public class Singleton {




 // Singleton is a creational design pattern, which ensures that only one object of its kind exists

    // and provides a single point of access to it for any other code.



    /*



    private static final Singleton instance = new Singleton(); // Eager



    public static Singleton getInstance() { //Eager

        return instance;

    }

    */
 private static volatile Singleton instance = null;


 private Singleton() {}




 // Lazy initialization


 public static Singleton getInstance() {
 if (instance == null) {
 instance = new Singleton();

        }
 return instance;

    }




 // static block initialization
 static {
 try {
 instance = new Singleton();

        } catch (Exception e) {
 throw new RuntimeException("Exception occurred in creating singleton instance");

        }

    }




 // Thread Safe


 public static synchronized  Singleton getInstanceWithThreadSafe() {
 if (instance == null) {
 instance = new Singleton();

        }
 return instance;

    }




 // Thread Safe with double lock
 public static Singleton getInstanceUsingDoubleLocking() {
 if (instance == null) {
 synchronized (Singleton.class) {
 if (instance == null) {
 instance = new Singleton();

                }

            }

        }
 return instance;

    }


 // Using Bill Pugh inner static helper class.


 private static class SingletonHelper {
 private static final Singleton INSTANCE = new Singleton();

    }


 public static Singleton getInstanceWithInnerStaticHelper() {
 return SingletonHelper.INSTANCE;

    }







}
 
 
 
 
 
public class SerializedSingleton implements Serializable {


 private static final long serialVersionUID = -7604766932017737115L;


 private SerializedSingleton(){}


 private static class SingletonHelper {
 private static final SerializedSingleton instance = new SerializedSingleton();

    }


 public static SerializedSingleton getInstance() {
 return SingletonHelper.instance;

    }
 protected Object readResolve() {
 return getInstance();

    }

}
 
 
public class SingletonSerializedTest {


 public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {
 SerializedSingleton instanceOne = SerializedSingleton.getInstance();
 ObjectOutput out = new ObjectOutputStream(new FileOutputStream(
 "filename.ser"));
 out.writeObject(instanceOne);
 out.close();


 // deserialize from file to object
 ObjectInput in = new ObjectInputStream(new FileInputStream(
 "filename.ser"));
 SerializedSingleton instanceTwo = (SerializedSingleton) in.readObject();
 in.close();


 System.out.println("instanceOne hashCode="+instanceOne.hashCode());
 System.out.println("instanceTwo hashCode="+instanceTwo.hashCode());



    }

}