Chapter: Java The Complete Reference - The Java Library - Using AWT Controls, Layout Managers, and Menus

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CheckboxGroup - AWT Controls

It is possible to create a set of mutually exclusive check boxes in which one and only one check box in the group can be checked at any one time.

CheckboxGroup

 

It is possible to create a set of mutually exclusive check boxes in which one and only one check box in the group can be checked at any one time. These check boxes are often called radio buttons, because they act like the station selector on a car radio—only one station can be selected at any one time. To create a set of mutually exclusive check boxes, you must first define the group to which they will belong and then specify that group when you construct the check boxes. Check box groups are objects of type CheckboxGroup. Only the default constructor is defined, which creates an empty group.

You can determine which check box in a group is currently selected by calling getSelectedCheckbox( ). You can set a check box by calling setSelectedCheckbox( ). These methods are as follows:

 

Checkbox getSelectedCheckbox( )

 

void setSelectedCheckbox(Checkbox which)

 

Here, which is the check box that you want to be selected. The previously selected check box will be turned off.

Here is a program that uses check boxes that are part of a group:

 

// Demonstrate check box group.

import java.awt.*;

 

import java.awt.event.*; import java.applet.*; /*

 

<applet code="CBGroup" width=240 height=200> </applet>

 

*/

 

public class CBGroup extends Applet implements ItemListener { String msg = "";

 

Checkbox windows, android, solaris, mac; CheckboxGroup cbg;

public void init() {

cbg = newCheckboxGroup();

windows = new Checkbox("Windows", cbg, true);

android = new Checkbox("Android", cbg, false);

solaris = new Checkbox("Solaris", cbg, false);

mac = new Checkbox("Mac OS", cbg, false);

 

add(windows);

 

add(android);

 

add(solaris);

 

add(mac);

 

windows.addItemListener(this);

 

android.addItemListener(this);

 

solaris.addItemListener(this);

 

mac.addItemListener(this);

 

}

 

public void itemStateChanged(ItemEvent ie) { repaint();

 

}

 

// Display current state of the check boxes.

public void paint(Graphics g) {

 

msg = "Current selection: ";

 

msg += cbg.getSelectedCheckbox().getLabel(); g.drawString(msg, 6, 100);

 

}

 

}

 

Sample output generated by the CBGroup applet is shown in Figure 26-3. Notice that the check boxes are now circular in shape.


Figure 26-3   Sample output from the CBGroup applet


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