Merge branch 'master' into BAEL-1559

.gitignore

@@ -43,3 +43,4 @@ spring-call-getters-using-reflection/.mvn/wrapper/maven-wrapper.properties
 spring-check-if-a-property-is-null/.mvn/wrapper/maven-wrapper.properties
 *.springBeans
 
+20171220-JMeter.csv

algorithms/src/main/java/com/baeldung/algorithms/maze/solver/BFSMazeSolver.java

@@ -0,0 +1,52 @@
+package com.baeldung.algorithms.maze.solver;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedList;
+import java.util.List;
+
+public class BFSMazeSolver {
+    private static final int[][] DIRECTIONS = { { 0, 1 }, { 1, 0 }, { 0, -1 }, { -1, 0 } };
+
+    public List<Coordinate> solve(Maze maze) {
+        LinkedList<Coordinate> nextToVisit = new LinkedList<>();
+        Coordinate start = maze.getEntry();
+        nextToVisit.add(start);
+
+        while (!nextToVisit.isEmpty()) {
+            Coordinate cur = nextToVisit.remove();
+
+            if (!maze.isValidLocation(cur.getX(), cur.getY()) || maze.isExplored(cur.getX(), cur.getY())) {
+                continue;
+            }
+
+            if (maze.isWall(cur.getX(), cur.getY())) {
+                maze.setVisited(cur.getX(), cur.getY(), true);
+                continue;
+            }
+
+            if (maze.isExit(cur.getX(), cur.getY())) {
+                return backtrackPath(cur);
+            }
+
+            for (int[] direction : DIRECTIONS) {
+                Coordinate coordinate = new Coordinate(cur.getX() + direction[0], cur.getY() + direction[1], cur);
+                nextToVisit.add(coordinate);
+                maze.setVisited(cur.getX(), cur.getY(), true);
+            }
+        }
+        return Collections.emptyList();
+    }
+
+    private List<Coordinate> backtrackPath(Coordinate cur) {
+        List<Coordinate> path = new ArrayList<>();
+        Coordinate iter = cur;
+
+        while (iter != null) {
+            path.add(iter);
+            iter = iter.parent;
+        }
+
+        return path;
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/maze/solver/Coordinate.java

@@ -0,0 +1,31 @@
+package com.baeldung.algorithms.maze.solver;
+
+public class Coordinate {
+    int x;
+    int y;
+    Coordinate parent;
+
+    public Coordinate(int x, int y) {
+        this.x = x;
+        this.y = y;
+        this.parent = null;
+    }
+
+    public Coordinate(int x, int y, Coordinate parent) {
+        this.x = x;
+        this.y = y;
+        this.parent = parent;
+    }
+
+    int getX() {
+        return x;
+    }
+
+    int getY() {
+        return y;
+    }
+
+    Coordinate getParent() {
+        return parent;
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/maze/solver/DFSMazeSolver.java

@@ -0,0 +1,48 @@
+package com.baeldung.algorithms.maze.solver;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class DFSMazeSolver {
+    private static final int[][] DIRECTIONS = { { 0, 1 }, { 1, 0 }, { 0, -1 }, { -1, 0 } };
+
+    public List<Coordinate> solve(Maze maze) {
+        List<Coordinate> path = new ArrayList<>();
+        if (explore(maze, maze.getEntry()
+            .getX(),
+            maze.getEntry()
+                .getY(),
+            path)) {
+            return path;
+        }
+        return Collections.emptyList();
+    }
+
+    private boolean explore(Maze maze, int row, int col, List<Coordinate> path) {
+        if (!maze.isValidLocation(row, col) || maze.isWall(row, col) || maze.isExplored(row, col)) {
+            return false;
+        }
+
+        path.add(new Coordinate(row, col));
+        maze.setVisited(row, col, true);
+
+        if (maze.isExit(row, col)) {
+            return true;
+        }
+
+        for (int[] direction : DIRECTIONS) {
+            Coordinate coordinate = getNextCoordinate(row, col, direction[0], direction[1]);
+            if (explore(maze, coordinate.getX(), coordinate.getY(), path)) {
+                return true;
+            }
+        }
+
+        path.remove(path.size() - 1);
+        return false;
+    }
+
+    private Coordinate getNextCoordinate(int row, int col, int i, int j) {
+        return new Coordinate(row + i, col + j);
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/maze/solver/Maze.java

@@ -0,0 +1,141 @@
+package com.baeldung.algorithms.maze.solver;
+
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Scanner;
+
+public class Maze {
+    private static final int ROAD = 0;
+    private static final int WALL = 1;
+    private static final int START = 2;
+    private static final int EXIT = 3;
+    private static final int PATH = 4;
+
+    private int[][] maze;
+    private boolean[][] visited;
+    private Coordinate start;
+    private Coordinate end;
+
+    public Maze(File maze) throws FileNotFoundException {
+        String fileText = "";
+        try (Scanner input = new Scanner(maze)) {
+            while (input.hasNextLine()) {
+                fileText += input.nextLine() + "\n";
+            }
+        }
+        initializeMaze(fileText);
+    }
+
+    private void initializeMaze(String text) {
+        if (text == null || (text = text.trim()).length() == 0) {
+            throw new IllegalArgumentException("empty lines data");
+        }
+
+        String[] lines = text.split("[\r]?\n");
+        maze = new int[lines.length][lines[0].length()];
+        visited = new boolean[lines.length][lines[0].length()];
+
+        for (int row = 0; row < getHeight(); row++) {
+            if (lines[row].length() != getWidth()) {
+                throw new IllegalArgumentException("line " + (row + 1) + " wrong length (was " + lines[row].length() + " but should be " + getWidth() + ")");
+            }
+
+            for (int col = 0; col < getWidth(); col++) {
+                if (lines[row].charAt(col) == '#')
+                    maze[row][col] = WALL;
+                else if (lines[row].charAt(col) == 'S') {
+                    maze[row][col] = START;
+                    start = new Coordinate(row, col);
+                } else if (lines[row].charAt(col) == 'E') {
+                    maze[row][col] = EXIT;
+                    end = new Coordinate(row, col);
+                } else
+                    maze[row][col] = ROAD;
+            }
+        }
+    }
+
+    public int getHeight() {
+        return maze.length;
+    }
+
+    public int getWidth() {
+        return maze[0].length;
+    }
+
+    public Coordinate getEntry() {
+        return start;
+    }
+
+    public Coordinate getExit() {
+        return end;
+    }
+
+    public boolean isExit(int x, int y) {
+        return x == end.getX() && y == end.getY();
+    }
+
+    public boolean isStart(int x, int y) {
+        return x == start.getX() && y == start.getY();
+    }
+
+    public boolean isExplored(int row, int col) {
+        return visited[row][col];
+    }
+
+    public boolean isWall(int row, int col) {
+        return maze[row][col] == WALL;
+    }
+
+    public void setVisited(int row, int col, boolean value) {
+        visited[row][col] = value;
+    }
+
+    public boolean isValidLocation(int row, int col) {
+        if (row < 0 || row >= getHeight() || col < 0 || col >= getWidth()) {
+            return false;
+        }
+        return true;
+    }
+
+    public void printPath(List<Coordinate> path) {
+        int[][] tempMaze = Arrays.stream(maze)
+            .map(int[]::clone)
+            .toArray(int[][]::new);
+        for (Coordinate coordinate : path) {
+            if (isStart(coordinate.getX(), coordinate.getY()) || isExit(coordinate.getX(), coordinate.getY())) {
+                continue;
+            }
+            tempMaze[coordinate.getX()][coordinate.getY()] = PATH;
+        }
+        System.out.println(toString(tempMaze));
+    }
+
+    public String toString(int[][] maze) {
+        StringBuilder result = new StringBuilder(getWidth() * (getHeight() + 1));
+        for (int row = 0; row < getHeight(); row++) {
+            for (int col = 0; col < getWidth(); col++) {
+                if (maze[row][col] == ROAD) {
+                    result.append(' ');
+                } else if (maze[row][col] == WALL) {
+                    result.append('#');
+                } else if (maze[row][col] == START) {
+                    result.append('S');
+                } else if (maze[row][col] == EXIT) {
+                    result.append('E');
+                } else {
+                    result.append('.');
+                }
+            }
+            result.append('\n');
+        }
+        return result.toString();
+    }
+
+    public void reset() {
+        for (int i = 0; i < visited.length; i++)
+            Arrays.fill(visited[i], false);
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/maze/solver/MazeDriver.java

@@ -0,0 +1,34 @@
+package com.baeldung.algorithms.maze.solver;
+
+import java.io.File;
+import java.util.List;
+
+public class MazeDriver {
+    public static void main(String[] args) throws Exception {
+        File maze1 = new File("src/main/resources/maze/maze1.txt");
+        File maze2 = new File("src/main/resources/maze/maze2.txt");
+
+        execute(maze1);
+        execute(maze2);
+    }
+
+    private static void execute(File file) throws Exception {
+        Maze maze = new Maze(file);
+        dfs(maze);
+        bfs(maze);
+    }
+
+    private static void bfs(Maze maze) {
+        BFSMazeSolver bfs = new BFSMazeSolver();
+        List<Coordinate> path = bfs.solve(maze);
+        maze.printPath(path);
+        maze.reset();
+    }
+
+    private static void dfs(Maze maze) {
+        DFSMazeSolver dfs = new DFSMazeSolver();
+        List<Coordinate> path = dfs.solve(maze);
+        maze.printPath(path);
+        maze.reset();
+    }
+}

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() {
+	}
+
+}

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);
+
+}

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 + "]";
+	}
+
+}

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 + "]";
+	}
+
+}

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 + "]";
+	}
+
+}

algorithms/src/main/java/com/baeldung/algorithms/sudoku/BacktrackingAlgorithm.java

@@ -0,0 +1,104 @@
+package com.baeldung.algorithms.sudoku;
+
+import java.util.stream.IntStream;
+
+public class BacktrackingAlgorithm {
+
+    private static final int BOARD_SIZE = 9;
+    private static final int SUBSECTION_SIZE = 3;
+    private static final int BOARD_START_INDEX = 0;
+
+    private static final int NO_VALUE = 0;
+    private static final int MIN_VALUE = 1;
+    private static final int MAX_VALUE = 9;
+
+    private static int[][] board = {
+      {8, 0, 0, 0, 0, 0, 0, 0, 0},
+      {0, 0, 3, 6, 0, 0, 0, 0, 0},
+      {0, 7, 0, 0, 9, 0, 2, 0, 0},
+      {0, 5, 0, 0, 0, 7, 0, 0, 0},
+      {0, 0, 0, 0, 4, 5, 7, 0, 0},
+      {0, 0, 0, 1, 0, 0, 0, 3, 0},
+      {0, 0, 1, 0, 0, 0, 0, 6, 8},
+      {0, 0, 8, 5, 0, 0, 0, 1, 0},
+      {0, 9, 0, 0, 0, 0, 4, 0, 0}
+    };
+
+    public static void main(String[] args) {
+        BacktrackingAlgorithm solver = new BacktrackingAlgorithm();
+        solver.solve(board);
+        solver.printBoard();
+    }
+
+    private void printBoard() {
+        for (int row = BOARD_START_INDEX; row < BOARD_SIZE; row++) {
+            for (int column = BOARD_START_INDEX; column < BOARD_SIZE; column++) {
+                System.out.print(board[row][column] + " ");
+            }
+            System.out.println();
+        }
+    }
+
+    private boolean solve(int[][] board) {
+        for (int row = BOARD_START_INDEX; row < BOARD_SIZE; row++) {
+            for (int column = BOARD_START_INDEX; column < BOARD_SIZE; column++) {
+                if (board[row][column] == NO_VALUE) {
+                    for (int k = MIN_VALUE; k <= MAX_VALUE; k++) {
+                        board[row][column] = k;
+                        if (isValid(board, row, column) && solve(board)) {
+                            return true;
+                        }
+                        board[row][column] = NO_VALUE;
+                    }
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+
+    private boolean isValid(int[][] board, int row, int column) {
+        return rowConstraint(board, row) &&
+          columnConstraint(board, column) &&
+          subsectionConstraint(board, row, column);
+    }
+
+    private boolean subsectionConstraint(int[][] board, int row, int column) {
+        boolean[] constraint = new boolean[BOARD_SIZE];
+        int subsectionRowStart = (row / SUBSECTION_SIZE) * SUBSECTION_SIZE;
+        int subsectionRowEnd = subsectionRowStart + SUBSECTION_SIZE;
+
+        int subsectionColumnStart = (column / SUBSECTION_SIZE) * SUBSECTION_SIZE;
+        int subsectionColumnEnd = subsectionColumnStart + SUBSECTION_SIZE;
+
+        for (int r = subsectionRowStart; r < subsectionRowEnd; r++) {
+            for (int c = subsectionColumnStart; c < subsectionColumnEnd; c++) {
+                if (!checkConstraint(board, r, constraint, c)) return false;
+            }
+        }
+        return true;
+    }
+
+    private boolean columnConstraint(int[][] board, int column) {
+        boolean[] constraint = new boolean[BOARD_SIZE];
+        return IntStream.range(BOARD_START_INDEX, BOARD_SIZE)
+          .allMatch(row -> checkConstraint(board, row, constraint, column));
+    }
+
+    private boolean rowConstraint(int[][] board, int row) {
+        boolean[] constraint = new boolean[BOARD_SIZE];
+        return IntStream.range(BOARD_START_INDEX, BOARD_SIZE)
+          .allMatch(column -> checkConstraint(board, row, constraint, column));
+    }
+
+    private boolean checkConstraint(int[][] board, int row, boolean[] constraint, int column) {
+        if (board[row][column] != NO_VALUE) {
+            if (!constraint[board[row][column] - 1]) {
+                constraint[board[row][column] - 1] = true;
+            } else {
+                return false;
+            }
+        }
+        return true;
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/sudoku/ColumnNode.java

@@ -0,0 +1,33 @@
+package com.baeldung.algorithms.sudoku;
+
+class ColumnNode extends DancingNode {
+    int size;
+    String name;
+
+    ColumnNode(String n) {
+        super();
+        size = 0;
+        name = n;
+        C = this;
+    }
+
+    void cover() {
+        unlinkLR();
+        for (DancingNode i = this.D; i != this; i = i.D) {
+            for (DancingNode j = i.R; j != i; j = j.R) {
+                j.unlinkUD();
+                j.C.size--;
+            }
+        }
+    }
+
+    void uncover() {
+        for (DancingNode i = this.U; i != this; i = i.U) {
+            for (DancingNode j = i.L; j != i; j = j.L) {
+                j.C.size++;
+                j.relinkUD();
+            }
+        }
+        relinkLR();
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/sudoku/DancingLinks.java

@@ -0,0 +1,133 @@
+package com.baeldung.algorithms.sudoku;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+public class DancingLinks {
+
+    private ColumnNode header;
+    private List<DancingNode> answer;
+
+    private void search(int k) {
+        if (header.R == header) {
+            handleSolution(answer);
+        } else {
+            ColumnNode c = selectColumnNodeHeuristic();
+            c.cover();
+
+            for (DancingNode r = c.D; r != c; r = r.D) {
+                answer.add(r);
+
+                for (DancingNode j = r.R; j != r; j = j.R) {
+                    j.C.cover();
+                }
+
+                search(k + 1);
+
+                r = answer.remove(answer.size() - 1);
+                c = r.C;
+
+                for (DancingNode j = r.L; j != r; j = j.L) {
+                    j.C.uncover();
+                }
+            }
+            c.uncover();
+        }
+    }
+
+    private ColumnNode selectColumnNodeHeuristic() {
+        int min = Integer.MAX_VALUE;
+        ColumnNode ret = null;
+        for (ColumnNode c = (ColumnNode) header.R; c != header; c = (ColumnNode) c.R) {
+            if (c.size < min) {
+                min = c.size;
+                ret = c;
+            }
+        }
+        return ret;
+    }
+
+    private ColumnNode makeDLXBoard(boolean[][] grid) {
+        final int COLS = grid[0].length;
+
+        ColumnNode headerNode = new ColumnNode("header");
+        List<ColumnNode> columnNodes = new ArrayList<>();
+
+        for (int i = 0; i < COLS; i++) {
+            ColumnNode n = new ColumnNode(Integer.toString(i));
+            columnNodes.add(n);
+            headerNode = (ColumnNode) headerNode.hookRight(n);
+        }
+        headerNode = headerNode.R.C;
+
+        for (boolean[] aGrid : grid) {
+            DancingNode prev = null;
+            for (int j = 0; j < COLS; j++) {
+                if (aGrid[j]) {
+                    ColumnNode col = columnNodes.get(j);
+                    DancingNode newNode = new DancingNode(col);
+                    if (prev == null)
+                        prev = newNode;
+                    col.U.hookDown(newNode);
+                    prev = prev.hookRight(newNode);
+                    col.size++;
+                }
+            }
+        }
+
+        headerNode.size = COLS;
+
+        return headerNode;
+    }
+
+    DancingLinks(boolean[][] cover) {
+        header = makeDLXBoard(cover);
+    }
+
+    public void runSolver() {
+        answer = new LinkedList<>();
+        search(0);
+    }
+
+    private void handleSolution(List<DancingNode> answer) {
+        int[][] result = parseBoard(answer);
+        printSolution(result);
+    }
+
+    private int size = 9;
+
+    private int[][] parseBoard(List<DancingNode> answer) {
+        int[][] result = new int[size][size];
+        for (DancingNode n : answer) {
+            DancingNode rcNode = n;
+            int min = Integer.parseInt(rcNode.C.name);
+            for (DancingNode tmp = n.R; tmp != n; tmp = tmp.R) {
+                int val = Integer.parseInt(tmp.C.name);
+                if (val < min) {
+                    min = val;
+                    rcNode = tmp;
+                }
+            }
+            int ans1 = Integer.parseInt(rcNode.C.name);
+            int ans2 = Integer.parseInt(rcNode.R.C.name);
+            int r = ans1 / size;
+            int c = ans1 % size;
+            int num = (ans2 % size) + 1;
+            result[r][c] = num;
+        }
+        return result;
+    }
+
+    private static void printSolution(int[][] result) {
+        int size = result.length;
+        for (int[] aResult : result) {
+            StringBuilder ret = new StringBuilder();
+            for (int j = 0; j < size; j++) {
+                ret.append(aResult[j]).append(" ");
+            }
+            System.out.println(ret);
+        }
+        System.out.println();
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/sudoku/DancingLinksAlgorithm.java

@@ -0,0 +1,121 @@
+package com.baeldung.algorithms.sudoku;
+
+import java.util.Arrays;
+
+public class DancingLinksAlgorithm {
+    private static final int BOARD_SIZE = 9;
+    private static final int SUBSECTION_SIZE = 3;
+    private static final int NO_VALUE = 0;
+    private static final int CONSTRAINTS = 4;
+    private static final int MIN_VALUE = 1;
+    private static final int MAX_VALUE = 9;
+    private static final int COVER_START_INDEX = 1;
+
+    private static int[][] board = {
+      {8, 0, 0, 0, 0, 0, 0, 0, 0},
+      {0, 0, 3, 6, 0, 0, 0, 0, 0},
+      {0, 7, 0, 0, 9, 0, 2, 0, 0},
+      {0, 5, 0, 0, 0, 7, 0, 0, 0},
+      {0, 0, 0, 0, 4, 5, 7, 0, 0},
+      {0, 0, 0, 1, 0, 0, 0, 3, 0},
+      {0, 0, 1, 0, 0, 0, 0, 6, 8},
+      {0, 0, 8, 5, 0, 0, 0, 1, 0},
+      {0, 9, 0, 0, 0, 0, 4, 0, 0}
+    };
+
+    public static void main(String[] args) {
+        DancingLinksAlgorithm solver = new DancingLinksAlgorithm();
+        solver.solve(board);
+    }
+
+    private void solve(int[][] board) {
+        boolean[][] cover = initializeExactCoverBoard(board);
+        DancingLinks dlx = new DancingLinks(cover);
+        dlx.runSolver();
+    }
+
+    private int getIndex(int row, int column, int num) {
+        return (row - 1) * BOARD_SIZE * BOARD_SIZE + (column - 1) * BOARD_SIZE + (num - 1);
+    }
+
+    private boolean[][] createExactCoverBoard() {
+        boolean[][] coverBoard = new boolean[BOARD_SIZE * BOARD_SIZE * MAX_VALUE][BOARD_SIZE * BOARD_SIZE * CONSTRAINTS];
+
+        int hBase = 0;
+        hBase = checkCellConstraint(coverBoard, hBase);
+        hBase = checkRowConstraint(coverBoard, hBase);
+        hBase = checkColumnConstraint(coverBoard, hBase);
+        checkSubsectionConstraint(coverBoard, hBase);
+        
+        return coverBoard;
+    }
+
+    private int checkSubsectionConstraint(boolean[][] coverBoard, int hBase) {
+        for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row += SUBSECTION_SIZE) {
+            for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column += SUBSECTION_SIZE) {
+                for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++, hBase++) {
+                    for (int rowDelta = 0; rowDelta < SUBSECTION_SIZE; rowDelta++) {
+                        for (int columnDelta = 0; columnDelta < SUBSECTION_SIZE; columnDelta++) {
+                            int index = getIndex(row + rowDelta, column + columnDelta, n);
+                            coverBoard[index][hBase] = true;
+                        }
+                    }
+                }
+            }
+        }
+        return hBase;
+    }
+
+    private int checkColumnConstraint(boolean[][] coverBoard, int hBase) {
+        for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++) {
+            for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++, hBase++) {
+                for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
+                    int index = getIndex(row, column, n);
+                    coverBoard[index][hBase] = true;
+                }
+            }
+        }
+        return hBase;
+    }
+
+    private int checkRowConstraint(boolean[][] coverBoard, int hBase) {
+        for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
+            for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++, hBase++) {
+                for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++) {
+                    int index = getIndex(row, column, n);
+                    coverBoard[index][hBase] = true;
+                }
+            }
+        }
+        return hBase;
+    }
+
+    private int checkCellConstraint(boolean[][] coverBoard, int hBase) {
+        for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
+            for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++, hBase++) {
+                for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++) {
+                    int index = getIndex(row, column, n);
+                    coverBoard[index][hBase] = true;
+                }
+            }
+        }
+        return hBase;
+    }
+
+    private boolean[][] initializeExactCoverBoard(int[][] board) {
+        boolean[][] coverBoard = createExactCoverBoard();
+        for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
+            for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++) {
+                int n = board[row - 1][column - 1];
+                if (n != NO_VALUE) {
+                    for (int num = MIN_VALUE; num <= MAX_VALUE; num++) {
+                        if (num != n) {
+                            Arrays.fill(coverBoard[getIndex(row, column, num)], false);
+                        }
+                    }
+                }
+            }
+        }
+        return coverBoard;
+    }
+}

algorithms/src/main/java/com/baeldung/algorithms/sudoku/DancingNode.java

@@ -0,0 +1,50 @@
+package com.baeldung.algorithms.sudoku;
+
+class DancingNode {
+    DancingNode L, R, U, D;
+    ColumnNode C;
+
+    DancingNode hookDown(DancingNode node) {
+        assert (this.C == node.C);
+        node.D = this.D;
+        node.D.U = node;
+        node.U = this;
+        this.D = node;
+        return node;
+    }
+
+    DancingNode hookRight(DancingNode node) {
+        node.R = this.R;
+        node.R.L = node;
+        node.L = this;
+        this.R = node;
+        return node;
+    }
+
+    void unlinkLR() {
+        this.L.R = this.R;
+        this.R.L = this.L;
+    }
+
+    void relinkLR() {
+        this.L.R = this.R.L = this;
+    }
+
+    void unlinkUD() {
+        this.U.D = this.D;
+        this.D.U = this.U;
+    }
+
+    void relinkUD() {
+        this.U.D = this.D.U = this;
+    }
+
+    DancingNode() {
+        L = R = U = D = this;
+    }
+
+    DancingNode(ColumnNode c) {
+        this();
+        C = c;
+    }
+}

algorithms/src/main/resources/maze/maze1.txt

@@ -0,0 +1,12 @@
+S ########
+#        #
+# ### ## #
+# #    # #
+# #  # # #
+# ## #####
+# #      #
+# # #  # #
+##### ####
+#   #    E
+# #    # #
+##########

algorithms/src/main/resources/maze/maze2.txt

@@ -0,0 +1,22 @@
+S ##########################
+# #      #                 #
+# # ####   ############### #
+# # #    # #               #
+# # #### # # ###############
+# # #    # # #             #
+# # # #### ### ########### #
+# # # #        #           #
+#       ################## #
+#########    # # #       # #
+# #       #### # ####### # #
+# # ### ###  # # #       # #
+#   #       ##   # ##### # #
+##### #######  # #     # # #
+#     #       ## ## #### # #
+# ##### #######     #      #
+#       #       ############
+####### #########        # #
+#       #         ######## #
+# ####### ###### ##   #    E
+#         #         #   ## #
+############################

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;
+		}
+	}
+
+}

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());
+	}
+
+}

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.
+				}
+			}
+		};
+	}
+
+}

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>

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();
+    }
+}

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;
+}

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);
+    }
+}

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>

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);
+    }
+
+}

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
+    }
+
+}

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) );
+    }
+
+}

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;
+    }
+
+}