Fix issue #179: Leader Followers Pattern (#1189)

* add leader followers pattern

* use var and streams instead in App::execute

* use logger instead of printing to system output stream

leader-followers/README.md

+---
+layout: pattern
+title: Leader/Followers
+folder: leader-followers
+permalink: /patterns/leader-followers/
+categories: Concurrency
+tags:
+ - Performance
+---
+
+## Intent
+The Leader/Followers pattern provides a concurrency model where multiple 
+threads can efficiently de-multiplex events and dispatch event handlers 
+that process I/O handles shared by the threads.
+
+## Class diagram
+![Leader/Followers class diagram](./etc/leader-followers.png)
+
+## Applicability
+Use Leader-Followers pattern when
+
+* multiple threads take turns sharing a set of event sources in order to detect, de-multiplex, dispatch and process service requests that occur on the event sources.
+
+## Real world examples
+
+* [ACE Thread Pool Reactor framework](https://www.dre.vanderbilt.edu/~schmidt/PDF/HPL.pdf)
+* [JAWS](http://www.dre.vanderbilt.edu/~schmidt/PDF/PDCP.pdf)
+* [Real-time CORBA](http://www.dre.vanderbilt.edu/~schmidt/PDF/RTS.pdf)
+
+## Credits
+
+* [Douglas C. Schmidt and Carlos O’Ryan - Leader/Followers](http://www.kircher-schwanninger.de/michael/publications/lf.pdf)

leader-followers/etc/leader-followers.urm.puml

+@startuml
+package com.iluwatar.leaderfollowers {
+  class App {
+    + App()
+    - addTasks(taskSet : TaskSet) {static}
+    - execute(workCenter : WorkCenter, taskSet : TaskSet) {static}
+    + main(args : String[]) {static}
+  }
+  class Task {
+    - finished : boolean
+    - time : int
+    + Task(time : int)
+    + getTime() : int
+    + isFinished() : boolean
+    + setFinished()
+  }
+  class TaskHandler {
+    + TaskHandler()
+    + handleTask(task : Task)
+  }
+  class TaskSet {
+    - queue : BlockingQueue<Task>
+    + TaskSet()
+    + addTask(task : Task)
+    + getSize() : int
+    + getTask() : Task
+  }
+  class WorkCenter {
+    - leader : Worker
+    - workers : List<Worker>
+    + WorkCenter()
+    + addWorker(worker : Worker)
+    + createWorkers(numberOfWorkers : int, taskSet : TaskSet, taskHandler : TaskHandler)
+    + getLeader() : Worker
+    + getWorkers() : List<Worker>
+    + promoteLeader()
+    + removeWorker(worker : Worker)
+  }
+  class Worker {
+    - id : long
+    - taskHandler : TaskHandler
+    - taskSet : TaskSet
+    - workCenter : WorkCenter
+    + Worker(id : long, workCenter : WorkCenter, taskSet : TaskSet, taskHandler : TaskHandler)
+    + equals(o : Object) : boolean
+    + hashCode() : int
+    + run()
+  }
+}
+Worker -->  "-taskSet" TaskSet
+Worker -->  "-taskHandler" TaskHandler
+TaskSet -->  "-queue" Task
+Worker -->  "-workCenter" WorkCenter
+@enduml
\ No newline at end of file

leader-followers/pom.xml

+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.23.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>leader-followers</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>
\ No newline at end of file

leader-followers/src/main/java/com.iluwatar.leaderfollowers/App.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import java.util.Random;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Leader/Followers pattern is a concurrency pattern. This pattern behaves like a taxi stand where
+ * one of the threads acts as leader thread which listens for event from event sources,
+ * de-multiplexes, dispatches and handles the event. It promotes the follower to be the new leader.
+ * When processing completes the thread joins the followers queue, if there are no followers then it
+ * becomes the leader and cycle repeats again.
+ *
+ * <p>In this example, one of the workers becomes Leader and listens on the {@link TaskSet} for
+ * work. {@link TaskSet} basically acts as the source of input events for the {@link Worker}, who
+ * are spawned and controlled by the {@link WorkCenter} . When {@link Task} arrives then the leader
+ * takes the work and calls the {@link TaskHandler}. It also calls the {@link WorkCenter} to
+ * promotes one of the followers to be the new leader, who can then process the next work and so
+ * on.
+ *
+ * <p>The pros for this pattern are:
+ * It enhances CPU cache affinity and eliminates unbound allocation and data buffer sharing between
+ * threads by reading the request into buffer space allocated on the stack of the leader or by using
+ * the Thread-Specific Storage pattern [22] to allocate memory. It minimizes locking overhead by not
+ * exchanging data between threads, thereby reducing thread synchronization. In bound handle/thread
+ * associations, the leader thread dispatches the event based on the I/O handle. It can minimize
+ * priority inversion because no extra queuing is introduced in the server. It does not require a
+ * context switch to handle each event, reducing the event dispatching latency. Note that promoting
+ * a follower thread to fulfill the leader role requires a context switch. Programming simplicity:
+ * The Leader/Followers pattern simplifies the programming of concurrency models where multiple
+ * threads can receive requests, process responses, and de-multiplex connections using a shared
+ * handle set.
+ */
+public class App {
+
+  /**
+   * The main method for the leader followers pattern.
+   */
+  public static void main(String[] args) throws InterruptedException {
+    var taskSet = new TaskSet();
+    var taskHandler = new TaskHandler();
+    var workCenter = new WorkCenter();
+    workCenter.createWorkers(4, taskSet, taskHandler);
+    execute(workCenter, taskSet);
+  }
+
+  /**
+   * Start the work, dispatch tasks and stop the thread pool at last.
+   */
+  private static void execute(WorkCenter workCenter, TaskSet taskSet) throws InterruptedException {
+    var workers = workCenter.getWorkers();
+    var exec = Executors.newFixedThreadPool(workers.size());
+    workers.forEach(exec::submit);
+    Thread.sleep(1000);
+    addTasks(taskSet);
+    exec.awaitTermination(2, TimeUnit.SECONDS);
+    exec.shutdownNow();
+  }
+
+  /**
+   * Add tasks.
+   */
+  private static void addTasks(TaskSet taskSet) throws InterruptedException {
+    var rand = new Random();
+    for (var i = 0; i < 5; i++) {
+      var time = Math.abs(rand.nextInt(1000));
+      taskSet.addTask(new Task(time));
+    }
+  }
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/Task.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+/**
+ * A unit of work to be processed by the Workers.
+ */
+public class Task {
+
+  private final int time;
+
+  private boolean finished;
+
+  public Task(int time) {
+    this.time = time;
+  }
+
+  public int getTime() {
+    return time;
+  }
+
+  public void setFinished() {
+    this.finished = true;
+  }
+
+  public boolean isFinished() {
+    return this.finished;
+  }
+
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/TaskHandler.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The TaskHandler is used by the {@link Worker} to process the newly arrived task.
+ */
+public class TaskHandler {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(TaskHandler.class);
+
+  /**
+   * This interface handles one task at a time.
+   */
+  public void handleTask(Task task) throws InterruptedException {
+    var time = task.getTime();
+    Thread.sleep(time);
+    LOGGER.info("It takes " + time + " milliseconds to finish the task");
+    task.setFinished();
+  }
+
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/TaskSet.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+
+/**
+ * A TaskSet is a collection of the tasks, the leader receives task from here.
+ */
+public class TaskSet {
+
+  private BlockingQueue<Task> queue = new ArrayBlockingQueue<>(100);
+
+  public void addTask(Task task) throws InterruptedException {
+    queue.put(task);
+  }
+
+  public Task getTask() throws InterruptedException {
+    return queue.take();
+  }
+
+  public int getSize() {
+    return queue.size();
+  }
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/WorkCenter.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+/**
+ * A WorkCenter contains a leader and a list of idle workers. The leader is responsible for
+ * receiving work when it arrives. This class also provides a mechanism to promote a new leader. A
+ * worker once he completes his task will add himself back to the center.
+ */
+public class WorkCenter {
+
+  private Worker leader;
+  private List<Worker> workers = new CopyOnWriteArrayList<>();
+
+  /**
+   * Create workers and set leader.
+   */
+  public void createWorkers(int numberOfWorkers, TaskSet taskSet, TaskHandler taskHandler) {
+    for (var id = 1; id <= numberOfWorkers; id++) {
+      var worker = new Worker(id, this, taskSet, taskHandler);
+      workers.add(worker);
+    }
+    promoteLeader();
+  }
+
+  public void addWorker(Worker worker) {
+    workers.add(worker);
+  }
+
+  public void removeWorker(Worker worker) {
+    workers.remove(worker);
+  }
+
+  public Worker getLeader() {
+    return leader;
+  }
+
+  /**
+   * Promote a leader.
+   */
+  public void promoteLeader() {
+    Worker leader = null;
+    if (workers.size() > 0) {
+      leader = workers.get(0);
+    }
+    this.leader = leader;
+  }
+
+  public List<Worker> getWorkers() {
+    return workers;
+  }
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/Worker.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import java.util.Objects;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class Worker implements Runnable {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Worker.class);
+
+  private final long id;
+  private final WorkCenter workCenter;
+  private final TaskSet taskSet;
+  private final TaskHandler taskHandler;
+
+  /**
+   * Constructor to create a worker which will take work from the work center.
+   */
+  public Worker(long id, WorkCenter workCenter, TaskSet taskSet, TaskHandler taskHandler) {
+    super();
+    this.id = id;
+    this.workCenter = workCenter;
+    this.taskSet = taskSet;
+    this.taskHandler = taskHandler;
+  }
+
+  /**
+   * The leader thread listens for task. When task arrives, it promotes one of the followers to be
+   * the new leader. Then it handles the task and add himself back to work center.
+   */
+  @Override
+  public void run() {
+    while (!Thread.interrupted()) {
+      try {
+        if (workCenter.getLeader() != null && !workCenter.getLeader().equals(this)) {
+          synchronized (workCenter) {
+            workCenter.wait();
+          }
+          continue;
+        }
+        final Task task = taskSet.getTask();
+        synchronized (workCenter) {
+          workCenter.removeWorker(this);
+          workCenter.promoteLeader();
+          workCenter.notifyAll();
+        }
+        taskHandler.handleTask(task);
+        LOGGER.info("The Worker with the ID " + id + " completed the task");
+        workCenter.addWorker(this);
+      } catch (InterruptedException e) {
+        LOGGER.warn("Worker interrupted");
+        return;
+      }
+    }
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (!(o instanceof Worker)) {
+      return false;
+    }
+    var worker = (Worker) o;
+    return id == worker.id;
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id);
+  }
+}

leader-followers/src/test/java/com.iluwatar.leaderfollowers/AppTest.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import org.junit.Test;
+
+/**
+ *
+ * Application test
+ *
+ */
+public class AppTest {
+
+    @Test
+    public void test() throws InterruptedException {
+        String[] args = {};
+        App.main(args);
+    }
+
+}

leader-followers/src/test/java/com.iluwatar.leaderfollowers/TaskHandlerTest.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Tests for TaskHandler
+ */
+public class TaskHandlerTest {
+
+    @Test
+    public void testHandleTask() throws InterruptedException {
+        var taskHandler = new TaskHandler();
+        var handle = new Task(100);
+        taskHandler.handleTask(handle);
+        Assert.assertTrue(handle.isFinished());
+    }
+
+}

leader-followers/src/test/java/com.iluwatar.leaderfollowers/TaskSetTest.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Tests for TaskSet
+ */
+public class TaskSetTest {
+
+    @Test
+    public void testAddTask() throws InterruptedException {
+        var taskSet = new TaskSet();
+        taskSet.addTask(new Task(10));
+        Assert.assertTrue(taskSet.getSize() == 1);
+    }
+
+    @Test
+    public void testGetTask() throws InterruptedException {
+        var taskSet = new TaskSet();
+        taskSet.addTask(new Task(100));
+        Task task = taskSet.getTask();
+        Assert.assertTrue(task.getTime() == 100);
+        Assert.assertTrue(taskSet.getSize() == 0);
+    }
+
+}

leader-followers/src/test/java/com.iluwatar.leaderfollowers/WorkCenterTest.java

+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+/**
+ * Tests for WorkCenter
+ */
+public class WorkCenterTest {
+
+  @Test
+  public void testCreateWorkers() {
+    var taskSet = new TaskSet();
+    var taskHandler = new TaskHandler();
+    var workCenter = new WorkCenter();
+    workCenter.createWorkers(5, taskSet, taskHandler);
+    Assert.assertEquals(workCenter.getWorkers().size(), 5);
+    Assert.assertEquals(workCenter.getWorkers().get(0), workCenter.getLeader());
+  }
+
+  @Test
+  public void testNullLeader() {
+    var workCenter = new WorkCenter();
+    workCenter.promoteLeader();
+    Assert.assertNull(workCenter.getLeader());
+  }
+
+  @Test
+  public void testPromoteLeader() {
+    var taskSet = new TaskSet();
+    var taskHandler = new TaskHandler();
+    var workCenter = new WorkCenter();
+    workCenter.createWorkers(5, taskSet, taskHandler);
+    workCenter.removeWorker(workCenter.getLeader());
+    workCenter.promoteLeader();
+    Assert.assertEquals(workCenter.getWorkers().size(), 4);
+    Assert.assertEquals(workCenter.getWorkers().get(0), workCenter.getLeader());
+  }
+}

pom.xml

@@ -183,6 +183,7 @@
         <module>game-loop</module>
         <module>combinator</module>
         <module>update-method</module>
+        <module>leader-followers</module>
     </modules>
 
     <repositories>