how to create a generic singleton class in java?

Question

i want to create a generic singleton class in java witch gives singleton object of class which i pass in method parameter. some thing like below code : please help

public final class Listener<T extends Object>  {
    private Listener() {
    }

    public static <T> Listener<?> getInstance(Class<T> clazz) {
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder {
        public static final Listener INSTANCE = new Listener();
        // private static final Map<Class<? extends Runnable>,Listener<? extends
        // Runnable> INSTANCE = new ...;
    }
}

Answer 1

Make an enum with just one value, INSTANCE or such. Instant singleton.

If I understand you correctly, I think you want your singleton to implement some generic interface? Something like:

interface ListenerInterface<T> {
    void handleEvent(T event);
}

In that case, you can use an unchecked cast to return your enum-as-singleton in a generified way. This is safe as long as your singleton doesn't actually use the instance (or else, only assume it's an Object).

public enum MySingleton implements ListenerInterface<Object> {
    INSTANCE;

    @SuppressWarnings("unchecked")
    public static <T> ListenerInterface<T> getListener() {
        return (ListenerInterface<T>) INSTANCE;
    }

    public void handleEvent(Object event) {
        System.out.println("I am a singleton. An event happened: " + event);
    }

}

If the interface is a producer instead of consumer -- that is, it returns a T -- then you can only return null safely. Otherwise, someone else will get a ClassCastException when they try to use their T, which they think is a Foo but is actually just an Object.

Answer 2

This was going to be comment but I think it better be an answer due to character limit,

Thinking little bit deeper will it be really possible to create a generic singleton class?

A singleton class can not be instantiated neither should it be cloned.

To give any class this behavior you will have to make all the constructors private and clone method will have to be overridden to throw an exception. Now if you are going to change all your classes to implement this behavior is your way of creating singleton really generic?

Please excuse me if I interpreted the question wrongly

Answer 3

You can do something like this, but is unreliable IMHO:

public class SimpleSingleton {
private Map<String, Object> counter = new HashMap<String, Object>();

public <T> T getInstance(Class<T> clazz) throws IllegalAccessException, InstantiationException {
    T singleton = (T) counter.get(clazz.getName());
    if (singleton == null) {
        singleton = clazz.newInstance();
        counter.put(clazz.getName(), singleton);
    }
    return singleton;
}

}

Answer 4

The solution for me to this problem was designing it like this:

public interface IListener<T extends Object>  {
    void handle(T event);
}

public final class Listener implements IListener<Object> {
    private static Listener instance = new Listener();

    private Listener() {
    }

    public static Listener getInstance() {
        return instance;
    }

    @Override
    public void handle(Object event) {
        //TODO Do stuff
    }
}