Creating SingleTON without static variables

[Rahul Krishnan]

Can we create SingleTon classes without static variables.

[Hebert Coelho]

Yes, you can have instance variables and only an instance as static.

[Rob Spoor]

But you will still need one static variable, to store the singleton instance itself.

[Hebert Coelho]

Yes, but without it how would you controll the state of you class?

Unless you have a getInstance returning a new instance every time.

He asks:

Can we create SingleTon classes without static variables.

I reduced to one static, no more. [=

[Joanne Neal]

Use an Enum

[John Jai]

Please check if this enum is a valid singleton ...

enum SingletonEnum {
	INSTANCE;
	private SingletonEnum(){
		
	}
	public int add(int a, int b){
		return a + b;
	}	
}

public class EnumTester {
	public static void main(String...args){
		System.out.println(SingletonEnum.INSTANCE.add(34, 34));
	}
}

[Joanne Neal]

You don't need the constructor.

[John Jai]

Thanks! that makes all enums a singleton by itself?

[Joanne Neal]

John Jai wrote:Thanks! that makes all enums a singleton by itself?

It's probably a question of semantics.

enum SingletonEnum {   
    INSTANCE1,   
    INSTANCE2,   
    INSTANCE3;   
} 

There will only ever be one instance each of SingletonEnum.INSTANCE1, SingletonEnum.INSTANCE2 and SingletonEnum.INSTANCE3, but I guess you could argue that there are 3 instances of SingletonEnum.

[Stephan van Hulst]

Regardless, it's a form in instance control. I was thinking Joanne's point is that you can easily make a singleton like this:

enum MySingleton {
  INSTANCE
}

[Joanne Neal]

Stephan van Hulst wrote:I was thinking Joanne's point is that you can easily make a singleton like this:

You think correctly.

[Campbell Ritchie]

. . . and welcome to the Ranch

[Rob Spoor]

Technically, enum constants are static final variables of the enum class. Just decompile such an enum with JAD or similar decompilers.