title: Map with key only - for contains checking
body: <p>I have to store a <em>close list</em> for this I defined a Map -</p>
  
  <p><code>Map&lt;Node, Boolean&gt; closeList = new HashMap&lt;Node, Boolean&gt;()</code>  </p>
  
  <p>now for checking whether a node is exist in this map I use -</p>
  
  <p><code>boolean binExists = closeList .containsKey(node)</code></p>
  
  <p>seems that the <code>value-boolean</code> of the map is unnecessary .</p>
  
  <p>Have you any better idea for this check using HashMap mode (O(1)) ? </p>