Question

Possible Duplicate:
What are the reasons why Map.get(Object key) is not (fully) generic
Why does Java's TreeSet<E> remove(Object) not take an E

Why does HashSet not restrict type of argument to E here:

public boolean contains(Object o)
public boolean remove(Object o)

like it does for add()

public boolean add(E e)

I mean if the compiler is enforcing that only objects of type E are being added, then the set can’t contain/remove any other type

Answer 1

The difference is that adding must be type-safe to preserve the integrity of the collection, while item checking/removal can afford to be “type-forgiving” without the risk of harming type safety of the collection. In other words, if you add an element of a wrong type, the set will become invalid; on the other hand, if you check for a presence of an element of a wrong type, you’ll simply get back a false. Same goes for remove: if you pass an element of an incompatible type, it’s not going to be in the set ⁺, so the removal is going to be a no-op.

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⁺ Unless you put it in through a hack that exploits type erasure.