How to wire one pane to another

Here’s an example using the observer pattern, also seen here, here and here. Note that it would also be possible to listen to the combo’s model.

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.*;

/**
* @see http://en.wikipedia.org/wiki/Observer_pattern
* @see https://stackoverflow.com/a/10523401/230513
*/
public class PropertyChangeDemo {

    public PropertyChangeDemo() {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.add(new ObserverPanel());
        f.pack();
        f.setLocationByPlatform(true);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                PropertyChangeDemo example = new PropertyChangeDemo();
            }
        });
    }
}

class ObserverPanel extends JPanel {

    private JLabel title = new JLabel("Value received: ");
    private JLabel label = new JLabel("null", JLabel.CENTER);

    public ObserverPanel() {
        this.setBorder(BorderFactory.createTitledBorder("ObserverPanel"));
        JPanel panel = new JPanel(new GridLayout(0, 1));
        panel.add(title);
        panel.add(label);
        this.add(panel);
        ObservedPanel observed = new ObservedPanel();
        observed.addPropertyChangeListener(new PropertyChangeListener() {

            @Override
            public void propertyChange(PropertyChangeEvent e) {
                if (e.getPropertyName().equals(ObservedPanel.PHYSICIST)) {
                    String value = e.getNewValue().toString();
                    label.setText(value);
                }
            }
        });
        this.add(observed);
    }
}

class ObservedPanel extends JPanel {

    public static final String PHYSICIST = "Physicist";
    private static final String[] items = new String[]{
        "Alpher", "Bethe", "Gamow", "Dirac", "Einstein"
    };
    private JComboBox combo = new JComboBox(items);
    private String oldValue;

    public ObservedPanel() {
        this.setBorder(BorderFactory.createTitledBorder("ObservedPanel"));
        combo.addActionListener(new ComboBoxListener());
        this.add(combo);
    }

    private class ComboBoxListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent ae) {
            String newValue = (String) combo.getSelectedItem();
            firePropertyChange(PHYSICIST, oldValue, newValue);
            oldValue = newValue;
        }
    }
}