Java generics, singletons and static methods

Question

So I have a few 'Manager' classes, for example GroupManager. All these Managers are singletons.

Using this method for instancing:

private static GroupManager groupManager = null;

private GroupManager()
{

}

public static GroupManager Instance()
{
    if (groupManager == null)
    {
        groupManager = new GroupManager();
    }
    return groupManager;
}

I'm thinking I should start to use some inheritance as they have a lot of copied methods.

The Instance() methods for each Manager is the same.

So for inheritance i can do this (obviously):

GroupManager extends Manager

Is it possible to use generics to use the same Instance method for all managers, something like:

public class Manager<E>
{
    private static E instance = null;

    public static E Instance()
    {
        if (instance == null)
        {
            instance = new E();
        }
        return instance;
    }

}

I think that makes sense :)

So then you would do GroupManager.Instance() like normal.

Answer 1

You don't understand how generics and statics work. If you have a static field or method (such as "instance" or instance()), which can be called without instantiating the class Manager, how do you expect the JVM (and the compiler even) to know what type E is supposed to be?

Here's an example, as per G_H's suggestion:

GeneralManager and AreaManager both extend Manager

The Manager class is the only one that has the getInstance() static method:

    public class Manager {

        private static Map<Class<? extends Manager>,Manager> INSTANCES_MAP = new java.util.HashMap<Class<? extends Manager>, Manager>();

//Also, you will want to make this method synchronized if your application is multithreaded,
//otherwise you mihgt have a race condition in which multiple threads will trick it into
//creating multiple instances
        public static <E extends Manager> E getInstance(Class<E> instanceClass) throws InstantiationException, IllegalAccessException {
            if(INSTANCES_MAP.containsKey(instanceClass)) {
                return (E) INSTANCES_MAP.get(instanceClass);
            } else {
                E instance = instanceClass.newInstance();
                INSTANCES_MAP.put(instanceClass, instance);
                return instance;
            }
        }
    }

Answer 2

Nope, it's not gonna work. Java uses generics at compile time for type checking, but doesn't generate extra classes or retain info regarding type parameters at runtime.

When you declare Manager<E> with that type parameter E, that's something that will only play a role in an actual instance. You could have a subclass like GroupManager extends Manager<String> or whatever, but that's not magically gonna generate a variety of the static method.

Static methods and members belong with a class, not an instance. So trying to use generics there, which are intended for typing instances, isn't gonna fly.