Question

I have the following code:

public static<F, S> void xyz() {
  class Pair<X, Y> {}    
  class Role<F> {}

  Map<?, List<Pair<?, ?>>> map = null;
  Role<F> role = null;

  List<Pair<F, S>> result = (List<Pair<F, S>>) map.get(role);
}

Unfortunately, Java complains about the cast on the last line. Why does this happen?

In my option, this is counterintuitive. A cast should be allowed in every place where there’s a possibility of a correct type correspondence.

Answer 1

“A cast should be allowed in every place where there’s a possibility of a correct type correspondence.” In this case, there is NO possibility of a correct type correspondence.

An object can never be an instance of both List<A> and List<B>, if A and B are concrete and not the same, even if A is a subtype of B or whatever. For example, you cannot have an object that is both List<String> and List<Object>. There is (conceptually) a specific type parameter for each object. You have to understand this part first.

Your example is just the same — List<Pair<F, S>> and List<Pair<?, ?>>. For the same reason as with String and Object above, an object that is of type List<Pair<?, ?>> can never also be List<Pair<F, S>>.

Update: More explanation in case it helps. @Konstantin: If you write Pair<?, ?>, then those type parameters at the top level, which are ? (wildcards), are flexible. Pair<T1, T2> is compatible with Pair<?, ?>. However, in List<Pair<?, ?>>, the ? is not at the top level. The type parameter at the top level is Pair<?, ?>, which is not a wildcard, and thus is not flexible. It doesn’t matter that you happen to have ? deeper down. If you have List<? extends Pair<?, ?>>, then the top level is a wildcard, and is flexible.

What you probably want is List<? extends Pair<?, ?>>. Perhaps it will help you to consider the difference between List<Pair<?, ?>> and List<? extends Pair<?, ?>>. List<Pair<?, ?>> says that this is a List, and it type parameter is exactly the type Pair<?, ?> (and nothing else, not Pair<T1, T2>, etc.). List<? extends Pair<?, ?>> says that this is a List, and it type parameter is something which is a subtype of Pair<?, ?> (and this includes Pair<T1, T2>).