Other Ways of Singleton in Java [duplicate]

Answer 1

Here's one more way to do it :

public enum Singleton{
  INSTANCE("xyz", 123);

  // Attributes
  private String str;
  private int i;

  // Constructor
  Singleton(String str, int i){
    this.str = str;
    this.i = i;
  }
}

According to Josh Bloch's Effective Java, this is the best way to implement Singleton in Java. Unlike implementations that involve a private static instance field, which can be multiply instantiated through the abuse of reflection and/or serialization, the enum is guaranteed to be a singleton.

The main limitation with enum singletons is that they are always instantiated at class loading time and can't be lazily instantiated. So if, for example, you want to instantiate a singleton using run-time arguments, you'll have to use a different implementation (preferably using double-checked locking).