title: TreeMap<Integer, String> returns an single value in java
body: <pre><code>TreeMap&lt;Integer, String&gt; map = new TreeMap&lt;Integer, String&gt;();
  
      map.put(1, "example");
      map.put(1, "example2");
      map.put(1, "example3");
  
      Iterator&lt;Integer&gt; itr = map.keySet().iterator();
  
      while (itr.hasNext()) {
          Object obj = itr.next();
          System.out.println(map.get(obj));
  
      }
  </code></pre>
  
  <p>It always returns <code>"example3"</code>.  May I know why?  Why can I not retrieve all the values?
  How are the keys in <code>Treemap</code> ordered?  The keys should be from 0,1,2...</p>