<E> and <? extends E>

[-]

I noticed some classes have "<E>" and "?". What does it mean?

e.g:

public class TreeSet<E>
extends AbstractSet<E>
implements SortedSet<E>, Cloneable, java.io.Serializable




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

 

[Tor Iver Wilhelmsen]

public class TreeSet<E>
extends AbstractSet<E>
implements SortedSet<E>, Cloneable, java.io.Serializable

Generic class declaration: The "parameter" E is replaced by the

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.