Learning Java - Chapter 7 : Java

The demo applets here will illustrate four more of the Swing JComponent classes that allow the user to select among a group of choices or to set values:

Each example uses the UiTestApplet class, which adds one of four types of JPanel subclasses. Each subclass has one of the above components and is used to set the background color of another blank panel (OutputPanel) in the top area of the applet. The panels are selected according to the applet tag param value.

CheckBoxesPanel
+ UiTestApplet + OutputPanel

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

/**
  * Display one of four types of component panels
  * to control the background color on a separate panel.
  * Applet parameter selects the type of panel.
**/
public class UiTestApplet extends JApplet
{
  /** Create the user interface with OutputPanel and ControlPanel.**/
  public void init () {
    //Container content_pane = getContentPane ();

    // Create an instance of OutputPanel
    OutputPanel output_panel = new OutputPanel ();

    // Find out from the applet tag which of the four
    // panels to use. Each panel demonstrates a different
    // type of control component.
    String panel_choice = getParameter ("Choose Panel");
    JPanel control_panel;

    // Create an instance of the control panel
    if (panel_choice.equals ("Checkboxes"))
        control_panel = new CheckBoxesPanel (output_panel);
    else if (panel_choice.equals ("Radiobuttons"))
        control_panel = new RadioButtonsPanel (output_panel);
    else if (panel_choice.equals ("List"))
        control_panel = new ListPanel (output_panel);
    else
        control_panel = new SliderPanel (output_panel);

    // Add the panels to applet's pane.
    content_pane.add (output_panel, BorderLayout.CENTER);
    content_pane.add (control_panel, BorderLayout.SOUTH);
  } // init

} // class UiTestApplet

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

/** Create a JPanel subclass called OutputPanel.
  * It provides a method to set its color plus
  * a paintComponent method.
**/
public class OutputPanel extends JPanel {
  Color fBgColor = Color.RED;

  /** Set the color by passing the 3 RGB component
    * values  (with ranges 0-255).
   **/
  void setColor (int cr, int cg, int cb) {
    fBgColor = new Color (cr,cg,cb);
    repaint ();
  }

  /** For Swing components you must override
    * paintComponent () rather than paint ().
   **/
  public void paintComponent (Graphics g) {
    super.paintComponent (g);

    // Now we fill a rectangle the size of the
    // panel with the chosen color.
    g.setColor (fBgColor);
    g.fillRect (0, 0, getWidth (), getHeight () );
  } // paintComponent

} // class OutputPanel

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

/**
  * The CheckBoxesPanel uses check boxes to set
  * the color of the output panel.
**/
public class CheckBoxesPanel extends JPanel
                   implements ItemListener
{
  JCheckBox fRed, fGreen, fBlue;
  OutputPanel fOutputPanel;

  /** Constructor adds 3 check boxes to the panel. **/
  CheckBoxesPanel (OutputPanel output_panel) {

    fOutputPanel = output_panel;

    // Initial color is red so select this button.
    fRed = new JCheckBox ("Red", true);
    fRed.addItemListener (this);
    add (fRed);

    fGreen = new JCheckBox ("Green");
    fGreen.addItemListener (this);
    add (fGreen);

    fBlue = new JCheckBox ("Blue");
    fBlue.addItemListener (this);
    add (fBlue);

  }// ctor

  /**
    * An item event from a check box comes here when
    * the user clicks on one.
   **/
  public void itemStateChanged (ItemEvent evt) {
    if (evt.getStateChange () == ItemEvent.SELECTED)
        System.out.println ("SELECTED");
    else
        System.out.println ("DESELECTED");
    int cr = (fRed.isSelected ()   ? 0xFF : 0);
    int cg = (fGreen.isSelected () ? 0xFF : 0);
    int cb = (fBlue.isSelected ()  ? 0xFF : 0);

    fOutputPanel.setColor (cr,cg,cb);

  } // itemStateChanged

} // class CheckBoxesPanel

JRadioButton

Radio buttons are for those cases where only one of several options can be selected:

RadioButtonsPanel
+ UITestApplet + ControlPanel

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

/**
  * The RadioButtonsPanel holds radio buttons that set
  * the color of the output panel.
**/
public class RadioButtonsPanel extends JPanel
                   implements ActionListener
{

  JRadioButton fRed = null, fGreen = null, fBlue = null;
  OutputPanel fOutputPanel = null;

  RadioButtonsPanel (OutputPanel output_panel) {

    fOutputPanel = output_panel;

    // RadioButtons need to be organized with a
    // ButtonGroup object.
    ButtonGroup group = new ButtonGroup ();

    fRed = new JRadioButton ("Red", true);
    fRed.addActionListener (this);

    // Add the JRadioButton instance to both the
    // ButtonGroup and the panel.
    group.add (fRed);
    add (fRed);

    fGreen = new JRadioButton ("Green", false);
    fGreen.addActionListener (this);
    group.add (fGreen);
    add (fGreen);

    fBlue = new JRadioButton ("Blue", false);
    fBlue.addActionListener (this);
    group.add (fBlue);
    add (fBlue);

  } // ctor

  /** Action events come here when the user clicks on a button.**/
  public void actionPerformed (ActionEvent evt)  {

    // Default color component values.
    int cr=0;
    int cg=0;
    int cb=0;
    // Only one button can be selected at a time
    // so choose one color and then set the output
    // panel color.
    Object source = evt.getSource ();
    if ( source == fRed)
        cr = 0xFF;
    else if (source == fGreen)
        cg = 0xFF;
    else if (source == fBlue)
        cb = 0xFF;

    fOutputPanel.setColor (cr,cg,cb);
  } // actionPerformed

} // class RadioButtonsPanel

JList

The JList component provides a way of offering a list of items from which the user can select.

Depending on how the JList instance is configured, either a single item can be selected, or multiple items selected.

ListPanel
+ UITestApplet + OutputPanel

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

/**
  * The ListPanel holds the JList component with items
  * that determine the color of the output panel.
**/
public class ListPanel extends JPanel
              implements ListSelectionListener
{

  OutputPanel fOutputPanel;

  /** Constructor adds a JList to the panel. **/
  ListPanel (OutputPanel output_panel) {

    fOutputPanel = output_panel;

    String [] colors = {
      "Red","Green","Blue","Yellow","White","Black"};

    JList color_list = new JList (colors);

    // Show only 4 items in the list at a time.
    color_list.setVisibleRowCount (4);

    // Allow only one of the items to be selected.
    color_list.setSelectionMode (
      ListSelectionModel.SINGLE_SELECTION);

    // Select initially the top item.
    color_list.setSelectedIndex (0);

    color_list.addListSelectionListener (this);

    // Add to a JScrollPane so that we can have
    // a scroller to view other items.
    JScrollPane scroll_pane = new JScrollPane (color_list);
    add (scroll_pane);

  } // ctor

  /** This class implements the ListSelectionListener
    * so events come to this valueChanged method.
   **/
  public void valueChanged (ListSelectionEvent evt)  {

    // Default color component values.
    int cr=0;
    int cg=0;
    int cb=0;

    // Get the reference to the JList object
    JList source =  (JList)evt.getSource ();
    // and get an array of the selected items.
    Object [] values = source.getSelectedValues ();

    // In this case only one value can be selected
    // so just look at first item in array.
    String color_selected =  (String)values[0];
    if ( color_selected.equals ("Red") )
        cr = 0xFF;
    else if (color_selected.equals ("Green") )
        cg = 0xFF;
    else if (color_selected.equals ("Blue") )
        cb = 0xFF;
    else if (color_selected.equals ("Yellow") ){
        cr = 0xFF;
        cg = 0xFF;
    }else if (color_selected.equals ("White") ){
        cr = 0xFF;
        cg = 0xFF;
        cb = 0xFF;
    }

    fOutputPanel.setColor (cr,cg,cb);

  } // valueChanged

} // class ListPanel

JSlider

Sliders allow the user to choose from a continuous range of values rather than from discrete values in a set of radio buttons or in a list.

SliderPanel
+ UITestApplet + OutputPanel

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

/**
  * The SliderPanel holds three JSlider widgets that set
  * the color of the output panel.
**/
public class SliderPanel extends JPanel
                           implements ChangeListener {

  OutputPanel fOutputPanel;
  JLabel fRedLabel,fGreenLabel,fBlueLabel;
  JSlider fRed,fGreen,fBlue;

  SliderPanel (OutputPanel output_panel) {

    fOutputPanel = output_panel;

    setLayout (new GridLayout (3, 2));

    add (fRedLabel = new JLabel ("Red 0  ",SwingConstants.RIGHT) );

    // The JSlider constructor parameters:
    //
    //   orientation, minimum, maximum, inital value
    //
    // The sliders are set horizontally.
    // The values range from 0 to 255.
    // Set the red slider to max initially to match the
    // initial Red color for the output panel.
    //
    add (fRed = new JSlider (JSlider.HORIZONTAL, 0, 255, 255));
    fRed.addChangeListener (this);

    add (fGreenLabel = new JLabel ("Green 0  ",SwingConstants.RIGHT));
    add (fGreen = new JSlider (Adjustable.HORIZONTAL, 0, 255, 0));
    fGreen.addChangeListener (this);

    add (fBlueLabel = new JLabel ("Blue 0  ",SwingConstants.RIGHT) );
    add (fBlue = new JSlider (Adjustable.HORIZONTAL, 0, 255, 0));
    fBlue.addChangeListener (this);

  } // ctor

  /** This class is the AdjustmentListener for the
    * scroll bar. So the events come here when the
    * scroll bar is moved.
   **/
  public void stateChanged (ChangeEvent evt){

    // Use the labels to show the numerical values of the
    // scroll bar settings.
    fRedLabel.setText (  "Red "   + fRed.getValue ()  );
    fGreenLabel.setText ("Green " + fGreen.getValue ());
    fBlueLabel.setText ( "Blue "  + fBlue.getValue () );

    // Get the values from each scroll bar and pass
    // them as the color component values.
    fOutputPanel.setColor (fRed.getValue (), fGreen.getValue (),
                           fBlue.getValue ());

    fOutputPanel.repaint ();

  } // stateChanged

} // class SliderPanel

The above examples give the essential steps for using these components. However, they provide many other options. For more information on the above components, see the API Specifications and the resources below.

References and Web Resources