Question

While we are creating a class using Singleton Pattern I am able to clone in the same class.
It means the Singleton Pattern rule was violated. Can any one please clarify my doubt.
My Code is:

class SingletonObject implements Cloneable
{
    private SingletonObject()
    {
        // no code req'd
    }

    public static SingletonObject getSingletonObject()
    {
        SingletonObject obj = null;
      if (ref == null){
          // it's ok, we can call this constructor
          ref = new SingletonObject();  

          try {
              obj = (SingletonObject)ref.clone();
            if(obj.equals(ref)){
                System.out.println("both objects are same");
            }else{
                System.out.println("both objects are not same");
            }
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

      }    
      return obj;
    }

    private static SingletonObject ref;
}

Answer 1

Good observation. Your code is a good example of why many Java developers consider clone() to be problematic:

It is a mechanism which circumvents many restrictions that you can place on the creation of objects. It can thus be used to duplicate singletons. This is (arguably) a design flaw of Java. The only solution is to not use clone, or not to implement a public clone() method in your class.

In practice it’s not much of a problem, because clone will only work if the class explicitly implements it, which you would not do for a singleton. It’s still ugly though.