Question

I’m not professional Java programmer, so this question could be simple, but I have searched the web for the answer but found nothing so far.

Lets say we have a generic class in Java:

public class C1 <T, U> {
  public /*TYPE*/ f(T t, U u) {  
    return t.g(u)
  }

The question is – how can I determine the type of the result of this generic example?
Is it somehow possible to write something like typeof(t.g(u))?

Answer 1

You cannot call t.g(u) as is because T is an unbounded type. You need to put a compile-time bound on it so the compiler knows what methods are available. Otherwise you’ll only be able to call Object methods since the only thing the compiler can infer about T is that is extends Object.

For example, if you have an interface G<U> with the g method you want to call:

public interface G<U> {
    R g(U u);
}

Then you would specify that T extends G<U>, which lets you call g(). And now you know what the return type of g() is: it’s R.

public class C1<T extends G<U>, U> {
    public R f(T t, U u) {  
        return t.g(u)
    }
}

If you want the result of g() to be dependent on types T and U then you could do something like this:

public interface G<T, U> {
    T g(U u);
}

public class C1<T, U> {
    public T f(G<T, U> g, U u) {  
        return g.g(u)
    }
}