Question

Take the PriorityQueue for example http://java.sun.com/j2se/1.5.0/docs/api/java/util/PriorityQueue.html#offer(E)

Can anyone give me an example of a Queue where the add and offer methods are different?

According to the Collection doc, the add method will often seek to ensure that an element exists within the Collection rather than adding duplicates. So my question is, what is the difference between the add and offer methods?

Is it that the offer method will add duplicates regardless? (I doubt that it is because if a Collection should only have distinct elements this would circumvent that).

EDIT:
In a PriorityQueue the add and offer methods are the same method (see my answer below). Can anyone give me an example of a class where the add and offer methods are different?

Answer 1

I guess the difference is in the contract, that when element can not be added to collection the add method throws an exception and offer doesn’t.

From: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#add%28E%29

If a collection refuses to add a
particular element for any reason
other than that it already contains
the element, it must throw an
exception (rather than returning
false). This preserves the invariant
that a collection always contains the
specified element after this call
returns.

From: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Queue.html#offer%28E%29

Inserts the specified element into
this queue, if possible. When using
queues that may impose insertion
restrictions (for example capacity
bounds), method offer is generally
preferable to method
Collection.add(E), which can fail to
insert an element only by throwing an
exception.