How to consume a List<Sub> being returned as List<Super>

Question

I have a method that returns List<Super> . say List<Super> myMethod();

When calling that method, I want to cast the returned list to List<Sub> , knowing that its runtime type will be a List of Sub .

I know that List<Sub> subs = (List<Sub>) myMethod() won't work because generics are invariant.

However, List<? super Sub> subs = myMethod() List<? super Sub> subs = myMethod() works, the compile time type becomes List<Object> , so subs.get(0).mySubMethod() won't work.

What I end up with is that I have to explicitly cast it as ((Sub) subs.get(0)).mySubMethod()

How do I do this correctly without explicit casting on each element?

Answer 1

The shortest version is:

List<Sub> subs = (List) myMethod();

List is a "raw type"; you can convert from List to any List<...> without a cast.

Note that this is circumventing the type system in a fundamentally incorrect way; the list was constructed as an ArrayList<Super> (or similar), so it's actually incorrect to trick the compiler into letting you refer to it with a reference of type List<Sub> , even if it so happens that all of the list elements are of type Sub . (The same is true of msandiford's longer version above; it may look more generics-friendly, but it's actually just another way of doing the same fundamentally incorrect thing.)

But, due to type erasure, you can generally get away with it.

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One correct approach would be to make the method itself generic:

<T extends Super> List<T> myMethod(final Class<T> clazz) {
    final List<T> ret = Collections.checkedList(new ArrayList<T>(), clazz);
    // ... add elements to ret ...
    return ret;
}