code: public boolean add (E e) {
  		if (e == null) throw new IllegalArgumentException("Element cannot be null.");
  		if (uniqueness && !set.add(e)) return false;
  		int i = size;
  		if (i >= queue.length) growToSize(i + 1);
  		size = i + 1;
  		if (i == 0)
  			queue[0] = e;
  		else
  			siftUp(i, e);
  		return true;
  	}
docs: Inserts the specified element into this priority queue. If {@code uniqueness} is enabled and this priority queue already
  contains the element, the call leaves the queue unchanged and returns false.
  
  @return true if the element was added to this queue, else false
  @throws ClassCastException if the specified element cannot be compared with elements currently in this priority queue
  according to the priority queue's ordering
  @throws IllegalArgumentException if the specified element is null