To listen to focus change events between components, install a listener with the keyboard focus manager. If you need the ability to veto (reject) a focus change, install a vetoable listener with the keyboard focus manager.

    KeyboardFocusManager.getCurrentKeyboardFocusManager()
        .addPropertyChangeListener(new FocusChangeListener());
    KeyboardFocusManager.getCurrentKeyboardFocusManager()
        .addVetoableChangeListener(new FocusVetoableChangeListener());
    
    class FocusChangeListener implements PropertyChangeListener {
        public void propertyChange(PropertyChangeEvent evt) {
            Component oldComp = (Component)evt.getOldValue();
            Component newComp = (Component)evt.getNewValue();
    
            if ("focusOwner".equals(evt.getPropertyName())) {
                if (oldComp == null) {
                    // the newComp component gained the focus
                } else {
                    // the oldComp component lost the focus
                }
            } else if ("focusedWindow".equals(evt.getPropertyName())) {
                if (oldComp == null) {
                    // the newComp window gained the focus
                } else {
                    // the oldComp window lost the focus
                }
            }
        }
    }
    
    class FocusVetoableChangeListener implements VetoableChangeListener {
        public void vetoableChange(PropertyChangeEvent evt) throws PropertyVetoException {
            Component oldComp = (Component)evt.getOldValue();
            Component newComp = (Component)evt.getNewValue();
    
            if ("focusOwner".equals(evt.getPropertyName())) {
                if (oldComp == null) {
                    // the newComp component will gain the focus
                } else {
                    // the oldComp component will lose the focus
                }
            } else if ("focusedWindow".equals(evt.getPropertyName())) {
                if (oldComp == null) {
                    // the newComp window will gain the focus
                } else {
                    // the oldComp window will lose the focus
                }
            }
    
            boolean vetoFocusChange = false;
            if (vetoFocusChange) {
                throw new PropertyVetoException("message", evt);
            }
        }

e609. Listening to All Focus Changes Between Components in an Application