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## Change Listeners

- Display adds itself as a change listener to the invoice
- Display updates itself when invoice object changes state

      final Invoice invoice = new Invoice();

      final JTextArea textArea = new JTextArea(20, 40);

      ChangeListener listener = new

         ChangeListener()

         {

            public void stateChanged(ChangeEvent event)

            {

               textArea.setText(...);

            }

         };

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