JCheckbox - Part 2

To make the Checkboxes actually work, we need to add an ItemListener, just like we did for JToggleButton. This is very simple, and works a lot like the ActionListener we used for the JButton.

This means that any time the JCheckBox is selected or deselected by the user, the ItemListener will catch this and throw it to a method called ItemStateChanged(). ItemStateChanged will then change the program however we decide.

Let's see the previous example edited slightly to incorporate the ItemListener.

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import javax.swing.*;
import java.awt.*;
import java.awt.Color;
import java.awt.event.*;

public class CheckBoxExample_Extended implements  ItemListener{
    
    JCheckBox redCB, blueCB, greenCB, yellowCB;
    JPanel redBox, blueBox, greenBox, yellowBox;

    public JPanel createContentPane (){
        
        JPanel totalGUI = new JPanel();
        
        // We create four checkboxes to control what is currently on-screen.
        // At the start, we set the red checkbox to 'ticked' or selected.
        // We also add an itemListener for each JCheckBox
        
        JPanel checkBoxPanel = new JPanel();
        checkBoxPanel.setLayout(new BoxLayout(checkBoxPanel, BoxLayout.PAGE_AXIS));
        checkBoxPanel.add(Box.createRigidArea(new Dimension(10,0)));

        redCB = new JCheckBox("Red");
        redCB.setSelected(true);
        redCB.addItemListener(this);
        
        checkBoxPanel.add(redCB);
        checkBoxPanel.add(Box.createHorizontalGlue());
        
        blueCB = new JCheckBox("Blue");
        blueCB.addItemListener(this);
        
        checkBoxPanel.add(blueCB);
        checkBoxPanel.add(Box.createHorizontalGlue());
        
        greenCB = new JCheckBox("Green");
        greenCB.addItemListener(this);
        
        checkBoxPanel.add(greenCB);
        checkBoxPanel.add(Box.createHorizontalGlue());
        
        yellowCB = new JCheckBox("Yellow");
        yellowCB.addItemListener(this);
        
        checkBoxPanel.add(yellowCB);
        checkBoxPanel.add(Box.createRigidArea(new Dimension(10, 0)));
        
        // Now we create a simple JPanel that displays our four coloured boxes.

        JPanel boxPanel = new JPanel(new GridLayout(2, 2, 20, 20));
        
        redBox = createSquareJPanel(Color.red, 50);
        blueBox = createSquareJPanel(Color.blue, 50);
        greenBox = createSquareJPanel(Color.green, 50);
        yellowBox = createSquareJPanel(Color.yellow, 50);
        
        // This sets all bar the red box to be hidden.
  
        blueBox.setVisible(false);
        greenBox.setVisible(false);
        yellowBox.setVisible(false);
        
        boxPanel.add(redBox);
        boxPanel.add(blueBox);
        boxPanel.add(greenBox);
        boxPanel.add(yellowBox);
        
        totalGUI.add(checkBoxPanel);
        totalGUI.add(boxPanel);
        
        totalGUI.setOpaque(true);
        return totalGUI;
    }

    // In this method, we create a square JPanel of a colour and set size
    // specified by the arguments.

    private JPanel createSquareJPanel(Color color, int size) {
        JPanel tempPanel = new JPanel();
        tempPanel.setBackground(color);
        tempPanel.setMinimumSize(new Dimension(size, size));
        tempPanel.setMaximumSize(new Dimension(size, size));
        tempPanel.setPreferredSize(new Dimension(size, size));
        return tempPanel;
    }
    
    // In itemStateChanged, we first determine if the item was selected
    // or deselected and set a temporary boolean depending.
    // Then we determine which item it was that was changed and display the 
    // coloured box to match.
    
    public void itemStateChanged(ItemEvent e){
        
        boolean visible = false;
        
        if (e.getStateChange() == ItemEvent.DESELECTED) {
            visible = false;
        }
        else
        {
            visible = true;
        }
        
        if(e.getItemSelectable() == redCB)
        {
            redBox.setVisible(visible);
        }
        else if(e.getItemSelectable() == blueCB)
        {
            blueBox.setVisible(visible);
        }
        else if(e.getItemSelectable() == greenCB)
        {
            greenBox.setVisible(visible);
        }
        else if(e.getItemSelectable() == yellowCB)
        {
            yellowBox.setVisible(visible);
        }
    }

    private static void createAndShowGUI() {

        JFrame.setDefaultLookAndFeelDecorated(true);
        JFrame frame = new JFrame("[=] JCheckBox [=]");

        CheckBoxExample_Extended demo = new CheckBoxExample_Extended();
        frame.setContentPane(demo.createContentPane());
        
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

This time, the class implements the ItemListener.

6	public class CheckBoxExample_Extended implements ItemListener{

Each JCheckBox has an ItemListener attached to it.

25	redCB.addItemListener(this);

We create the method itemStateChanged to deal with the change in state of the JCheckBoxes and pass the argument in of an ItemEvent. Firstly, we use the .getStateChange() to determine if the event is a selection or de-selection of the JCheckBox. We set a temporary variable depending on its outcome.

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        if (e.getStateChange() == ItemEvent.DESELECTED) {
            visible = false;
        }
        else
        {
            visible = true;
        }

Then we determine which box was pressed using the e.getItemSelectable() command. Using this information we set the visibility of the corresponding box.
You can get more information for these commands from the Java API

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        if(e.getItemSelectable() == redCB)
        {
            redBox.setVisible(visible);
        }
        else if(e.getItemSelectable() == blueCB)
        {
            blueBox.setVisible(visible);
        }
        else if(e.getItemSelectable() == greenCB)
        {
            greenBox.setVisible(visible);
        }
        else if(e.getItemSelectable() == yellowCB)
        {
            yellowBox.setVisible(visible);
        }

There is another way to find out if a JCheckbox is selected or not, using the .isSelected(); method. You can use the syntax .isSelected(); which will return a boolean depending if it is selected or not. Here's a change to the previous program that only refreshes the box on a button press.

Note the changes.

The addition of a refresh button. (Line 64)
The removal of ItemListener from the class and Checkboxes. (Line 6, 23, 29, 33, 37)
The addition of ActionListener to the class and refresh button. (Line 6, 65)
The actionPerformed method. (Line 90)

import javax.swing.*;
import java.awt.*;
import java.awt.Color;
import java.awt.event.*;

public class CheckBoxExample_UltraExtended implements  ActionListener{
    
    JCheckBox redCB, blueCB, greenCB, yellowCB;
    JPanel redBox, blueBox, greenBox, yellowBox;
    JButton refresh;

    public JPanel createContentPane (){
        
        JPanel totalGUI = new JPanel();
        
        // We create four checkboxes to control what is currently on-screen.
        // At the start, we set the red checkbox to 'ticked' or selected.
        
        JPanel checkBoxPanel = new JPanel();
        checkBoxPanel.setLayout(new BoxLayout(checkBoxPanel, BoxLayout.PAGE_AXIS));
        checkBoxPanel.add(Box.createRigidArea(new Dimension(10,0)));

        redCB = new JCheckBox("Red");
        redCB.setSelected(true);
        
        checkBoxPanel.add(redCB);
        checkBoxPanel.add(Box.createHorizontalGlue());
        
        blueCB = new JCheckBox("Blue");        
        checkBoxPanel.add(blueCB);
        checkBoxPanel.add(Box.createHorizontalGlue());
        
        greenCB = new JCheckBox("Green");
        checkBoxPanel.add(greenCB);
        checkBoxPanel.add(Box.createHorizontalGlue());
        
        yellowCB = new JCheckBox("Yellow");
        checkBoxPanel.add(yellowCB);
        checkBoxPanel.add(Box.createRigidArea(new Dimension(10, 0)));
        
        // Now we create a simple JPanel that displays our four coloured boxes.

        JPanel boxPanel = new JPanel(new GridLayout(2, 2, 20, 20));
        
        redBox = createSquareJPanel(Color.red, 50);
        blueBox = createSquareJPanel(Color.blue, 50);
        greenBox = createSquareJPanel(Color.green, 50);
        yellowBox = createSquareJPanel(Color.yellow, 50);
        
        // This sets all bar the red box to be hidden.
  
        blueBox.setVisible(false);
        greenBox.setVisible(false);
        yellowBox.setVisible(false);
        
        boxPanel.add(redBox);
        boxPanel.add(blueBox);
        boxPanel.add(greenBox);
        boxPanel.add(yellowBox);
        
        // Now lets quickly add a refresh button with an actionListener to determine
		// when it is pressed.
        
        refresh = new JButton("Refresh");
        refresh.addActionListener(this);
        
        totalGUI.add(checkBoxPanel);
        totalGUI.add(boxPanel);
        totalGUI.add(refresh);
        
        totalGUI.setOpaque(true);
        return totalGUI;
    }

    // In this method, we create a square JPanel of a colour and set size
    // specified by the arguments.

    private JPanel createSquareJPanel(Color color, int size) {
        JPanel tempPanel = new JPanel();
        tempPanel.setBackground(color);
        tempPanel.setMinimumSize(new Dimension(size, size));
        tempPanel.setMaximumSize(new Dimension(size, size));
        tempPanel.setPreferredSize(new Dimension(size, size));
        return tempPanel;
    }
    
    // This actionPerformed simply takes sets the visibility of each
    // coloured box to the state of each checkbox.
        
    public void actionPerformed(ActionEvent e) {
        
        if(e.getSource() == refresh)
        {
            redBox.setVisible(redCB.isSelected());
            blueBox.setVisible(blueCB.isSelected());
            greenBox.setVisible(greenCB.isSelected());
            yellowBox.setVisible(yellowCB.isSelected());
        }
    }


    private static void createAndShowGUI() {

        JFrame.setDefaultLookAndFeelDecorated(true);
        JFrame frame = new JFrame("[=] JCheckBox [=]");

        CheckBoxExample_UltraExtended demo = new CheckBoxExample_UltraExtended();
        frame.setContentPane(demo.createContentPane());
        
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

Let's see if you can tick all the boxes.

Hopefully this gives you a good insight into how to instantiate JCheckBoxes and really bring them to life!.

Code Style

Required Lessons

External Links