Example of Singleton Pattern

Author: dyriusdev

.idea/DesignPatterns.iml

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+package creational.singleton_pattern;
+
+public class BillPughSingleton {
+
+    private BillPughSingleton() {}
+
+    private static class SingletonHelper {
+        private static final BillPughSingleton INSTANCE = new BillPughSingleton();
+    }
+
+    public static BillPughSingleton GetInstance() {
+        return SingletonHelper.INSTANCE;
+    }
+}

SerializedSingleton.ser

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+package creational.singleton_pattern;
+
+public class EagerInitializedSingleton {
+
+    private static final EagerInitializedSingleton instance = new EagerInitializedSingleton();
+
+    private EagerInitializedSingleton() {}
+
+
+
+    public static EagerInitializedSingleton GetInstance() {
+        return instance;
+    }
+}

src/creational/singleton_pattern/BillPughSingleton.java

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+package creational.singleton_pattern;
+
+public enum EnumSingleton {
+
+    INSTANCE;
+
+    public static void DoSomething() {}
+}

src/creational/singleton_pattern/EagerInitializedSingleton.java

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+package creational.singleton_pattern;
+
+public class LazyInitializedSingleton {
+
+    private static LazyInitializedSingleton instance;
+
+    private LazyInitializedSingleton() {}
+
+
+
+    public static LazyInitializedSingleton GetInstance() {
+        if (instance == null) {
+            instance = new LazyInitializedSingleton();
+        }
+        return instance;
+    }
+}

src/creational/singleton_pattern/EnumSingleton.java

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+package creational.singleton_pattern;
+
+import java.lang.reflect.Constructor;
+
+public class ReflectionSingletonTest {
+
+    public static void main(String[] args) {
+        EagerInitializedSingleton instanceA = EagerInitializedSingleton.GetInstance();
+        EagerInitializedSingleton instanceB = null;
+
+        try {
+            Constructor[] constructors = EagerInitializedSingleton.class.getDeclaredConstructors();
+            for (Constructor constructor : constructors) {
+                // This destroy the singleton pattern
+                constructor.setAccessible(true);
+                instanceB = (EagerInitializedSingleton)constructor.newInstance();
+                break;
+            }
+        } catch (Exception e) {
+            System.out.println(e.getMessage());
+        }
+        System.out.println(instanceA.hashCode());
+        System.out.println(instanceB.hashCode());
+    }
+}

src/creational/singleton_pattern/LazyInitializedSingleton.java

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+package creational.singleton_pattern;
+
+import java.io.Serializable;
+
+public class SerializedSingleton implements Serializable {
+
+    private SerializedSingleton() {}
+
+    private static class SingletonHelper {
+        private static final SerializedSingleton instance = new SerializedSingleton();
+    }
+
+    public static SerializedSingleton GetInstance() {
+        return SingletonHelper.instance;
+    }
+}

src/creational/singleton_pattern/ReflectionSingletonTest.java

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+package creational.singleton_pattern;
+
+import java.io.*;
+
+public class SerializedSingletonTest {
+
+    public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {
+        SerializedSingleton instanceA = SerializedSingleton.GetInstance();
+        ObjectOutput out = new ObjectOutputStream(new FileOutputStream("SerializedSingleton.ser"));
+        out.writeObject(instanceA);
+        out.close();
+
+        // Deserialize from file to object
+        ObjectInput input = new ObjectInputStream(new FileInputStream("SerializedSingleton.ser"));
+        SerializedSingleton instanceB = (SerializedSingleton)input.readObject();
+        input.close();
+
+        System.out.println(instanceA.hashCode());
+        System.out.println(instanceB.hashCode());
+    }
+}

src/creational/singleton_pattern/SerializedSingleton.java

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+package creational.singleton_pattern;
+
+public class StaticBlockSingleton {
+
+    private static StaticBlockSingleton instance;
+
+    private StaticBlockSingleton() {}
+
+
+
+    // static block initialization for exception handling
+    static {
+        try {
+            instance = new StaticBlockSingleton();
+        } catch (Exception e) {
+            throw new RuntimeException("Exception occurred in creating singleton instance");
+        }
+    }
+
+    public static StaticBlockSingleton GetInstance() {
+        return instance;
+    }
+}

src/creational/singleton_pattern/SerializedSingletonTest.java

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+package creational.singleton_pattern;
+
+public class ThreadSafeSingleton {
+
+    private static ThreadSafeSingleton instance;
+
+    private ThreadSafeSingleton() {}
+
+
+    public static synchronized ThreadSafeSingleton GetInstance() {
+        if (instance == null) { instance = new ThreadSafeSingleton(); }
+        return instance;
+    }
+}