Added tests for singleton pattern

Author: fluxw42

singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java

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+package com.iluwatar.singleton;
+
+/**
+ * Date: 12/29/15 - 19:20 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class EnumIvoryTowerTest extends SingletonTest<EnumIvoryTower> {
+
+  /**
+   * Create a new singleton test instance using the given 'getInstance' method
+   */
+  public EnumIvoryTowerTest() {
+    super(() -> EnumIvoryTower.INSTANCE);
+  }
+
+}

singleton/src/test/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiomTest.java

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+package com.iluwatar.singleton;
+
+/**
+ * Date: 12/29/15 - 19:22 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class InitializingOnDemandHolderIdiomTest extends SingletonTest<InitializingOnDemandHolderIdiom> {
+
+  /**
+   * Create a new singleton test instance using the given 'getInstance' method
+   */
+  public InitializingOnDemandHolderIdiomTest() {
+    super(InitializingOnDemandHolderIdiom::getInstance);
+  }
+
+}

singleton/src/test/java/com/iluwatar/singleton/IvoryTowerTest.java

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+package com.iluwatar.singleton;
+
+/**
+ * Date: 12/29/15 - 19:23 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class IvoryTowerTest extends SingletonTest<IvoryTower> {
+
+  /**
+   * Create a new singleton test instance using the given 'getInstance' method
+   */
+  public IvoryTowerTest() {
+    super(IvoryTower::getInstance);
+  }
+
+}

singleton/src/test/java/com/iluwatar/singleton/LazyLoadedSingletonThreadSafetyTest.java

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+package com.iluwatar.singleton;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.function.Supplier;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertSame;
+
+/**
+ * This class provides several test case that test singleton construction.
+ *
+ * The first proves that multiple calls to the singleton getInstance object are the same when called
+ * in the SAME thread. The second proves that multiple calls to the singleton getInstance object are
+ * the same when called in the DIFFERENT thread.
+ *
+ * Date: 12/29/15 - 19:25 PM
+ *
+ * @author Jeroen Meulemeester
+ * @author Richard Jones
+ */
+public abstract class SingletonTest<S> {
+
+  /**
+   * The singleton's getInstance method
+   */
+  private final Supplier<S> singletonInstanceMethod;
+
+  /**
+   * Create a new singleton test instance using the given 'getInstance' method
+   *
+   * @param singletonInstanceMethod The singleton's getInstance method
+   */
+  public SingletonTest(final Supplier<S> singletonInstanceMethod) {
+    this.singletonInstanceMethod = singletonInstanceMethod;
+  }
+
+  /**
+   * Test the singleton in a non-concurrent setting
+   */
+  @Test
+  public void testMultipleCallsReturnTheSameObjectInSameThread() {
+    // Create several instances in the same calling thread
+    S instance1 = this.singletonInstanceMethod.get();
+    S instance2 = this.singletonInstanceMethod.get();
+    S instance3 = this.singletonInstanceMethod.get();
+    // now check they are equal
+    assertSame(instance1, instance2);
+    assertSame(instance1, instance3);
+    assertSame(instance2, instance3);
+  }
+
+  /**
+   * Test singleton instance in a concurrent setting
+   */
+  @Test(timeout = 10000)
+  public void testMultipleCallsReturnTheSameObjectInDifferentThreads() throws Exception {
+
+    // Create 10000 tasks and inside each callable instantiate the singleton class
+    final List<Callable<S>> tasks = new ArrayList<>();
+    for (int i = 0; i < 10000; i++) {
+      tasks.add(this.singletonInstanceMethod::get);
+    }
+
+    // Use up to 8 concurrent threads to handle the tasks
+    final ExecutorService executorService = Executors.newFixedThreadPool(8);
+    final List<Future<S>> results = executorService.invokeAll(tasks);
+
+    // wait for all of the threads to complete
+    final S expectedInstance = this.singletonInstanceMethod.get();
+    for (Future<S> res : results) {
+      final S instance = res.get();
+      assertNotNull(instance);
+      assertSame(expectedInstance, instance);
+    }
+
+    // tidy up the executor
+    executorService.shutdown();
+
+  }
+
+}

singleton/src/test/java/com/iluwatar/singleton/SingletonTest.java

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+package com.iluwatar.singleton;
+
+/**
+ * Date: 12/29/15 - 19:26 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class ThreadSafeDoubleCheckLockingTest extends SingletonTest<ThreadSafeDoubleCheckLocking> {
+
+  /**
+   * Create a new singleton test instance using the given 'getInstance' method
+   */
+  public ThreadSafeDoubleCheckLockingTest() {
+    super(ThreadSafeDoubleCheckLocking::getInstance);
+  }
+
+}

singleton/src/test/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLockingTest.java

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+package com.iluwatar.singleton;
+
+/**
+ * Date: 12/29/15 - 19:26 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class ThreadSafeLazyLoadedIvoryTowerTest extends SingletonTest<ThreadSafeLazyLoadedIvoryTower> {
+
+  /**
+   * Create a new singleton test instance using the given 'getInstance' method
+   */
+  public ThreadSafeLazyLoadedIvoryTowerTest() {
+    super(ThreadSafeLazyLoadedIvoryTower::getInstance);
+  }
+
+}