I always had a hard time with understanding wildcarded generics in Java, as in <? extends A> or <? super A>. While More Fun with Wildcards is a good place for start, you come to a TL;DR point after some time. Lucky for me, Heikki (fizzie) Kallasjoki provided me a succinct and complete code snippet that summarizes the entire issue on freenode ##java channel.

First, let’s assume we have two classes A and B as follows.

class A {}

class B extends A {}

And here is the code snippet that summarizes the entire issue with wildcarded generics in Java.

static void f(List<? extends A> l) {
    A a = l.get(0); /* okay */
    l.add(new A()); /* not okay, l can be of type List<B> */
}

static void g(List<A> l) {
    A a = l.get(0); /* okay */
    l.add(new A()); /* okay */
}

static void h(List<? super A> l) {
    A a = l.get(0); /* not okay, l can be of type List<Object> */
    l.add(new A()); /* okay */
}

static {
    List<B> l = new ArrayList<>();
    f(l);           /* okay */
    g(l);           /* not okay, List<B> is not of type List<A> */
    h(l);           /* not okay, List<B> is not of type List<? super A> */

    List<Object> l2 = new ArrayList<>();
    f(l2);          /* not okay, List<Object> is not of type List<? extends A> */
    g(l2);          /* not okay, List<Object> is not of type List<A> */
    h(l2);          /* okay */
}

Hope that helps.