ArrayList<T> vs ArrayList<?>

http://download.oracle.com/javase/tutorial/java/generics/wildcards.html

Note: It's also possible to specify a lower bound by using the super keyword instead of extends. The code <? super Animal>, therefore, would be read as "an unknown type that is a supertype of Animal, possibly Animal itself". You can also specify an unknown type with an unbounded wildcard, which simply looks like <?>. An unbounded wildcard is essentially the same as saying <? extends Object>.