Question on Singleton + Private Inner Classes

[fatabass]

Hi everyone,

public class Stage {
	
	private Stage(){
		System.out.println("Stage crated!");		
	}
	
	private static class StageSingletonHolder {
		static Stage instance = new Stage();
	}
	
	public static Stage getInstance(){
		return StageSingletonHolder.instance;
	}

}

I saw this example in a book and with a simple test I can see that no matter how many times I call the getInstance method, I only see one "Stage created!" in the console. So this is definitely a Singleton pattern, I can see that.

What I do not understand is how this works.

I can see that StageSingletonHolder is an "inner-class".

But what does it mean to be a static class? Is it like a static field that is shared by all the instances of the class?

So when I say Stage.getInstance -> static Stage instance = new Stage(); returns me a new Stage.
But how does it reference to the same Stage instance when I run Stage stage2 = Stage.getInstance? Why doesn't it give me a new Stage object?

Thank you.

[java.java]

StageSingletonHolder is a static inner class, in java if you want to make a class static it should always be a inner class , the top class can never be static.
Within this method the Stage object is instantiated with a static keyword, which means that at any point of time only one object of type Stage() will be available in Heap memory and every time this method is called the same object reference is returned and no new object is created. If you remove the static keyword you can create as many objects as you can. Hope this clears your doubt.

[fatabass]

Thank you.
I guess I understand it.

[Tolls]

Just as a pointer, I have never seen a situation where you actually need to write your singleton like that.
The only reason to use this is if there are more static methods in the singleton class and getInstance is normally the only static method in the singleton class, thus rendering the inner class entirely unecessary.

All you normally need is:

public class Stage {
    private static final Stage instance;

    private Stage() {
        System.out.println("Stage created");
    }

    public static Stage getInstance() {
        return instance;
    }
}

This will create the instance when the class is loaded, normally on the first access of the class, which will be the first call to getInstance.

[fatabass]

Thanks, it was confusing for me as well, it is from the book Spring in Action.

[Tolls]

It was a fad for a while, until it was pointed out how pointless it was.

If you had another static method on the code then it may make sense, since it means the singleton would not get created until getInstance was called, but I've not seen any singletons with more than getInstance as a static method.