Question

I have a JFrame that contains a ‘display’ JPanel with JTextField and a ‘control’ JPanel with buttons that should access the contents of the display JPanel. I think my problem is related on how to use the observer pattern, which in principle I understand. You need to place listeners and update messages, but I don’t have a clue where to put these, how to get access from one panel to the other and maybe if necessary to introduce a ‘datamodel’ class. For example, I want to access the contents of the JTextField from the control panel and I use an anonymous action listener as follows:

JButton openfile = new JButton('Convert file'); openfile.addActionListener(new ActionListener() {      public void actionPerformed(ActionEvent e) {          openButtonPressed();     }            }); 

Answer 1

You need to reduce the coupling between these objects.

You can have a master object, that owns all the text fields and the button ( the panels are irrelevant )

Then a separete actionlistener within that master object ( I call it mediator see mediator pattern )

That action listener performs a method on the mediator which in turn take the values from the textfields and create perhaps a transfer object.

This way you reduce the coupling between the panels, textfields etc. and let the control in one place ( the mediator ) that is, you don’t let them know each other.

You can take a look at the code in this question: https://stackoverflow.com/questions/324554/#324559

It shows these concepts in running code.

BTW the observer pattern is already implemented in the JTextField, JButton, ActionListener etc. You just need to add the hooks.

I hope this helps.

EDIT Joined two answers into one.

This is the code.

class App { // this is the mediator      // GUI components.     private JFrame frame;     private JTextField name;     private JTextField count;     private JTextField date;     // Result is displayed here.     private JTextArea textArea;      // Fired by this button.     private JButton   go;      private ActionListener actionListener;       public App(){         actionListener = new ActionListener(){             public void actionPerformed( ActionEvent e ){                 okButtonPressed();             }         };     }      private void okButtonPressed(){          // template is an object irrelevant to this code.          template.setData( getData() );          textArea.setText( template.getTransformedData() );     }       public void initialize(){          frame     = new JFrame('Code challenge v0.1');         frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );          name     = new JTextField();         count    = new JTextField();         date     = new JTextField();         textArea = new JTextArea();         go       = new JButton('Go');         go.addActionListener( actionListener ); // prepare the button.          layoutComponents(); // a lot of panels are created here. Irrelevant.     } } 

Complete and running code can be retrieved here:

It is important to favor composition over inheritance when possible.