Fixes missing eager/lazy initialization examples for Hacktoberfest contributors

Author: Th-Shivam

singleton/README.md

@@ -39,6 +39,51 @@ Sequence diagram
 
 ## Programmatic Example of Singleton Pattern in Java
 
+### 1. Eager Initialization
+
+The instance is created at class loading time. Thread-safe but may waste memory if never used.
+
+```java
+public final class EagerInitializedSingleton {
+    private static final EagerInitializedSingleton INSTANCE = new EagerInitializedSingleton();
+    
+    private EagerInitializedSingleton() {
+        if (INSTANCE != null) {
+            throw new IllegalStateException("Singleton already initialized");
+        }
+    }
+    
+    public static EagerInitializedSingleton getInstance() {
+        return INSTANCE;
+    }
+}
+```
+
+### 2. Lazy Initialization
+
+The instance is created only when first requested. Memory efficient but requires synchronization.
+
+```java
+public final class LazyInitializedSingleton {
+    private static LazyInitializedSingleton instance;
+    
+    private LazyInitializedSingleton() {
+        if (instance != null) {
+            throw new IllegalStateException("Singleton already initialized");
+        }
+    }
+    
+    public static synchronized LazyInitializedSingleton getInstance() {
+        if (instance == null) {
+            instance = new LazyInitializedSingleton();
+        }
+        return instance;
+    }
+}
+```
+
+### 3. Enum Singleton (Recommended)
+
 Joshua Bloch, Effective Java 2nd Edition p.18
 
 > A single-element enum type is the best way to implement a singleton
@@ -49,7 +94,7 @@ public enum EnumIvoryTower {
 }
 ```
 
-Then in order to use:
+Usage example:
 
 ```java
     var enumIvoryTower1 = EnumIvoryTower.INSTANCE;

singleton/src/main/java/com/iluwatar/singleton/App.java

@@ -72,6 +72,22 @@ public class App {
    */
   public static void main(String[] args) {
 
+    // Eager Initialization - Simple and clear example
+    LOGGER.info("=== Eager Initialization ===");
+    var eager1 = EagerInitializedSingleton.getInstance();
+    var eager2 = EagerInitializedSingleton.getInstance();
+    LOGGER.info("eager1={}", eager1);
+    LOGGER.info("eager2={}", eager2);
+    LOGGER.info("Same instance: {}", eager1 == eager2);
+
+    // Lazy Initialization - Simple and clear example
+    LOGGER.info("=== Lazy Initialization ===");
+    var lazy1 = LazyInitializedSingleton.getInstance();
+    var lazy2 = LazyInitializedSingleton.getInstance();
+    LOGGER.info("lazy1={}", lazy1);
+    LOGGER.info("lazy2={}", lazy2);
+    LOGGER.info("Same instance: {}", lazy1 == lazy2);
+
     // eagerly initialized singleton
     var ivoryTower1 = IvoryTower.getInstance();
     var ivoryTower2 = IvoryTower.getInstance();

singleton/src/main/java/com/iluwatar/singleton/EagerInitializedSingleton.java

@@ -0,0 +1,53 @@
+/*
+ * 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;
+
+/**
+ * Eager Initialization Singleton.
+ * Instance is created at class loading time.
+ * Thread-safe by default but may waste memory if instance is never used.
+ */
+public final class EagerInitializedSingleton {
+
+  /** Instance created at class loading time. */
+  private static final EagerInitializedSingleton INSTANCE = new EagerInitializedSingleton();
+
+  /** Private constructor to prevent instantiation. */
+  private EagerInitializedSingleton() {
+    // Prevent reflection attacks
+    if (INSTANCE != null) {
+      throw new IllegalStateException("Singleton already initialized");
+    }
+  }
+
+  /**
+   * Returns the singleton instance.
+   *
+   * @return the singleton instance
+   */
+  public static EagerInitializedSingleton getInstance() {
+    return INSTANCE;
+  }
+}

singleton/src/main/java/com/iluwatar/singleton/LazyInitializedSingleton.java

@@ -0,0 +1,57 @@
+/*
+ * 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;
+
+/**
+ * Lazy Initialization Singleton.
+ * Instance is created only when first requested.
+ * Memory efficient but requires synchronization for thread safety.
+ */
+public final class LazyInitializedSingleton {
+
+  /** Instance variable, initialized only when needed. */
+  private static LazyInitializedSingleton instance;
+
+  /** Private constructor to prevent instantiation. */
+  private LazyInitializedSingleton() {
+    // Prevent reflection attacks
+    if (instance != null) {
+      throw new IllegalStateException("Singleton already initialized");
+    }
+  }
+
+  /**
+   * Returns the singleton instance, creating it if necessary.
+   * Synchronized to ensure thread safety.
+   *
+   * @return the singleton instance
+   */
+  public static synchronized LazyInitializedSingleton getInstance() {
+    if (instance == null) {
+      instance = new LazyInitializedSingleton();
+    }
+    return instance;
+  }
+}

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

@@ -0,0 +1,42 @@
+/*
+ * 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;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link EagerInitializedSingleton}.
+ */
+class EagerInitializedSingletonTest {
+
+  @Test
+  void testSingletonInstance() {
+    var instance1 = EagerInitializedSingleton.getInstance();
+    var instance2 = EagerInitializedSingleton.getInstance();
+    assertSame(instance1, instance2);
+  }
+}

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

@@ -0,0 +1,42 @@
+/*
+ * 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;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link LazyInitializedSingleton}.
+ */
+class LazyInitializedSingletonTest {
+
+  @Test
+  void testSingletonInstance() {
+    var instance1 = LazyInitializedSingleton.getInstance();
+    var instance2 = LazyInitializedSingleton.getInstance();
+    assertSame(instance1, instance2);
+  }
+}