Extending a singleton in Java

Question

What I'd like to do is just make a class that when you extend it, you automatically get the getInstance class. The problem is, when i extend it, I can't reference the child class. The only way I could see to do it is by typecasting ((ClassName)class.getInstance()) but its not very user-friendly. Any suggestions?

Answer 1

You cannot extend a proper Singleton since it's supposed to have a private constructor:

Effective Java Item 2: Enforce the singleton property with a private constructor

Answer 2

The only way to override a singleton is to have a singleton that expects to be overridden. The simplest way to do this is to provide Singleton that implements an interface (or is otherwise fully abstract itself) that internally instantiates an injected singleton upon the first use of getInstance().

public interface SingletonMethods // use a better name
{
    String getName();

    void doSomething(Object something);
}

public class Singleton // use a better name
{
    private Singleton() { /* hidden constructor */ }

    public static SingletonMethods getInstance()
    {
        return SingletonContainer.INSTANCE;
    }

    /**
     * Thread safe container for instantiating a singleton without locking.
     */
    private static class SingletonContainer
    {
        public static final SingletonMethods INSTANCE;

        static
        {
            SingletonMethods singleton = null;
            // SPI load the type
            Iterator<SingletonMethods> loader =
                ServiceLoader.load(SingletonMethods.class).iterator();

            // alternatively, you could add priority to the interface, or
            //  load a builder than instantiates the singleton based on
            //  priority (if it's a heavy object)
            // then, you could loop through a bunch of SPI provided types
            //  and find the "highest" priority one
            if (loader.hasNext())
            {
                singleton = loader.next();
            }
            else
            {
                // the standard singleton to use if not overridden
                singleton = new DefaultSingletonMethods();
            }

            // remember singleton
            INSTANCE = singleton;
        }
    }
}