Java create map between classes and classes with generic type of key classes

Question

I have a class Saver<T> with one generic type argument <T>. In my Saver<T> class, I'm attempting to define the following static map:

public abstract class Saver<T> {

    private static final Map<Class<E>, Class<? extends Saver<E>>> DEFAULT_SAVERS = new HashMap<>();

}

Of course, this gives me an error because the compiler doesn't know what E is. How can I create this map? The map should have Classes for keys, and Savers with that class as its generic type as its values. Is this possible? How can I do this?

Answer 1

There's no typesafe way to declare it. You'll have to use wildcards and control how it's accessed to prevent heap pollution:

private static Map<Class<?>, Saver<?>> DEFAULT_SAVERS = new HashMap<>();

public static <T> void put(Class<T> clazz, Saver<T> saver) {
    DEFAULT_SAVERS.put(clazz, saver);
}

public static <T> Saver<T> get(Class<T> clazz) {
    return (Saver<T>)DEFAULT_SAVERS.get(clazz);
}