Java singleton with inheritance

Question

I have a set of singleton classes and I want to avoid boilerplate code. Here is what I have now:

public class Mammal {
    protected Mammal() {}
}

public class Cat extends Mammal {
    static protected Cat instance = null;
    static public Cat getInstance() {
        if (null == instance) {
            instance = new Cat();
        }
        return instance;
    }

    private Cat() {
        // something cat-specific
    }
}

This works and there's nothing wrong with it, except that I have many subclasses of Mammal that must replicate the getInstance() method. I would prefer something like this, if possible:

public class Mammal {
    protected Mammal() {}

    static protected Mammal instance = null;

    static public Mammal getInstance() {
        if (null == instance) {
            instance = new Mammal();
        }
        return instance;
    }
}

public class Cat extends Mammal {
    private Cat() {
        // something cat-specific
    }
}

How do I do that?

Horsey

Your singleton getInstance() method is not properly implemented -- in a multi-threaded environment you can end up with multiple instances being created! – Accepted

Not an answer to your question, but your singleton pattern isn't thread-safe. There are examples in Java Concurrency in Practice. – Haire

Is a singleton really the best approach here? Singletons are best used to represent services or other things that there really is only one instance in the real world. Make sure this pattern is really suited for your application domain because I will assume your code isn't really using cats and dogs. If singletons are appropriate, I recommend using Jim Garrison's solution of using Spring; spring beans are singleton by default. – Megargee

Answer 1

You can't since constructors are neither inherited nor overridable. The new Mammal() in your desired example creates only a Mammal, not one of its subclasses. I suggest you look at the Factory pattern, or go to something like Spring, which is intended for just this situation.

Keeton

Answer 2

You can use enum to create singletons.

public enum Mammal {
    Cat {
        // put custom fields and methods here
    }
}

Dunsany

Answer 3

Make your Constructor private, this will prevent your class to be inherited by other classes.

Or the best way to create a singleton is by creating your class an enum with one enumeration.(Josh Block)

Ensepulcher