inheritance - 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 instance;
    }
    //...rest of my abstract methods...
}

And here is the concrete implementation:

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

Unfortunately I can't get the static code in class B to execute, so the instance variable never gets set. I have tried this:

Class c = B.class;
A.getInstance() - returns null;

and this

ClassLoader.getSystemClassLoader().loadClass("B");
A.getInstance() - return null;

Running both these in the eclipse debugger the static code never gets executed. The only way I could find to get the static code executed is to change the accessibility on B's constructor to public, and to call it.

I'm using sun-java6-jre on Ubuntu 32bit to run these tests.

Answer 1

Abstract Singleton? Doesn't sound viable to me. The Singleton pattern requires a private constructor and this already makes subclassing impossible. You'll need to rethink your design. The Abstract Factory pattern may be more suitable for the particular purpose.