Question

Possible Duplicate:
Why aren’t Java Collections remove methods generic?

The Java Collection<E> interface has a contains method with the following signature:

boolean contains(Object o)

Since the interface is generic with type E shouldn’t the signature be
boolean contains(E o)

to reflect that and only allow arguments of type E.

The same question can be asked regarding the remove(Object o) method

Answer 1

The contains and remove methods accept any object because the they accept (and can succeed with) objects that might not be instances of E. The contract for contains is:

returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e))

Note that o need not actually be an object in the collection; it must merely pass the equals test.

The same idea goes for remove.

See also this thread, where it is pointed out that making contains and remove generic would have broken a lot of existing, perfectly valid code.