public static void addListener(final DiagnosticListener listener){
  if (listener != null) {
    LISTENERS.add(listener);
  }
}

public static void addListener ( final DiagnosticListener listener ) { if ( listener != null ) { LISTENERS . add ( listener ) ; } }
add the given listener to the those notified whenever a diagnostic occurs . requests to add null are ignored . adding the same listener multiple times is the same as adding it once .
