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 are both subclasses 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>(); //fine
lb = new ArrayList<C>(); //will not compile

public 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.