Question

I have the following two classes:

class Key<T1, T2> {}
class Pair<F, S> {}

I want to create a map from Key<T1, T2> to List<Pair<T1, T2>> where T1 and T2 are different for each map’s entry. I am trying to implement it this way:

class KeyToListMap {
  private Map<Key<?, ?>, List<Pair<?, ?>>> myItems = new HashMap<Key<?, ?>, List<Pair<?, ?>>>();


  public<T1, T2> List<Pair<T1, T2>> getList(Key<T1, T2> key) {
    if (!myItems.containsKey(key)) {
      myItems.put(key, new ArrayList<Pair<T1, T2>>());
    }
    return (List<Pair<T1, T2>>) myItems.get(key);
  }
}

However I got two type errors:

put(test.Key<?,?>,java.util.List<test.Pair<?,?>>) in java.util.Map<test.Key<?,?>,java.util.List<test.Pair<?,?>>> cannot be applied to (test.Key<T1,T2>,java.util.ArrayList<test.Pair<T1,T2>>)

inconvertible types
found   : java.util.List<test.Pair<?,?>>
required: java.util.List<test.Pair<T1,T2>>

How can I express this in Java without resorting to using raw types?

Answer 1

Use

? extends

in Map declaration