Question

Why does this code print 2.0 and not 1.0?

abstract class B<T extends Number> {
    abstract Number f(T j);
}

class A<T extends Number> extends B<T> {
    public Number f(Float j) {
        return 1f;
    }

    public Number f(T j) {
        return j;
    }
}

public class J {
    public static void main(String[] args) {
        B<Float> a = new A<>();
        Number r = a.f(2f);
        System.out.println(r);
    }
}

Answer 1

So the heart of the problem here is that you have declared the variable a to be of type B. Since the B class has only one method, that’s the one that wins. However, in your main, if you change the type of a to be of type A, you’ll notice that it will not compile because it is ambiguous. However, if you did change the method in the class A to accept a primitive instead, and in the main() method defined the variable a to be of type A, it would result in 1.0. I.e., the following will result in printing 1.0:

class A<T extends Number> extends B<T> {
    public Number f(float j) {
        return 1f;
    }

    public Number f(T j) {
        return j;
    }
}


public class J {
    public static void main(String[] args) {
        A<Float> a = new A<>();
        Number r = a.f(2f);
        System.out.println(r);
    }
}