Java – the difference between the add and offer methods in a Queue in Java

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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?

Best Answer

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.

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