What is the difference between the add and offer methods in a Queue in Java?

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.

• offer method - tries to add an element to a queue, and returns false if the element can't be added (like in case when a queue is full), or true if the element was added, and doesn't throw any specific exception.
• add method - tries to add an element to a queue, returns true if the element was added, or throws an IllegalStateException if no space is currently available.

There is no difference for the implementation of PriorityQueue.add:

public boolean add(E e) {
    return offer(e);
}

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For AbstractQueue there actually is a difference:

public boolean add(E e) {
    if (offer(e))
        return true;
    else
        throw new IllegalStateException("Queue full");
}