Java Tips - How to implement a graphical list selection monitor

This Java Swing tip illustrates a method of implementing a graphical list selection monitor. This includes a listener that responds to the events and correspondingly updates the buttons.

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import javax.swing.event.*;



public class SelectionMonitor extends JPanel {

    

    String label[] = { "Zero","One","Two","Three","Four","Five","Six",

    "Seven","Eight","Nine","Ten","Eleven","Twelve" };

    

    JCheckBox checks[] = new JCheckBox[label.length];

    JList list;

    

    public SelectionMonitor() {

        

        setLayout(new BorderLayout());

        

        list = new JList(label);

        JScrollPane pane = new JScrollPane(list);

        

        //  Format the list and the buttons in a vertical box

        Box rightBox = new Box(BoxLayout.Y_AXIS);

        Box leftBox = new Box(BoxLayout.Y_AXIS);

        

        //  Monitor all list selections

        list.addListSelectionListener(new RadioUpdater());

        

        for(int i=0; i < label.length; i++) {

            checks[i] = new JCheckBox("Selection " + i);

            checks[i].setEnabled(false);

            rightBox.add(checks[i]);

        }

        

        leftBox.add(pane);

        add(rightBox, BorderLayout.EAST);

        add(leftBox, BorderLayout.WEST);

    }

    

    public static void main(String s[]) {

        

        JFrame frame = new JFrame("Selection Monitor");

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.setContentPane(new SelectionMonitor());

        frame.pack();

        frame.setVisible(true);

        

    }

    

    // Inner class that responds to selection events to update the buttons

    class RadioUpdater implements ListSelectionListener {

        

        public void valueChanged(ListSelectionEvent e) {

            //  If either of these are true, the event can be ignored.

            if ((!e.getValueIsAdjusting()) || (e.getFirstIndex() == -1))

                return;

            

            //  Change the radio button to match the current selection state

            //  for each list item that reported a change.

            for (int i = e.getFirstIndex(); i <= e.getLastIndex(); i++) {

                checks[i].setSelected(

                        ((JList)e.getSource()).isSelectedIndex(i));

            }

        }

    }

}
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