Question

Can someone tell me why this code gives me a compile-error?

public class Main {

    public static void main(String[] args) {
        System.out.println(sum(2, 6.9));
    }

    public static <T extends Number<T>> T sum(T a, T b) {
        T result = a + b;      // compile-error here
        return result;
    }
}

Answer 1

Number is not a generic class, so you can’t parameterize it:

public abstract class Number implements java.io.Serializable {
...
}

Further, the + operator only works on primitive types like int, long, etc., rather than Number subtypes like Integer, Long, etc. (EDIT: It will operate on these by unboxing, yes, but it cannot automatically box the result in the appropriate wrapper class.)

(You’ve stumbled upon one of the reasons Number is a poor example of polymorphism. It really only performs object-to-primitive conversions.)