How does the JLS specify that wildcards cannot be formally used within methods?

Question

I\'ve always been wondering about some weird aspect of Java generics and the use of wildcards. Let\'s say, I have the following API:

public interface X

        

Answer 1

Not having been involved in writing the JLS, my best guess is simplicity. Imagine silly, yet (syntactically) valid expressions such as (x = new X()).set(x.get());. Tracking captures of wildcards can get tricky.

I think of capturing wildcards not as boilerplate, but as having done something right. It's a sign that your API is free of type parameters which client code does not need.

Edit: The current behavior is specified in JLS §4.5.2. The x gets capture-converted for each member access -- once for x.set(...) and once for x.get(). The captures are distinct and thus not known to represent the same type.