哦~谢谢！

> public class TreeSet<E>
> extends AbstractSet<E>
> implements SortedSet<E>, Cloneable,

Generic class declaration: The "parameter" E is replaced by the programmer when using the class, e.g. TreeSet<String>.

> public boolean addAll(Collection<? extends E> c)

Wildcard generic parameter: It means that the parameter can be any
generic Collection where the parametrized type extends the collection.

So if you have

class Foo { ...

class Fie extends Foo { ...

set = new TreeSet<Foo>();

otherCollection = new ArrayList<Fie>();

you can do a

set.addAll(otherCollection);

because the generic parameter for the second declaration is a class
that extends the parameter class of the first.

http://www.

[ 本帖最后由 sunboy_2050 于 2011-11-18 18:03 编辑 ]

Collection<? extends E>中？是E的一个子类，例如Collection<? extends List>，此时？可以取ArrayList、LinkedList等