Higher-kinded generics in Java

Question

Suppose I have the following class:

public class FixExpr {
  Expr in;
}

Now I want to introduce a generic argument, abstract

Answer 1

I think what you're trying to do is simply not supported by Java generics. The simpler case of

public class Foo {
    public T bar() { return null; }
}

also does not compile using javac.

Since Java does not know at compile-time what T is, it can't guarantee that T is at all meaningful. For example if you created a Foo, bar would have the signature

public BufferedImage bar()

which is nonsensical. Since there is no mechanism to force you to only instantiate Foos with generic Ts, it refuses to compile.