feat: Add Bill Pugh Impl & Test (iluwatar#2538)

* Add Bill Pugh Impl & Test

Add Bill Pugh Impl & Test

* Fix formatting

* Fix checkstyle error

* Reformat file

* Reformat file

* Fix indentation

* Fix comment indent

Author: Owen-CH-Leung

Diff for: singleton/src/main/java/com/iluwatar/singleton/App.java

@@ -100,5 +100,11 @@ public static void main(String[] args) {
     LOGGER.info(demandHolderIdiom.toString());
     var demandHolderIdiom2 = InitializingOnDemandHolderIdiom.getInstance();
     LOGGER.info(demandHolderIdiom2.toString());
+
+    // initialize singleton using Bill Pugh's implementation
+    var billPughSingleton = BillPughImplementation.getInstance();
+    LOGGER.info(billPughSingleton.toString());
+    var billPughSingleton2 = BillPughImplementation.getInstance();
+    LOGGER.info(billPughSingleton2.toString());
   }
 }

Diff for: singleton/src/main/java/com/iluwatar/singleton/BillPughImplementation.java

@@ -0,0 +1,73 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.singleton;
+
+/**
+ * <p>Bill Pugh Singleton Implementation.</p>
+ *
+ * <p>This implementation of the singleton design pattern takes advantage of the
+ * Java memory model's guarantees about class initialization. Each class is
+ * initialized only once, when it is first used. If the class hasn't been used
+ * yet, it won't be loaded into memory, and no memory will be allocated for
+ * a static instance. This makes the singleton instance lazy-loaded and thread-safe.</p>
+ *
+ * @author [email protected]
+ */
+public final class BillPughImplementation {
+
+  /**
+   * Private constructor to prevent instantiation from outside the class.
+   */
+  private BillPughImplementation() {
+    // private constructor
+  }
+
+  /**
+   * The InstanceHolder is a static inner class and it holds the Singleton instance.
+   * It is not loaded into memory until the getInstance() method is called.
+   */
+  private static class InstanceHolder {
+    /**
+     * Singleton instance of the class.
+     */
+    private static BillPughImplementation instance = new BillPughImplementation();
+  }
+
+  /**
+   * Public accessor for the singleton instance.
+   *
+   * <p>
+   * When this method is called, the InstanceHolder is loaded into memory
+   * and creates the Singleton instance. This method provides a global access point
+   * for the singleton instance.
+   * </p>
+   *
+   * @return an instance of the class.
+   */
+  // global access point
+  public static BillPughImplementation getInstance() {
+    return InstanceHolder.instance;
+  }
+}

Diff for: singleton/src/main/java/com/iluwatar/singleton/EnumIvoryTower.java

@@ -32,6 +32,9 @@
  */
 public enum EnumIvoryTower {
 
+  /**
+   * The singleton instance of the class, created by the Java enum singleton pattern.
+   */
   INSTANCE;
 
   @Override

Diff for: singleton/src/main/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiom.java

@@ -58,6 +58,10 @@ public static InitializingOnDemandHolderIdiom getInstance() {
    * Provides the lazy-loaded Singleton instance.
    */
   private static class HelperHolder {
+
+    /**
+     * Singleton instance of the class.
+     */
     private static final InitializingOnDemandHolderIdiom INSTANCE =
         new InitializingOnDemandHolderIdiom();
   }

Diff for: singleton/src/main/java/com/iluwatar/singleton/ThreadSafeDoubleCheckLocking.java

@@ -34,7 +34,9 @@
  * @author [email protected]
  */
 public final class ThreadSafeDoubleCheckLocking {
-
+  /**
+   * Singleton instance of the class, declared as volatile to ensure atomic access by multiple threads.
+   */
   private static volatile ThreadSafeDoubleCheckLocking instance;
 
   /**
@@ -73,7 +75,8 @@ public static ThreadSafeDoubleCheckLocking getInstance() {
           // The instance is still not initialized so we can safely
           // (no other thread can enter this zone)
           // create an instance and make it our singleton instance.
-          instance = result = new ThreadSafeDoubleCheckLocking();
+          result = new ThreadSafeDoubleCheckLocking();
+          instance = result;
         }
       }
     }

Diff for: singleton/src/main/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTower.java

@@ -31,8 +31,14 @@
  */
 public final class ThreadSafeLazyLoadedIvoryTower {
 
+  /**
+   * Singleton instance of the class, declared as volatile to ensure atomic access by multiple threads.
+   */
   private static volatile ThreadSafeLazyLoadedIvoryTower instance;
 
+  /**
+   * Private constructor to prevent instantiation from outside the class.
+   */
   private ThreadSafeLazyLoadedIvoryTower() {
     // Protect against instantiation via reflection
     if (instance != null) {
@@ -42,6 +48,8 @@ private ThreadSafeLazyLoadedIvoryTower() {
 
   /**
    * The instance doesn't get created until the method is called for the first time.
+   *
+   * @return an instance of the class.
    */
   public static synchronized ThreadSafeLazyLoadedIvoryTower getInstance() {
     if (instance == null) {

Diff for: singleton/src/main/java/com/iluwatar/singleton/package-info.java

@@ -0,0 +1,25 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.singleton;

Diff for: singleton/src/test/java/com/iluwatar/singleton/BillPughImplementationTest.java

@@ -0,0 +1,16 @@
+package com.iluwatar.singleton;
+
+/**
+ * Date: 06/18/23 - 16:29 PM.
+ *
+ * @author Owen Leung
+ */
+public class BillPughImplementationTest
+        extends SingletonTest<BillPughImplementation>{
+    /**
+     * Create a new singleton test instance using the given 'getInstance' method.
+     */
+    public BillPughImplementationTest() {
+        super(BillPughImplementation::getInstance);
+    }
+}