Difference between add(E e) and offer(E e) of ArrayDqueue Class

Question

Hi I used add and offer to add my element in last pace. Both are returning boolean and both does not throw any exception apart from NPE.

 public class ArrayDequeDemo {

  public static void main(String[] args) {


    // Create ArrayDeque elements.
    ArrayDeque<Integer> deque = new ArrayDeque<>();
    deque.add(10);
    deque.offer(30);

   }
 }

Both will add element in last place by returning a boolean.

JAVA IMPLEMENTATION

//For Add and Offer Both
   public void addLast(E e) {
    if (e == null)
        throw new NullPointerException();
    elements[tail] = e;
    if ( (tail = (tail + 1) & (elements.length - 1)) == head)
        doubleCapacity();
}

Answer 1

The two methods are equivalent.

The reason that they both exist is that the java.util.Queue interface specifies both.

The reason that java.util.Queue specifies both is that an implementation of java.util.Queue is allowed to implement capacity restrictions, and the two methods are specified to behave differently in the case that adding the element would violate that restriction; specifically, add(...) is specified to throw IllegalStateException in that case, whereas offer(...) simply returns false.

java.util.ArrayDeque, however, does not implement any capacity restrictions, so this situation does not arise with it, so the distinction does not apply.

Answer 2

The Queue documentation does a fairly good job of explaining the difference.

• add(E e) has the capability of throwing an exception if an element can't be added into the queue. This happens in case the queue is full.

• offer(E e) will return a special value (in this case, a boolean) if the value can't be added into the queue. This is useful if you're dealing with a size-limited queue but do not want to throw an exception.