code: @Override
      public E peek() {
          @SuppressWarnings("unchecked") E result = (E) elements[head];
          // elements[head] is null if queue empty
          return result;
      }
docs: Retrieves, but does not remove, the head of the queue represented by
  this queue, or returns <tt>null</tt> if this queue is empty.
  
  @return the head of the queue represented by this queue, or
  <tt>null</tt> if this queue is empty