Is it safe to cast from List<Optional> to List<Optional<?>>?

Question

If you have a raw type in Java, you can safely assign/cast this to the same type with an unbounded wildcard. For example a List can be safely cast to a List<?>, which removes its raw nature and allows you to use it in a safe (type-checked) manner¹.

On the other hand, Java doesn't let you cast from a List itself parameterized with a raw type, like List<Optional> to a list of the same type parameter with an unbounded wildcard, like List<Optional<?>>.

You can still do it by dropping all the way down a raw List and back up again (implicitly via the assignment):

List<Optional> rawOptionalList = null;
List<Optional<?>> wildcardOptionalList = (List)rawOptionalList;

Of course, this triggers warning about unchecked conversion (from List to List<Optional<?>>).

It seems to me though that this conversion is guaranteed safe: isn't List<Optional<?>> just as safe as List<Optional> in the same way that casting a raw Optional to an Optional<?> is safe?

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¹ ... but you'll never be able to add anything to this list, since nothing will match the capture of ? for the add(?) method. That's the price you pay for safety.

Answer 1

Yes, it's safe. The generic check is just at compile time.