Read Notes:[Effective Java] Enforce the singleton with a private constructor or an enum type

// Singleton with public final field
public class Elvis {
    public static final Elvis INSTANCE = new Elvis();
    private Elvis() { ... }
    public void leaveTheBuilding() { ... }
}

// Singleton with static factory
public class Elvis {
    private static final Elvis INSTANCE = new Elvis();
    private Elvis() { ... }
    public static Elvis getInstance() { return INSTANCE; }
    public void leaveTheBuilding() { ... }
}

  

    It is not sufficient merely to add implements Serializable to its declaration. To maintain the singleton guarantee, you have to declare all instance fields transient and provide a readResolve method. Otherwise, each time a serialized instance is deserialized, a new instance will be created.

// readResolve method to preserve singleton property
private Object readResolve() {
    // Return the one true Elvis and let the garbage collector
    // take care of Elvis impersonator
    return INSTANCE;
}

As a release 1.5, there is a third approach to implementing singletons. Simply make an enum type with one element:

// Enum singleton - the preferred approach
public enum Elvis {
    INSTANCE;
    public void leaveTheBuilding() { ... }
}

    While this approach has yet to be widely adopted, a single-element enum type is the best way to implement a singleton.