Assign a subclass of a Generic class to a super class of this class

Question

I have couple of supplied interfaces

public interface Finder,T extends Item> {
    public S find(S s, int a);
}

public interf         


        

Answer 1

Since you can't change the interfaces, you have no choice but to do brute cast.

In a more general discussion, what we need here is "self type", we want to say that a method invocation foo.bar() should return the static type of foo. Usually self type is wanted for fluent API where the method should return foo itself. In your case you want to return a new object.

In java there's no satisfactory answer for self type. One trick is through self referenced type paramter like Foo>, however it is very ugly, and it cannot really enforce that any subtype Bar must be a Foo. And the trick won't help in your case at all.

Another trick may work

public interface Stack {
    > X getCopy();
}

here, the caller supplies the exact return type.

     S stack = ....;
     ...
     stack = s.getCopy();
     // compiles, because X is inferred to be S

This trick helps to simplify call sites. However brute casts still exists, hidden in implementations of getCopy(). This trick is dangerous and caller must know what it is doing. Personally I wouldn't do it; it's better for force caller to do the cast.