title: Storing data in HashSets
body: <p>I am trying to store the data of <code>roomNo</code> in the <code>occupiedRooms</code> <code>HashSet</code>. I'm unsure how to store the value. I have this so far: a validation condition which then, if correct, should store the <code>roomNo</code> in the <code>HashSet</code>; however, it does not.</p>
  
  <pre><code>public void checkIn (String roomNo, String guest) {
  
      if(!(validRoom(roomNo) &amp;&amp; roomTaken(roomNo))) {
          System.out.println(guest + " booked in to room number: " + roomNo);
          occupiedRooms.add(roomNo);
      }
  }
  </code></pre>