code: public boolean offer(E e) {
          if (e == null)
              throw new NullPointerException();
          modCount++;
          int i = size;
          if (i >= queue.length)
              grow(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.
  
  @return {@code true} (as specified by {@link Queue#offer})
  @throws ClassCastException if the specified element cannot be
  compared with elements currently in this priority queue
  according to the priority queue's ordering
  @throws NullPointerException if the specified element is null