Question

Is it possible to declare a Map that maps from keys of a specific subclass to values of a specific subclass but ensuring that both classes share the same Type parameter?

For the background:

both ClassA and ClassB implement behaviour for a common resource

public abstract  class ClassA<T> {
      public abstract T getResource() ;
}

public abstract class classB<T> {
       public  abstract void consoumeResource(T resource);
}

i want to map from implementations of ClassA and ClassB and ensure only “compatible” pairs can be put together in one entry.

Answer 1

Another way would be to provide your own Map implementation. There’s not much code needed if you extend an existing implementation, and use your new type:

public class CompatibleHashMap<T> extends HashMap<ClassA<T>, ClassB<T>> {

}

Now, a CompatibleHashMap<String> only lets you put ClassA<String> as keys and ClassB<String> as values.

EDIT:

As you mentioned in your comment, this way you are tying yourself to a Map implementation. You can overcome this by doing something like the following:

public class CompatibleMap<T> implements Map<ClassA<T>, ClassB<T>> {

    private Map<ClassA<T>, ClassB<T>> map;

    public CompatibleMap(Map<ClassA<T>, ClassB<T>> map) {
        this.map = map;
    }

    @Override
    public Set<List<T>> keySet() {
        return map.keySet();
    }
    // ... implement all other Map methods by calling the method on map.
}

You can then instantiate it like

CompatibleMap<String> map = new CompatibleMap<>(new HashMap<ClassA<String>, ClassB<String>>());

This way, you are not tied to a specific Map implementation, and the compiler will throw an error if the generic types of the map, ClassA and ClassB are not the same.