Difference between <? super T> and <? extends T> in Java [duplicate]

Answer 1

The most confusing thing here is that whatever type restrictions we specify, assignment works only one way:

baseClassInstance = derivedClassInstance;

You may think that Integer extends Number and that an Integer would do as a , but the compiler will tell you that cannot be converted to Integer (that is, in human parlance, it is wrong that anything that extends number can be converted to Integer):

class Holder {
    T v;
    T get() { return v; }
    void set(T n) { v=n; }
}
class A {
    public static void main(String[]args) {
        Holder he = new Holder();
        Holder hs = new Holder();

        Integer i;
        Number n;
        Object o;

        // Producer Super: always gives an error except
        //       when consumer expects just Object
        i = hs.get(); //  cannot be converted to Integer
        n = hs.get(); //  cannot be converted to Number
                      //  cannot be converted to ... (but
                      //       there is no class between Number and Object)
        o = hs.get();

        // Consumer Super
        hs.set(i);
        hs.set(n);
        hs.set(o); // Object cannot be converted to 

        // Producer Extends
        i = he.get(); //  cannot be converted to Integer
        n = he.get();
        o = he.get();

        // Consumer Extends: always gives an error
        he.set(i); // Integer cannot be converted to 
        he.set(n); // Number cannot be converted to 
        he.set(o); // Object cannot be converted to 
    }
}

hs.set(i); is ok because Integer can be converted to any superclass of Number (and not because Integer is a superclass of Number, which is not true).

EDIT added a comment about Consumer Extends and Producer Super -- they are not meaningful because they specify, correspondingly, nothing and just Object. You are advised to remember PECS because CEPS is never useful.