ui/src/main/java/ucar/util/prefs/ui/PrefPanel.java
public void addActionListener(ActionListener l) {
    listenerList.add(java.awt.event.ActionListener.class, l);
  }
public void addActionListener(ActionListener l) {
    listenerList.add(java.awt.event.ActionListener.class, l);
  }
Add listener: action event sent if "apply" button is pressed