title: Is there a java hash structure with keys only and no values?
body: <p>I'm looking for a structure which hashes keys without requiring a value.  When queried, it should return true if the key is found and false otherwise.  I'm looking for something similar to 
  <code>Hashtable&lt;MyClass, Boolean&gt;</code> 
  except insertion requires only a key and queries only ever return true or false, never null.</p>