How to declare a map with variable generics?

Question

I have a Map whose keys are of generic type Key, and values are of type List. If the key is an instance of Key

Answer 1

This is what you want:

public class Test<T> extends HashMap<T, List<T>>
{
}

If you don't want a HashMap as the super class then change it to whatever concrete class you want.

Answer 2

I'm not aware of any existing library that does precisely this but it is not too hard to implement yourself. I've done something similar a few times in the past. You cannot use the standard Map interface but you can use a hash map inside to implement your class. To start, it might look something like this:

public class KeyMap {
  public static class Key<T> { }

  private final HashMap<Object,List<?>> values = new HashMap<Object,List<?>>();

  public <T> void put(Key<T> k, List<T> v) {
    values.put(k, v);
  }

  public <T> List<T> get(Key<T> k) {
    return (List<T>)values.get(k);
  }

  public static void main(String[] args) {
    KeyMap a = new KeyMap();
    a.put(new Key<String>(), new ArrayList<String>());
    a.get(new Key<Integer>());
  }
}