 0;
+                for (int i = 0; i < preMerged.size() - 1; ++i) {
+                    Integer first = preMerged.get(i);
+                    Integer second = preMerged.get(i + 1);
+                    if (first.equals(second)) {
+                        ++numMerges;
+                    } else if (first > second) {
+                        monotonicityLeft += first - second;
+                    } else {
+                        monotonicityRight += second - first;
+                    }
+                }
+
+                int score = 1000;
+                score += 250 * row.stream().filter(value -> value == 0).count();
+                score += 750 * numMerges;
+                score -= 10 * row.stream().mapToInt(value -> value).sum();
+                score -= 50 * Math.min(monotonicityLeft, monotonicityRight);
+                return score;
+            })
+            .sum();
+    }
+}

algorithms-miscellaneous-2/src/main/java/com/baeldung/algorithms/play2048/Move.java

@@ -0,0 +1,8 @@
+package com.baeldung.algorithms.play2048;
+
+public enum Move {
+    UP,
+    DOWN,
+    LEFT,
+    RIGHT
+}

algorithms-miscellaneous-2/src/main/java/com/baeldung/algorithms/play2048/Play2048.java

@@ -0,0 +1,73 @@
+package com.baeldung.algorithms.play2048;
+
+public class Play2048 {
+    private static final int SIZE = 3;
+    private static final int INITIAL_NUMBERS = 2;
+
+    public static void main(String[] args) {
+        // The board and players
+        Board board = new Board(SIZE);
+        Computer computer = new Computer();
+        Human human = new Human();
+
+        // The computer has two moves first
+        System.out.println("Setup");
+        System.out.println("=====");
+        for (int i = 0; i < INITIAL_NUMBERS; ++i) {
+            board = computer.makeMove(board);
+        }
+
+        printBoard(board);
+        do {
+            board = human.makeMove(board);
+            System.out.println("Human move");
+            System.out.println("==========");
+            printBoard(board);
+
+            board = computer.makeMove(board);
+            System.out.println("Computer move");
+            System.out.println("=============");
+            printBoard(board);
+        } while (!board.emptyCells().isEmpty());
+
+        System.out.println("Final Score: " + board.getScore());
+
+    }
+
+    private static void printBoard(Board board) {
+        StringBuilder topLines = new StringBuilder();
+        StringBuilder midLines = new StringBuilder();
+        for (int x = 0; x < board.getSize(); ++x) {
+            topLines.append("+--------");
+            midLines.append("|        ");
+        }
+        topLines.append("+");
+        midLines.append("|");
+
+
+        for (int y = 0; y < board.getSize(); ++y) {
+            System.out.println(topLines);
+            System.out.println(midLines);
+            for (int x = 0; x < board.getSize(); ++x) {
+                Cell cell = new Cell(x, y);
+                System.out.print("|");
+                if (board.isEmpty(cell)) {
+                    System.out.print("        ");
+                } else {
+                    StringBuilder output = new StringBuilder(Integer.toString(board.getCell(cell)));
+                    while (output.length() < 8) {
+                        output.append(" ");
+                        if (output.length() < 8) {
+                            output.insert(0, " ");
+                        }
+                    }
+                    System.out.print(output);
+                }
+            }
+            System.out.println("|");
+            System.out.println(midLines);
+        }
+        System.out.println(topLines);
+        System.out.println("Score: " + board.getScore());
+    }
+}

core-groovy-2/src/main/groovy/com/baeldung/determinedatatype/Person.groovy

@@ -0,0 +1,14 @@
+package com.baeldung.determinedatatype
+
+class Person {
+    
+    private int ageAsInt
+    private Double ageAsDouble
+    private String ageAsString
+    
+    Person() {}
+    Person(int ageAsInt) { this.ageAsInt =  ageAsInt}
+    Person(Double ageAsDouble) { this.ageAsDouble =  ageAsDouble}
+    Person(String ageAsString) { this.ageAsString =  ageAsString}    
+}
+class Student extends Person {}

core-groovy-2/src/test/groovy/com/baeldung/determinedatatype/PersonTest.groovy

@@ -0,0 +1,56 @@
+package com.baeldung.determinedatatype
+
+import org.junit.Assert
+import org.junit.Test
+import com.baeldung.determinedatatype.Person
+
+public class PersonTest {
+    
+    @Test
+    public void givenWhenParameterTypeIsInteger_thenReturnTrue() {
+        Person personObj = new Person(10)
+        Assert.assertTrue(personObj.ageAsInt instanceof Integer)
+    }
+    
+    @Test
+    public void givenWhenParameterTypeIsDouble_thenReturnTrue() {
+        Person personObj = new Person(10.0)
+        Assert.assertTrue((personObj.ageAsDouble).getClass() == Double)
+    }
+    
+    @Test
+    public void givenWhenParameterTypeIsString_thenReturnTrue() {
+        Person personObj = new Person("10 years")
+        Assert.assertTrue(personObj.ageAsString.class == String)
+    }
+    
+    @Test
+    public void givenClassName_WhenParameterIsInteger_thenReturnTrue() {
+        Assert.assertTrue(Person.class.getDeclaredField('ageAsInt').type == int.class)
+    }
+    
+    @Test
+    public void givenWhenObjectIsInstanceOfType_thenReturnTrue() {
+        Person personObj = new Person()
+        Assert.assertTrue(personObj instanceof Person)
+    }
+
+    @Test
+    public void givenWhenInstanceIsOfSubtype_thenReturnTrue() {
+        Student studentObj = new Student()
+        Assert.assertTrue(studentObj in Person)
+    }
+    
+    @Test
+    public void givenGroovyList_WhenFindClassName_thenReturnTrue() {
+      def ageList = ['ageAsString','ageAsDouble', 10]   
+      Assert.assertTrue(ageList.class == ArrayList)     
+      Assert.assertTrue(ageList.getClass() == ArrayList)
+    }
+    
+    @Test
+    public void givenGrooyMap_WhenFindClassName_thenReturnTrue() {
+      def ageMap = [ageAsString: '10 years', ageAsDouble: 10.0]
+      Assert.assertFalse(ageMap.class == LinkedHashMap)
+    }
+}

core-java-modules/core-java-14/src/main/java/com/baeldung/java14/record/Person.java

@@ -4,19 +4,19 @@ import java.util.Objects;
 
 public record Person (String name, String address) {
 	
-	public static String UNKWOWN_ADDRESS = "Unknown";
-	public static String UNNAMED = "Unnamed";
+	  public static String UNKNOWN_ADDRESS = "Unknown";
+	  public static String UNNAMED = "Unnamed";
 
-	public Person {
-		Objects.requireNonNull(name);
-		Objects.requireNonNull(address);
-	}
+	  public Person {
+		    Objects.requireNonNull(name);
+		    Objects.requireNonNull(address);
+	  }
 	
-	public Person(String name) {
-		this(name, UNKWOWN_ADDRESS);
-	}
+	  public Person(String name) {
+		    this(name, UNKNOWN_ADDRESS);
+	  }
 	
-	public static Person unnamed(String address) {
-		return new Person(UNNAMED, address);
-	}
-}
+	  public static Person unnamed(String address) {
+		    return new Person(UNNAMED, address);
+	  }
+}

core-java-modules/core-java-14/src/test/java/com/baeldung/java14/record/PersonTest.java

@@ -134,7 +134,7 @@ public class PersonTest {
         Person person = new Person(name);
 
         assertEquals(name, person.name());
-        assertEquals(Person.UNKWOWN_ADDRESS, person.address());
+        assertEquals(Person.UNKNOWN_ADDRESS, person.address());
     }
 
     @Test
@@ -147,4 +147,4 @@ public class PersonTest {
         assertEquals(Person.UNNAMED, person.name());
         assertEquals(address, person.address());
     }
-}
+}

core-java-modules/core-java-8-datetime-2/pom.xml

@@ -4,7 +4,7 @@
     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-java-8-datetime</artifactId>
+    <artifactId>core-java-8-datetime-2</artifactId>
     <version>${project.parent.version}</version>
     <name>core-java-8-datetime</name>
     <packaging>jar</packaging>
@@ -41,7 +41,6 @@
     </dependencies>
 
     <build>
-        <finalName>core-java-datetime-java8</finalName>
         <resources>
             <resource>
                 <directory>src/main/resources</directory>

core-java-modules/core-java-arrays-guides/src/test/java/com/baeldung/arrays/JavaArraysToStringUnitTest.java

@@ -0,0 +1,42 @@
+package com.baeldung.arrays;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class JavaArraysToStringUnitTest {
+
+    @Test
+    public void givenInstanceOfArray_whenTryingToConvertToString_thenNameOfClassIsShown() {
+        Object[] arrayOfObjects = { "John", 2, true };
+        assertTrue(arrayOfObjects.toString().startsWith("[Ljava.lang.Object;"));
+    }
+
+    @Test
+    public void givenInstanceOfArray_whenUsingArraysToStringToConvert_thenValueOfObjectsAreShown() {
+        Object[] arrayOfObjects = { "John", 2, true };
+        assertEquals(Arrays.toString(arrayOfObjects), "[John, 2, true]");
+    }
+
+    @Test
+    public void givenInstanceOfDeepArray_whenUsingArraysDeepToStringToConvert_thenValueOfInnerObjectsAreShown() {
+        Object[] innerArray = { "We", "Are", "Inside" };
+        Object[] arrayOfObjects = { "John", 2, innerArray };
+        assertEquals(Arrays.deepToString(arrayOfObjects), "[John, 2, [We, Are, Inside]]");
+    }
+
+    @Test
+    public void givenInstanceOfDeepArray_whenUsingStreamsToConvert_thenValueOfObjectsAreShown() {
+        Object[] arrayOfObjects = { "John", 2, true };
+        List<String> listOfString = Stream.of(arrayOfObjects)
+          .map(Object::toString)
+          .collect(Collectors.toList());
+        assertEquals(listOfString.toString(), "[John, 2, true]");
+    }
+}

core-java-modules/core-java-collections-3/README.md

@@ -9,3 +9,4 @@
 - [Differences Between Collection.clear() and Collection.removeAll()](https://www.baeldung.com/java-collection-clear-vs-removeall)
 - [Performance of contains() in a HashSet vs ArrayList](https://www.baeldung.com/java-hashset-arraylist-contains-performance)
 - [Fail-Safe Iterator vs Fail-Fast Iterator](https://www.baeldung.com/java-fail-safe-vs-fail-fast-iterator)
+- [Quick Guide to the Java Stack](https://www.baeldung.com/java-stack)

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/abaproblem/Account.java

@@ -0,0 +1,66 @@
+package com.baeldung.abaproblem;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class Account {
+
+    private AtomicInteger balance;
+    private AtomicInteger transactionCount;
+    private ThreadLocal<Integer> currentThreadCASFailureCount;
+
+    public Account() {
+        this.balance = new AtomicInteger(0);
+        this.transactionCount = new AtomicInteger(0);
+        this.currentThreadCASFailureCount = new ThreadLocal<>();
+        this.currentThreadCASFailureCount.set(0);
+    }
+
+    public int getBalance() {
+        return balance.get();
+    }
+
+    public int getTransactionCount() {
+        return transactionCount.get();
+    }
+
+    public int getCurrentThreadCASFailureCount() {
+        return currentThreadCASFailureCount.get();
+    }
+
+    public boolean withdraw(int amount) {
+        int current = getBalance();
+        maybeWait();
+        boolean result = balance.compareAndSet(current, current - amount);
+        if (result) {
+            transactionCount.incrementAndGet();
+        } else {
+            int currentCASFailureCount = currentThreadCASFailureCount.get();
+            currentThreadCASFailureCount.set(currentCASFailureCount + 1);
+        }
+        return result;
+    }
+
+    private void maybeWait() {
+        if ("thread1".equals(Thread.currentThread().getName())) {
+            try {
+                TimeUnit.SECONDS.sleep(2);
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+            }
+        }
+    }
+
+    public boolean deposit(int amount) {
+        int current = balance.get();
+        boolean result = balance.compareAndSet(current, current + amount);
+        if (result) {
+            transactionCount.incrementAndGet();
+        } else {
+            int currentCASFailureCount = currentThreadCASFailureCount.get();
+            currentThreadCASFailureCount.set(currentCASFailureCount + 1);
+        }
+        return result;
+    }
+
+}

core-java-modules/core-java-concurrency-advanced-3/src/test/java/com/baeldung/abaproblem/AccountUnitTest.java

@@ -0,0 +1,98 @@
+package com.baeldung.abaproblem;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class AccountUnitTest {
+
+    private Account account;
+
+    @BeforeEach
+    public void setUp() {
+        account = new Account();
+    }
+
+    @Test
+    public void zeroBalanceInitializationTest() {
+        assertEquals(0, account.getBalance());
+        assertEquals(0, account.getTransactionCount());
+        assertEquals(0, account.getCurrentThreadCASFailureCount());
+    }
+
+    @Test
+    public void depositTest() {
+        final int moneyToDeposit = 50;
+
+        assertTrue(account.deposit(moneyToDeposit));
+
+        assertEquals(moneyToDeposit, account.getBalance());
+    }
+
+    @Test
+    public void withdrawTest() throws InterruptedException {
+        final int defaultBalance = 50;
+        final int moneyToWithdraw = 20;
+
+        account.deposit(defaultBalance);
+
+        assertTrue(account.withdraw(moneyToWithdraw));
+
+        assertEquals(defaultBalance - moneyToWithdraw, account.getBalance());
+    }
+
+    @Test
+    public void abaProblemTest() throws InterruptedException {
+        final int defaultBalance = 50;
+
+        final int amountToWithdrawByThread1 = 20;
+        final int amountToWithdrawByThread2 = 10;
+        final int amountToDepositByThread2 = 10;
+
+        assertEquals(0, account.getTransactionCount());
+        assertEquals(0, account.getCurrentThreadCASFailureCount());
+        account.deposit(defaultBalance);
+        assertEquals(1, account.getTransactionCount());
+
+        Thread thread1 = new Thread(() -> {
+
+            // this will take longer due to the name of the thread
+            assertTrue(account.withdraw(amountToWithdrawByThread1));
+
+            // thread 1 fails to capture ABA problem
+            assertNotEquals(1, account.getCurrentThreadCASFailureCount());
+
+        }, "thread1");
+
+        Thread thread2 = new Thread(() -> {
+
+            assertTrue(account.deposit(amountToDepositByThread2));
+            assertEquals(defaultBalance + amountToDepositByThread2, account.getBalance());
+
+            // this will be fast due to the name of the thread
+            assertTrue(account.withdraw(amountToWithdrawByThread2));
+
+            // thread 1 didn't finish yet, so the original value will be in place for it
+            assertEquals(defaultBalance, account.getBalance());
+
+            assertEquals(0, account.getCurrentThreadCASFailureCount());
+        }, "thread2");
+
+        thread1.start();
+        thread2.start();
+        thread1.join();
+        thread2.join();
+
+        // compareAndSet operation succeeds for thread 1
+        assertEquals(defaultBalance - amountToWithdrawByThread1, account.getBalance());
+
+        //but there are other transactions
+        assertNotEquals(2, account.getTransactionCount());
+
+        // thread 2 did two modifications as well
+        assertEquals(4, account.getTransactionCount());
+    }
+}

core-java-modules/core-java-console/README.md

@@ -0,0 +1,5 @@
+#Core Java Console
+
+[Read and Write User Input in Java](http://www.baeldung.com/java-console-input-output)
+[Formatting with printf() in Java](https://www.baeldung.com/java-printstream-printf)
+[ASCII Art in Java](http://www.baeldung.com/ascii-art-in-java)

core-java-modules/core-java-console/pom.xml

@@ -0,0 +1,142 @@
+<?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-java-console</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-console</name>
+    <packaging>jar</packaging>
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../</relativePath>
+    </parent>
+
+    <build>
+        <finalName>core-java-console</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-dependency-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <id>copy-dependencies</id>
+                        <phase>prepare-package</phase>
+                        <goals>
+                            <goal>copy-dependencies</goal>
+                        </goals>
+                        <configuration>
+                            <outputDirectory>${project.build.directory}/libs</outputDirectory>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+
+            <plugin>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>exec-maven-plugin</artifactId>
+                <version>${exec-maven-plugin.version}</version>
+                <configuration>
+                    <executable>java</executable>
+                    <mainClass>com.baeldung.outofmemoryerror.OutOfMemoryGCLimitExceed</mainClass>
+                    <arguments>
+                        <argument>-Xmx300m</argument>
+                        <argument>-XX:+UseParallelGC</argument>
+                        <argument>-classpath</argument>
+                        <classpath />
+                        <argument>com.baeldung.outofmemoryerror.OutOfMemoryGCLimitExceed</argument>
+                    </arguments>
+                </configuration>
+            </plugin>
+
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-javadoc-plugin</artifactId>
+                <version>${maven-javadoc-plugin.version}</version>
+                <configuration>
+                    <source>${source.version}</source>
+                    <target>${target.version}</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <profiles>
+        <profile>
+            <id>integration</id>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-surefire-plugin</artifactId>
+                        <executions>
+                            <execution>
+                                <phase>integration-test</phase>
+                                <goals>
+                                    <goal>test</goal>
+                                </goals>
+                                <configuration>
+                                    <excludes>
+                                        <exclude>**/*ManualTest.java</exclude>
+                                    </excludes>
+                                    <includes>
+                                        <include>**/*IntegrationTest.java</include>
+                                        <include>**/*IntTest.java</include>
+                                    </includes>
+                                </configuration>
+                            </execution>
+                        </executions>
+                        <configuration>
+                            <systemPropertyVariables>
+                                <test.mime>json</test.mime>
+                            </systemPropertyVariables>
+                        </configuration>
+                    </plugin>
+                    <plugin>
+                        <groupId>org.codehaus.mojo</groupId>
+                        <artifactId>exec-maven-plugin</artifactId>
+                        <version>${exec-maven-plugin.version}</version>
+                        <executions>
+                            <execution>
+                                <id>run-benchmarks</id>
+                                <!-- <phase>integration-test</phase> -->
+                                <phase>none</phase>
+                                <goals>
+                                    <goal>exec</goal>
+                                </goals>
+                                <configuration>
+                                    <classpathScope>test</classpathScope>
+                                    <executable>java</executable>
+                                    <arguments>
+                                        <argument>-classpath</argument>
+                                        <classpath />
+                                        <argument>org.openjdk.jmh.Main</argument>
+                                        <argument>.*</argument>
+                                    </arguments>
+                                </configuration>
+                            </execution>
+                        </executions>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
+    </profiles>
+
+    <properties>
+        <maven-javadoc-plugin.version>3.0.0-M1</maven-javadoc-plugin.version>
+        <exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
+        <source.version>1.8</source.version>
+        <target.version>1.8</target.version>
+    </properties>
+
+</project>

core-java-modules/core-java-console/src/test/java/com/baeldung/asciiart/AsciiArtIntegrationTest.java

@@ -1,10 +1,9 @@
 package com.baeldung.asciiart;
 
-import java.awt.Font;
-
+import com.baeldung.asciiart.AsciiArt.Settings;
 import org.junit.Test;
 
-import com.baeldung.asciiart.AsciiArt.Settings;
+import java.awt.*;
 
 public class AsciiArtIntegrationTest {
 
@@ -16,5 +15,4 @@ public class AsciiArtIntegrationTest {
         
         asciiArt.drawString(text, "*", settings);
     }
-    
 }

core-java-modules/core-java-date-operations-2/README.md

@@ -8,4 +8,5 @@ This module contains articles about date operations in Java.
 - [Converting Java Date to OffsetDateTime](https://www.baeldung.com/java-convert-date-to-offsetdatetime)
 - [How to Set the JVM Time Zone](https://www.baeldung.com/java-jvm-time-zone)
 - [How to determine day of week by passing specific date in Java?](https://www.baeldung.com/java-get-day-of-week)
+- [Finding Leap Years in Java](https://www.baeldung.com/java-leap-year)
 - [[<-- Prev]](/core-java-modules/core-java-date-operations-1)

core-java-modules/core-java-jvm/README.md

@@ -12,3 +12,4 @@ This module contains articles about working with the Java Virtual Machine (JVM).
 - [Guide to System.gc()](https://www.baeldung.com/java-system-gc)
 - [Runtime.getRuntime().halt() vs System.exit() in Java](https://www.baeldung.com/java-runtime-halt-vs-system-exit)
 - [Adding Shutdown Hooks for JVM Applications](https://www.baeldung.com/jvm-shutdown-hooks)
+- [How to Get the Size of an Object in Java](http://www.baeldung.com/java-size-of-object)

core-java-modules/core-java-lang-oop-types/README.md

@@ -7,3 +7,6 @@ This module contains articles about types in Java
 - [Guide to the this Java Keyword](https://www.baeldung.com/java-this)
 - [Nested Classes in Java](https://www.baeldung.com/java-nested-classes)
 - [Marker Interfaces in Java](https://www.baeldung.com/java-marker-interfaces)
+- [Iterating Over Enum Values in Java](https://www.baeldung.com/java-enum-iteration)
+- [Attaching Values to Java Enum](https://www.baeldung.com/java-enum-values)
+- [A Guide to Java Enums](https://www.baeldung.com/a-guide-to-java-enums)

core-java-modules/core-java-lang-syntax/src/test/java/com/baeldung/varargs/HeapPollutionUnitTest.java

@@ -0,0 +1,40 @@
+package com.baeldung.varargs;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+
+public class HeapPollutionUnitTest {
+
+    @Test(expected = ClassCastException.class)
+    public void givenGenericVararg_whenUsedUnsafe_shouldThrowClassCastException() {
+        String one = firstOfFirst(Arrays.asList("one", "two"), Collections.emptyList());
+
+        assertEquals("one", one);
+    }
+
+    @Test(expected = ClassCastException.class)
+    public void givenGenericVararg_whenRefEscapes_mayCauseSubtleBugs() {
+        String[] args = returnAsIs("One", "Two");
+    }
+
+    private static String firstOfFirst(List<String>... strings) {
+        List<Integer> ints = Collections.singletonList(42);
+        Object[] objects = strings;
+        objects[0] = ints;
+
+        return strings[0].get(0);
+    }
+
+    private static <T> T[] toArray(T... arguments) {
+        return arguments;
+    }
+
+    private static <T> T[] returnAsIs(T a, T b) {
+        return toArray(a, b);
+    }
+}

core-java-modules/core-java-lang/README.md

@@ -4,7 +4,6 @@ This module contains articles about core features in the Java language
 
 ### Relevant Articles: 
 - [Generate equals() and hashCode() with Eclipse](https://www.baeldung.com/java-eclipse-equals-and-hashcode)
-- [Iterating Over Enum Values in Java](https://www.baeldung.com/java-enum-iteration)
 - [Comparator and Comparable in Java](https://www.baeldung.com/java-comparator-comparable)
 - [Recursion In Java](https://www.baeldung.com/java-recursion)
 - [A Guide to the finalize Method in Java](https://www.baeldung.com/java-finalize)
@@ -12,8 +11,6 @@ This module contains articles about core features in the Java language
 - [Using Java Assertions](https://www.baeldung.com/java-assert)
 - [Synthetic Constructs in Java](https://www.baeldung.com/java-synthetic)
 - [Retrieving a Class Name in Java](https://www.baeldung.com/java-class-name)
-- [Attaching Values to Java Enum](https://www.baeldung.com/java-enum-values)
 - [The Java continue and break Keywords](https://www.baeldung.com/java-continue-and-break)
-- [A Guide to Java Enums](https://www.baeldung.com/a-guide-to-java-enums)
 - [Infinite Loops in Java](https://www.baeldung.com/infinite-loops-java)
 - [[More --> ]](/core-java-modules/core-java-lang-2)

core-java-modules/core-java-reflection/README.MD

@@ -8,3 +8,5 @@
 - [Changing Annotation Parameters At Runtime](http://www.baeldung.com/java-reflection-change-annotation-params)
 - [Dynamic Proxies in Java](http://www.baeldung.com/java-dynamic-proxies)
 - [What Causes java.lang.reflect.InvocationTargetException?](https://www.baeldung.com/java-lang-reflect-invocationtargetexception)
+- [How to Find all Getters Returning Null](http://www.baeldung.com/java-getters-returning-null)
+- [How to Get a Name of a Method Being Executed?](http://www.baeldung.com/java-name-of-executing-method)

core-java-modules/core-java-reflection/src/test/java/com/baeldung/reflection/java/reflection/ReflectionUnitTest.java

@@ -200,7 +200,7 @@ public class ReflectionUnitTest {
     @Test
     public void givenClassField_whenSetsAndGetsValue_thenCorrect() throws Exception {
         final Class<?> birdClass = Class.forName("com.baeldung.java.reflection.Bird");
-        final Bird bird = (Bird) birdClass.newInstance();
+        final Bird bird = (Bird) birdClass.getConstructor().newInstance();
         final Field field = birdClass.getDeclaredField("walks");
         field.setAccessible(true);
 
@@ -266,7 +266,7 @@ public class ReflectionUnitTest {
     @Test
     public void givenMethod_whenInvokes_thenCorrect() throws Exception {
         final Class<?> birdClass = Class.forName("com.baeldung.java.reflection.Bird");
-        final Bird bird = (Bird) birdClass.newInstance();
+        final Bird bird = (Bird) birdClass.getConstructor().newInstance();
         final Method setWalksMethod = birdClass.getDeclaredMethod("setWalks", boolean.class);
         final Method walksMethod = birdClass.getDeclaredMethod("walks");
         final boolean walks = (boolean) walksMethod.invoke(bird);

core-java-modules/core-java-string-conversions/src/test/java/com/baeldung/stringtoint/StringToIntOrIntegerUnitTest.java

@@ -26,6 +26,17 @@ public class StringToIntOrIntegerUnitTest {
         assertThat(result).isEqualTo(new Integer(42));
     }
 
+    @Test
+    public void givenString_whenCallingValueOf_shouldCacheSomeValues() {
+        for (int i = -128; i <= 127; i++) {
+            String value = i + "";
+            Integer first = Integer.valueOf(value);
+            Integer second = Integer.valueOf(value);
+
+            assertThat(first).isSameAs(second);
+        }
+    }
+
     @Test
     public void givenString_whenCallingIntegerConstructor_shouldConvertToInt() {
         String givenString = "42";

core-java-modules/core-java-string-operations-2/README.md

@@ -12,4 +12,6 @@ This module contains articles about string operations.
 - [L-Trim and R-Trim Alternatives in Java](https://www.baeldung.com/java-trim-alternatives)
 - [Java Convert PDF to Base64](https://www.baeldung.com/java-convert-pdf-to-base64)
 - [Encode a String to UTF-8 in Java](https://www.baeldung.com/java-string-encode-utf-8)
+- [Guide to Character Encoding](https://www.baeldung.com/java-char-encoding)
+- [Convert Hex to ASCII in Java](https://www.baeldung.com/java-convert-hex-to-ascii) #remove additional readme file
 - More articles: [[<-- prev]](../core-java-string-operations)

core-java-modules/core-java-time-measurements/README.md

@@ -6,3 +6,4 @@ This module contains articles about the measurement of time in Java.
 - [Guide to the Java Clock Class](http://www.baeldung.com/java-clock)
 - [Measure Elapsed Time in Java](http://www.baeldung.com/java-measure-elapsed-time)
 - [Overriding System Time for Testing in Java](https://www.baeldung.com/java-override-system-time)
+- [Java Timer](http://www.baeldung.com/java-timer-and-timertask)

core-java-modules/core-java-time-measurements/src/test/java/com/baeldung/clock/ClockUnitTest.java

@@ -1,4 +1,4 @@
-package com.baeldung.java.clock;
+package com.baeldung.clock;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;

core-java-modules/core-java/README.md

@@ -1,35 +1,13 @@
 ## Core Java Cookbooks and Examples
 
 ### Relevant Articles: 
-- [Java Timer](http://www.baeldung.com/java-timer-and-timertask)
-- [Getting Started with Java Properties](http://www.baeldung.com/java-properties)
-- [Introduction to Nashorn](http://www.baeldung.com/java-nashorn)
-- [Java Money and the Currency API](http://www.baeldung.com/java-money-and-currency)
-- [JVM Log Forging](http://www.baeldung.com/jvm-log-forging)
-- [How to Find all Getters Returning Null](http://www.baeldung.com/java-getters-returning-null)
-- [How to Get a Name of a Method Being Executed?](http://www.baeldung.com/java-name-of-executing-method)
-- [Introduction to Java Serialization](http://www.baeldung.com/java-serialization)
-- [Guide to UUID in Java](http://www.baeldung.com/java-uuid)
-- [Creating a Java Compiler Plugin](http://www.baeldung.com/java-build-compiler-plugin)
-- [Quick Guide to the Java Stack](https://www.baeldung.com/java-stack)
-- [Compiling Java *.class Files with javac](http://www.baeldung.com/javac)
-- [Introduction to Javadoc](http://www.baeldung.com/javadoc)
-- [Guide to the Externalizable Interface in Java](http://www.baeldung.com/java-externalizable)
-- [ASCII Art in Java](http://www.baeldung.com/ascii-art-in-java)
-- [What is the serialVersionUID?](http://www.baeldung.com/java-serial-version-uid)
-- [A Guide to the ResourceBundle](http://www.baeldung.com/java-resourcebundle)
-- [Java Global Exception Handler](http://www.baeldung.com/java-global-exception-handler)
-- [How to Get the Size of an Object in Java](http://www.baeldung.com/java-size-of-object)
-- [Common Java Exceptions](http://www.baeldung.com/java-common-exceptions)
-- [Merging java.util.Properties Objects](https://www.baeldung.com/java-merging-properties)
-- [Java – Try with Resources](https://www.baeldung.com/java-try-with-resources)
-- [Guide to Character Encoding](https://www.baeldung.com/java-char-encoding)
-- [Graphs in Java](https://www.baeldung.com/java-graphs)
-- [Read and Write User Input in Java](http://www.baeldung.com/java-console-input-output)
-- [Formatting with printf() in Java](https://www.baeldung.com/java-printstream-printf)
-- [Retrieve Fields from a Java Class Using Reflection](https://www.baeldung.com/java-reflection-class-fields)
-- [Using Curl in Java](https://www.baeldung.com/java-curl)
-- [Finding Leap Years in Java](https://www.baeldung.com/java-leap-year)
-- [Making a JSON POST Request With HttpURLConnection](https://www.baeldung.com/httpurlconnection-post)
-- [How to Find an Exception’s Root Cause in Java](https://www.baeldung.com/java-exception-root-cause)
-- [Convert Hex to ASCII in Java](https://www.baeldung.com/java-convert-hex-to-ascii)
+[Getting Started with Java Properties](http://www.baeldung.com/java-properties)
+[Java Money and the Currency API](http://www.baeldung.com/java-money-and-currency)
+[Introduction to Java Serialization](http://www.baeldung.com/java-serialization)
+[Guide to UUID in Java](http://www.baeldung.com/java-uuid)
+[Compiling Java *.class Files with javac](http://www.baeldung.com/javac)
+[Introduction to Javadoc](http://www.baeldung.com/javadoc)
+[Guide to the Externalizable Interface in Java](http://www.baeldung.com/java-externalizable)
+[What is the serialVersionUID?](http://www.baeldung.com/java-serial-version-uid)
+[A Guide to the ResourceBundle](http://www.baeldung.com/java-resourcebundle)
+[Merging java.util.Properties Objects](https://www.baeldung.com/java-merging-properties)

core-java-modules/core-java/src/main/java/com/baeldung/uuid/UUIDGenerator.java

@@ -3,30 +3,57 @@ package com.baeldung.uuid;
 import java.io.UnsupportedEncodingException;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
+import java.time.Duration;
+import java.time.LocalDateTime;
 import java.util.Arrays;
+import java.util.Random;
 import java.util.UUID;
 
 public class UUIDGenerator {
 
-    /**
-     * These are predefined UUID for name spaces
-     */
-    private static final String NAMESPACE_DNS = "6ba7b810-9dad-11d1-80b4-00c04fd430c8";
-    private static final String NAMESPACE_URL = "6ba7b811-9dad-11d1-80b4-00c04fd430c8";
-    private static final String NAMESPACE_OID = "6ba7b812-9dad-11d1-80b4-00c04fd430c8";
-    private static final String NAMESPACE_X500 = "6ba7b814-9dad-11d1-80b4-00c04fd430c8";
-
     private static final char[] hexArray = "0123456789ABCDEF".toCharArray();
 
-    public static void main(String[] args) {
-        try {
-            System.out.println("Type 3 : " + generateType3UUID(NAMESPACE_DNS, "google.com"));
-            System.out.println("Type 4 : " + generateType4UUID());
-            System.out.println("Type 5 : " + generateType5UUID(NAMESPACE_URL, "google.com"));
-            System.out.println("Unique key  : " + generateUniqueKeysWithUUIDAndMessageDigest());
-        } catch (NoSuchAlgorithmException | UnsupportedEncodingException e) {
-            e.printStackTrace();
-        }
+    /**
+     * Type 1 UUID Generation
+     */
+    public static UUID generateType1UUID() {
+
+        long most64SigBits = get64MostSignificantBitsForVersion1();
+        long least64SigBits = get64LeastSignificantBitsForVersion1();
+
+        return new UUID(most64SigBits, least64SigBits);
+    }
+
+    private static long get64LeastSignificantBitsForVersion1() {
+        Random random = new Random();
+        long random63BitLong = random.nextLong() & 0x3FFFFFFFFFFFFFFFL;
+        long variant3BitFlag = 0x8000000000000000L;
+        return random63BitLong + variant3BitFlag;
+    }
+
+    private static long get64MostSignificantBitsForVersion1() {
+        LocalDateTime start = LocalDateTime.of(1582, 10, 15, 0, 0, 0);
+        Duration duration = Duration.between(start, LocalDateTime.now());
+        long seconds = duration.getSeconds();
+        long nanos = duration.getNano();
+        long timeForUuidIn100Nanos = seconds * 10000000 + nanos * 100;
+        long least12SignificatBitOfTime = (timeForUuidIn100Nanos & 0x000000000000FFFFL) >> 4;
+        long version = 1 << 12;
+        return (timeForUuidIn100Nanos & 0xFFFFFFFFFFFF0000L) + version + least12SignificatBitOfTime;
+    }
+
+    /**
+     * Type 3 UUID Generation
+     *
+     * @throws UnsupportedEncodingException
+     */
+    public static UUID generateType3UUID(String namespace, String name) throws UnsupportedEncodingException {
+
+        byte[] nameSpaceBytes = bytesFromUUID(namespace);
+        byte[] nameBytes = name.getBytes("UTF-8");
+        byte[] result = joinBytes(nameSpaceBytes, nameBytes);
+
+        return UUID.nameUUIDFromBytes(result);
     }
 
     /**
@@ -37,28 +64,18 @@ public class UUIDGenerator {
         return uuid;
     }
 
-    /**
-     * Type 3 UUID Generation
-     * 
-     * @throws UnsupportedEncodingException 
-     */
-    public static UUID generateType3UUID(String namespace, String name) throws UnsupportedEncodingException {
-        String source = namespace + name;
-        byte[] bytes = source.getBytes("UTF-8");
-        UUID uuid = UUID.nameUUIDFromBytes(bytes);
-        return uuid;
-    }
-
     /**
      * Type 5 UUID Generation
-     * 
-     * @throws UnsupportedEncodingException 
+     *
+     * @throws UnsupportedEncodingException
      */
     public static UUID generateType5UUID(String namespace, String name) throws UnsupportedEncodingException {
-        String source = namespace + name;
-        byte[] bytes = source.getBytes("UTF-8");
-        UUID uuid = type5UUIDFromBytes(bytes);
-        return uuid;
+
+        byte[] nameSpaceBytes = bytesFromUUID(namespace);
+        byte[] nameBytes = name.getBytes("UTF-8");
+        byte[] result = joinBytes(nameSpaceBytes, nameBytes);
+
+        return type5UUIDFromBytes(result);
     }
 
     public static UUID type5UUIDFromBytes(byte[] name) {
@@ -91,20 +108,20 @@ public class UUIDGenerator {
 
     /**
      * Unique Keys Generation Using Message Digest and Type 4 UUID
-     * 
-     * @throws NoSuchAlgorithmException 
-     * @throws UnsupportedEncodingException 
+     *
+     * @throws NoSuchAlgorithmException
+     * @throws UnsupportedEncodingException
      */
     public static String generateUniqueKeysWithUUIDAndMessageDigest() throws NoSuchAlgorithmException, UnsupportedEncodingException {
         MessageDigest salt = MessageDigest.getInstance("SHA-256");
         salt.update(UUID.randomUUID()
-            .toString()
-            .getBytes("UTF-8"));
+                .toString()
+                .getBytes("UTF-8"));
         String digest = bytesToHex(salt.digest());
         return digest;
     }
 
-    public static String bytesToHex(byte[] bytes) {
+    private static String bytesToHex(byte[] bytes) {
         char[] hexChars = new char[bytes.length * 2];
         for (int j = 0; j < bytes.length; j++) {
             int v = bytes[j] & 0xFF;
@@ -114,4 +131,37 @@ public class UUIDGenerator {
         return new String(hexChars);
     }
 
-}
+    private static byte[] bytesFromUUID(String uuidHexString) {
+        String normalizedUUIDHexString = uuidHexString.replace("-","");
+
+        assert normalizedUUIDHexString.length() == 32;
+
+        byte[] bytes = new byte[16];
+        for (int i = 0; i < 16; i++) {
+            byte b = hexToByte(normalizedUUIDHexString.substring(i*2, i*2+2));
+            bytes[i] = b;
+        }
+        return bytes;
+    }
+
+    public static byte hexToByte(String hexString) {
+        int firstDigit = Character.digit(hexString.charAt(0),16);
+        int secondDigit = Character.digit(hexString.charAt(1),16);
+        return (byte) ((firstDigit << 4) + secondDigit);
+    }
+
+    public static byte[] joinBytes(byte[] byteArray1, byte[] byteArray2) {
+        int finalLength = byteArray1.length + byteArray2.length;
+        byte[] result = new byte[finalLength];
+
+        for(int i = 0; i < byteArray1.length; i++) {
+            result[i] = byteArray1[i];
+        }
+
+        for(int i = 0; i < byteArray2.length; i++) {
+            result[byteArray1.length+i] = byteArray2[i];
+        }
+
+        return result;
+    }
+}

core-java-modules/core-java/src/test/java/com/baeldung/uuid/UUIDGeneratorUnitTest.java

@@ -0,0 +1,64 @@
+package com.baeldung.uuid;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.UnsupportedEncodingException;
+import java.util.UUID;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class UUIDGeneratorUnitTest {
+
+    private static final String NAMESPACE_URL = "6ba7b811-9dad-11d1-80b4-00c04fd430c8";
+    private static final String NAMESPACE_DNS = "6ba7b810-9dad-11d1-80b4-00c04fd430c8";
+
+    @Test
+    public void version_1_UUID_is_generated_with_correct_length_version_and_variant() {
+
+        UUID uuid = UUIDGenerator.generateType1UUID();
+
+        assertEquals(36, uuid.toString().length());
+        assertEquals(1, uuid.version());
+        assertEquals(2, uuid.variant());
+    }
+
+    @Test
+    public void version_3_UUID_is_correctly_generated_for_domain_baeldung_com() throws UnsupportedEncodingException {
+
+        UUID uuid = UUIDGenerator.generateType3UUID(NAMESPACE_DNS, "baeldung.com");
+
+        assertEquals("23785b78-0132-3ac6-aff6-cfd5be162139", uuid.toString());
+        assertEquals(3, uuid.version());
+        assertEquals(2, uuid.variant());
+    }
+
+    @Test
+    public void version_3_UUID_is_correctly_generated_for_domain_d() throws UnsupportedEncodingException {
+
+        UUID uuid = UUIDGenerator.generateType3UUID(NAMESPACE_DNS, "d");
+
+        assertEquals("dbd41ecb-f466-33de-b309-1468addfc63b", uuid.toString());
+        assertEquals(3, uuid.version());
+        assertEquals(2, uuid.variant());
+    }
+
+    @Test
+    public void version_4_UUID_is_generated_with_correct_length_version_and_variant() {
+
+        UUID uuid = UUIDGenerator.generateType4UUID();
+
+        assertEquals(36, uuid.toString().length());
+        assertEquals(4, uuid.version());
+        assertEquals(2, uuid.variant());
+    }
+
+    @Test
+    public void version_5_UUID_is_correctly_generated_for_domain_baeldung_com() throws UnsupportedEncodingException {
+
+        UUID uuid = UUIDGenerator.generateType5UUID(NAMESPACE_URL, "baeldung.com");
+
+        assertEquals("aeff44a5-8a61-52b6-bcbe-c8e5bd7d0300", uuid.toString());
+        assertEquals(5, uuid.version());
+        assertEquals(2, uuid.variant());
+    }
+}

core-kotlin-modules/core-kotlin-2/README.md

@@ -1,9 +0,0 @@
-## Core Kotlin 2
-
-This module contains articles about Kotlin core features.
-
-### Relevant articles:
-- [Working with Dates in Kotlin](https://www.baeldung.com/kotlin-dates)
-- [Kotlin Ternary Conditional Operator](https://www.baeldung.com/kotlin-ternary-operator)
-- [Sequences in Kotlin](https://www.baeldung.com/kotlin/sequences)
-- [[<-- Prev]](/core-kotlin-modules/core-kotlin)

core-kotlin-modules/core-kotlin-collections/README.md

@@ -10,3 +10,4 @@ This module contains articles about core Kotlin collections.
 - [Filtering Kotlin Collections](https://www.baeldung.com/kotlin-filter-collection)
 - [Collection Transformations in Kotlin](https://www.baeldung.com/kotlin-collection-transformations)
 - [Difference between fold and reduce in Kotlin](https://www.baeldung.com/kotlin/fold-vs-reduce)
+- [Guide to Sorting in Kotlin](https://www.baeldung.com/kotlin-sort)

core-kotlin-modules/core-kotlin-datastructures/README.md

@@ -0,0 +1,6 @@
+## Core Kotlin
+
+This module contains articles about data structures in Kotlin
+
+### Relevant articles:
+[Implementing a Binary Tree in Kotlin](https://www.baeldung.com/kotlin-binary-tree)

core-kotlin-modules/core-kotlin-datastructures/pom.xml

@@ -0,0 +1,29 @@
+<?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-kotlin-datastructures</artifactId>
+    <name>core-kotlin-datastructures</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-kotlin-modules</groupId>
+        <artifactId>core-kotlin-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit.platform.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <junit.platform.version>1.1.1</junit.platform.version>
+    </properties>
+
+</project>

core-kotlin-modules/core-kotlin-datastructures/src/test/kotlin/com/binarytree/NodeTest.kt

@@ -1,4 +1,4 @@
-package com.baeldung.binarytree
+package com.binarytree
 
 import org.junit.After
 import org.junit.Assert.assertEquals

core-kotlin-modules/core-kotlin-date-time/README.md

@@ -0,0 +1,6 @@
+## Core Kotlin Date and Time
+
+This module contains articles about Kotlin core date/time features.
+
+### Relevant articles:
+[Working with Dates in Kotlin](https://www.baeldung.com/kotlin-dates)

core-kotlin-modules/core-kotlin-date-time/pom.xml

@@ -0,0 +1,36 @@
+<?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-kotlin-date-time</artifactId>
+    <name>core-kotlin-date-time</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-kotlin-modules</groupId>
+        <artifactId>core-kotlin-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${org.assertj.core.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit.platform.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <junit.platform.version>1.1.1</junit.platform.version>
+        <org.assertj.core.version>3.9.0</org.assertj.core.version>
+    </properties>
+
+</project>

core-kotlin-modules/core-kotlin-design-patterns/README.md

@@ -0,0 +1,6 @@
+## Core Kotlin Design Patterns
+
+This module contains articles about design patterns in Kotlin
+
+### Relevant articles:
+- [Creational Design Patterns in Kotlin: Builder](https://www.baeldung.com/kotlin-builder-pattern)

core-kotlin-modules/core-kotlin-design-patterns/pom.xml

@@ -0,0 +1,29 @@
+<?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-kotlin-design-patterns</artifactId>
+    <name>core-kotlin-design-patterns</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-kotlin-modules</groupId>
+        <artifactId>core-kotlin-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit.platform.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <junit.platform.version>1.1.1</junit.platform.version>
+    </properties>
+
+</project>