added singleton-design-pattern java programs

Java/singleton-design-pattern/EagerInitializedSingleton.java

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+public class EagerInitializedSingleton {
+
+    private static final EagerInitializedSingleton instance = new EagerInitializedSingleton();
+
+    //private constructor to avoid client applications to use constructor
+    private EagerInitializedSingleton(){}
+
+    public static EagerInitializedSingleton getInstance(){
+        return instance;
+    }
+
+	public void test() {
+		System.out.println("This is Singleton instance");
+	}
+
+	public static void main(String[] args) {
+		EagerInitializedSingleton.getInstance().test();
+	}
+}

Java/singleton-design-pattern/LazyInitializedSingleton.java

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+public class LazyInitializedSingleton {
+
+    private static LazyInitializedSingleton instance;
+
+    private LazyInitializedSingleton(){}
+
+    public static LazyInitializedSingleton getInstance(){
+        if(instance == null){
+            instance = new LazyInitializedSingleton();
+        }
+        return instance;
+    }
+
+	public void test() {
+		System.out.println("This is LazyInitializedSingleton instance");
+	}
+
+	public static void main(String[] args) {
+		LazyInitializedSingleton.getInstance().test();
+	}
+}

Java/singleton-design-pattern/StaticBlockSingleton.java

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+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 occured in creating singleton instance");
+        }
+    }
+
+    public static StaticBlockSingleton getInstance(){
+        return instance;
+    }
+
+	public void test() {
+		System.out.println("This is StaticBlockSingleton instance");
+	}
+
+	public static void main(String[] args) {
+		StaticBlockSingleton.getInstance().test();
+	}
+}

Java/singleton-design-pattern/ThreadSafeSingleton.java

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+public class ThreadSafeSingleton {
+
+    private static ThreadSafeSingleton instance;
+
+    private ThreadSafeSingleton(){}
+
+    public static synchronized ThreadSafeSingleton getInstance(){
+        if(instance == null){
+            instance = new ThreadSafeSingleton();
+        }
+        return instance;
+    }
+
+	public void test() {
+		System.out.println("This is ThreadSafeSingleton instance");
+	}
+
+	public static void main(String[] args) {
+		ThreadSafeSingleton.getInstance().test();
+	}
+
+}