Question

Is there a way to say “this method returns this” using Generics?

Of course, I want to override this method in subclasses, so the declaration should work well with @Override.

Here is an example:

class Base {
    public Base copyTo (Base dest) {
        ... copy all fields to dest ...
        return this;
    }
}
class X extends Base {
    @Override
    public X copyTo (X dest) {
        super.copyTo (dest);
        ... copy all fields to dest ...
        return this;
    }
}

public <T extends Base> T copyTo (Base dest) doesn’t work at all: I get “Type mismatch: Can’t convert from Base to T”. If I force it with a cast, the override fails.

Answer 1

No, there’s no way of expressing that. Just declare the method to return the type of the class. Java has covariant return types, so you can override a method to return a more specific type anyway.

If you wanted to have some marker for this, you could always introduce your own annotation – but don’t expect any other tools to take any particular notice of it.

EDIT: the answer from oxbow_lakes does indeed give something which will work in most cases, but I believe there are ways of fooling it such that you’re actually dealing with a different type. (From memories of experimentation, anyway.) Note that this is similar to how Java enums work.