Sams Teach Yourself Visual J++ 6 in 21 Days
(Publisher: Macmillan Computer Publishing)
Author(s): Rick Leinecker
ISBN: 0672313510
Publication Date: 11/01/98

When this program runs, it will look identical to the earlier program. Check boxes one, two, and three will generate no events, whereas check boxes four, five, and six will generate events. The text string in the applet window will show the last event that was handled.

Once again, we’ll start about halfway down the program with the analysis because it’s very important. At line 36, we declare our own class, named MyListener, which implements the ItemListener interface. At line 38, we add the itemStateChanged() method. This is where all the events will go when they are triggered. Inside the itemStateChanged() method, we use the Item event’s getSource() method to find out which instantiated object triggered the event. We make a comparison, and depending on which one triggered the event, we respond by displaying the event to the window. Lines 9–14 show where we create the six check boxes that we add to the applet window. They’re all normal Checkbox objects; none of them is an extended Checkbox object. In line 16, we instantiate one of the MyListener classes. In lines 20–22, we add the ItemListener by using the addItemListener() method to check boxes four, five, and six. Just as in the earlier program, you can see that there’s no way that check boxes one, two, and three will handle events because we don’t add the ItemListener to them. Finally, lines 23–28 simply add the check boxes to the applet.

Checkbox Groups

If you combine check boxes into a Checkbox group, you’ll get check boxes with a different look. Whereas check boxes are normally square, the Checkbox groups (usually referred to as “radio buttons”) are normally round. Besides the difference in appearance, they function differently. After you add check boxes into a Checkbox group, only one of the check boxes can be selected at a time.

Constructors

There is only one Checkbox group constructor. It takes no arguments. The following line of code shows how to create a Checkbox group:

CheckboxGroup group = new CheckboxGroup();

Before we go on, though, let’s revisit the Checkbox constructors. There’s a Checkbox constructor that’s especially meant for creating check boxes that will be grouped together. It takes three arguments: a string, a Checkbox group, and an initial state. If you specify an initial state of true for more than one check box in a group, the last one for which you specify true will be shown as selected.

Grouping Check Boxes

Checkbox groups handle no events on their own. However, the check boxes that are grouped into them handle events in the exact same manner as was described in the preceding section. There’s absolutely no difference in event handling for check boxes, whether they’re in a group or not.

Here are the steps to creating check boxes and associating them all in a group:

1.  Create a Checkbox group object.

2.  Create all the Checkbox objects that will be grouped.

3.  In the Checkbox constructor, give it these items:

•  The Checkbox label

•  The group to which it will belong

•  The initial state of that particular check box

The following short example shows how to group three check boxes:

CheckboxGroup group = new CheckboxGroup();
Checkbox Checkbox1 = new Checkbox( "One", group, true );
Checkbox Checkbox2 = new Checkbox( "Two", group, false );
Checkbox Checkbox3 = new Checkbox( "Three", group, false );

Sample Program

Because handling events for check boxes that are grouped in a Checkbox group is exactly the same for check boxes that are not in a group, the purpose of this example is not to show you how to handle events, but to show you how to group the check boxes together. I’ve taken the first sample program from the Checkbox section and altered it so that the first three buttons are grouped together and the last three buttons are grouped together. This altered program is given in Listing 7.5.

No substantive change to the event-handling code was made. Therefore, check boxes one, two, and three will not trigger any events, but check boxes four, five, and six will.

Listing 7.5 A Modified Program Showing How to Group Check Boxes

1   import java.awt.*;
2   import java.applet.*;
3   import java.awt.event.*;
4
5   public class Applet1 extends Applet
6   {
7     String m_strDisplay = "No events";
8
9     CheckboxGroup m_Group1 = new CheckboxGroup();
10    CheckboxGroup m_Group2 = new CheckboxGroup();
11    Checkbox m_Checkbox1 = new Checkbox( "One", m_Group1, true );
12    Checkbox m_Checkbox2 = new Checkbox( "Two", m_Group1, false );
13    Checkbox m_Checkbox3 = new Checkbox( "Three", m_Group1, false );
14    MyCheckbox m_Checkbox4 = new MyCheckbox( "Four", m_Group2, true );
15    MyCheckbox m_Checkbox5 = new MyCheckbox( "Five", m_Group2, false );
16    MyCheckbox m_Checkbox6 = new MyCheckbox( "Six", m_Group2, false );
17
18    public void init()
19      {
20          add( m_Checkbox1 );
21          add( m_Checkbox2 );
22          add( m_Checkbox3 );
23          add( m_Checkbox4 );
24          add( m_Checkbox5 );
25          add( m_Checkbox6 );
26      }
27
28    public void paint( Graphics g )
29      {
30          g.drawString( m_strDisplay, 20, 100 );
31      }
32
33    public class MyCheckbox extends Checkbox
34      {
35          String m_strText;
36
37          MyCheckbox( String s, CheckboxGroup group, boolean state )
38          {
39              super( s, group, state );
40              m_strText = s;
41              enableEvents( AWTEvent.ITEM_EVENT_MASK );
42          }
43
44          public void processItemEvent( ItemEvent ie )
45          {
46              m_strDisplay = "'" + m_strText + "' was just pressed. ";
47              getParent().repaint();
48              super.processItemEvent( ie );
49          }
50
51      }
52
53  }

When the program runs, as shown in Figure 7.3, you’ll notice right away that the check boxes have a different appearance. When you start clicking on the check boxes, you’ll also notice that their behavior is very different. Because the first three check boxes are grouped together, only one of those first three can be selected at once. The same holds true for the fourth, fifth, and sixth check boxes. You’ll also see the text string in the applet window change only when you select fourth, fifth, and sixth radio-button check boxes.

Figure 7.3  This program groups check boxes together.

There aren’t many differences between the source code for this program and that for the first check box sample program, but notice that lines 9 and 10 create two Checkbox groups. Then notice that lines 11, 12, and 13 use the Checkbox constructor that takes three arguments, one of which is the group to which the check box will belong. Lines 14, 15, and 16 call the MyCheckbox constructor that also takes three arguments, one of which is the group to which this Checkbox belongs. Remember that previously the constructor took only a single argument—the label String. You’ll notice in line 37 that we had to modify the constructor.

So now, not only does it take a String, but it also takes a Checkbox group in an initial state. Lines 20–25, as always, have the Checkbox controls to the applet; and line 33, as before, is where the declaration of the extended Checkbox class begins.