code: public void addListener(ISUPListener listener) {
          if (listener == null) {
              throw new NullPointerException("Listener must not be null!");
          }
          if (this.listeners.contains(listener)) {
              throw new IllegalArgumentException("Listener already present: " + listener + " !");
          } else {
              this.listeners.add(listener);
          }
  
      }
docs: /*
  (non-Javadoc)
  
  @see org.mobicents.isup.ISUPProvider#addListener(org.mobicents.isup.ISUPListener )