How can I implement an abstract singleton class in Java?

Question

Here is my sample abstract singleton class:

public abstract class A {
    protected static A instance;
    public static A getInstance() {
        return ins         


        

Answer 1

In addition to problems others have pointed out, having the instance field in A means that you can only have one singleton in the entire VM. If you also have:

public class C extends A {
    private C() { }
    static {
        instance = new C();
    }
    //...implementations of my abstract methods...
}

... then whichever of B or C gets loaded last will win, and the other's singleton instance will be lost.

This is just a bad way to do things.