Java Generics (Wildcards)

In your first question, <? extends T> and <? super T> are examples of bounded wildcards. An unbounded wildcard looks like <?>, and basically means <? extends Object>. It loosely means the generic can be any type. A bounded wildcard (<? extends T> or <? super T>) places a restriction on the type by saying that it either has to extend a specific type (<? extends T> is known as an upper bound), or has to be an ancestor of a specific type (<? super T> is known as a lower bound).

The Java Tutorials have some pretty good explanations of generics in the articles Wildcards and More Fun with Wildcards.

If you have a class hierarchy A, B is a subclass of A, and C and D both are subclass of B like below

class A {}class B extends A {}class C extends B {}class D extends B {}

Then

List<? extends A> la;la = new ArrayList<B>();la = new ArrayList<C>();la = new ArrayList<D>();List<? super B> lb;lb = new ArrayList<A>(); //finelb = new ArrayList<C>(); //will not compilepublic void someMethod(List<? extends B> lb) {    B b = lb.get(0); // is fine    lb.add(new C()); //will not compile as we do not know the type of the list, only that it is bounded above by B}public void otherMethod(List<? super B> lb) {    B b = lb.get(0); // will not compile as we do not know whether the list is of type B, it may be a List<A> and only contain instances of A    lb.add(new B()); // is fine, as we know that it will be a super type of A }

A bounded wildcard is like ? extends B where B is some type. That is, the type is unknown but a "bound" can be placed on it. In this case, it is bounded by some class, which is a subclass of B.

Josh Bloch also has a good explanation of when to use super and extends in this google io video talk where he mentions the Producer extends Consumer super mnemonic.

From the presentation slides:

Suppose you want to add bulk methods to Stack<E>

void pushAll(Collection<? extends E> src);

– src is an E producer

void popAll(Collection<? super E> dst);

– dst is an E consumer