Question

Look at the following code:

public class Foo {
    private static Foo sigleton=new Foo();
    private static int count1;
    private static int count2=0;

    private Foo (){
        count1++;
        count2++;
    }

    public static Foo getInstance(){
        return sigleton;
    }

    public static void main(String[] args) {
        //Foo f= Foo.getInstance(); // case 1
        //Foo f= new Foo(); // case 2
        System.out.println(f.count1);
        System.out.println(f.count2);
    } 
}

For each run, uncomment one of the lines in the main method.

Why is the output in case 1 and 2 different?

Answer 1

Simply because in the first case, one Foo object is constructed, but in the second case, two Foo objects are constructed.

You initialise the sigleton field statically – so when the class is loaded, the Foo constructor is always called (as you specified for the field initialiser).

Now in case 1, you’re just calling a method, which returns the sigleton object which has already been constructed – so no further constructors are called. In case 2, you explicitly construct a new Foo object – but the sigleton will still be constructed. Hence in this latter case, two objects are created, the constructor is run twice in total, and so count1 and count2 will be one greater.