Is this the right way to implement Singleton design pattern?

[Monica Shiralkar]

Singleton design pattern is to be used when we want to ensure that only 1 instance of the class should be created. For e.g if we have only 1 liscence for something. Is the below the right way to implement it ?

public class Singleton {

   private static Singleton singleton = new Singleton( );

 // to prevent instantiation from outside.
   private Singleton() { }

   public static Singleton getInstance( ) {
      return singleton;
   }
}

thanks.

[Junilu Lacar]

Yes, although the field is usually called instance not singleton

[Monica Shiralkar]

Thanks. I have seen the below kind of code too for singleton but did not understand whether it will have any benefit over the one I originally posted.

public class Singleton {

   private static Singleton instance = null;
   private Singleton() {
   }

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

[Paul Clapham]

Or this way:

public enum Singleton {

  INSTANCE;

}

[Campbell Ritchie]

Monica Shiralkar wrote:. . . whether it will have any benefit over the one I originally posted. . . .

No, it hasn't. It isn't thead‑safe. I think the only good way to create a singleton is what Paul showed you.
There is doubt about whether a singleton is a pattern or an anti‑pattern.

[Junilu Lacar]

Campbell Ritchie wrote:There is doubt about whether a singleton is a pattern or an anti‑pattern.

It can be a pattern or anti-pattern; it just depends on how you apply it. That said, many applications of Singleton are anti-patterns because they cause more problems than they solve.

[Monica Shiralkar]

Or this way:

public enum Singleton {

  INSTANCE;

}

And how would rest of the code be?

[Campbell Ritchie]

Monica Shiralkar wrote:.. .And how would rest of the code be?

Like this:-

[Junilu Lacar]

Monica Shiralkar wrote:

public enum Singleton {
  INSTANCE;

  // implement singleton-specific behavior here
  ...
}

And how would rest of the code be?

That is the bare minimum you need to implement an enum singleton in Java. Obviously, if your singleton has some behavior that it needs to exhibit, the rest of the class would be whatever is needed to implement that behavior.

[Monica Shiralkar]

Thanks. So this means that code which I had originally pasted is required only for pre java 5 compiler and from java 5 onwards the code is a very short one using Enum as shown.

[Paul Clapham]

Yes.

[Monica Shiralkar]

Thanks.

[Paul Clapham]

This is a good use case for the mport static statement. You can either write

import package.Singleton;

...

Singleton.INSTANCE.doSomething();

or you can write

import static package.Singleton.INSTANCE;

...

INSTANCE.doSomething();

Which suggests that you might want to name the singleton instance something which describes its purpose more accurately than just "INSTANCE".

[Campbell Ritchie]

Monica Shiralkar wrote:. . . pre java 5 compiler

Have you got a pre‑Java5 compiler? Remember Java5 is over sixteen years old.

. . . using Enum as shown.

Not Enum, which is a class, but enum please.