code: public void setFilterExists( final String attributeName )
      {
          final StringBuilder sb = new StringBuilder();
          sb.append( "(" );
          sb.append( new FilterSequence( attributeName, "*", FilterSequence.MatchingRuleEnum.EQUALS ) );
          sb.append( ")" );
          this.filter = sb.toString();
      }
docs: Set up an exists filter for attribute name.  Consider the following example.
  
  <table border="1"><caption>Example Values</caption>
  <tr><td><b>Value</b></td></tr>
  <tr><td>givenName</td></tr>
  </table>
  <p><i>Result</i></p>
  <code>(givenName=*)</code>
  
  @param attributeName A valid attribute name