APPLYING CHECK BOXES

A check box is a control that is used to turn an option on or off. It consists of a small box
that can either contain a check mark or not. There is a label associated with each check
box that describes what option the box represents. You change the state of a check box
by clicking on it. Check boxes can be used individually or as part of a group. Check
boxes are objects of the Checkbox class.
Checkbox supports these constructors:
Checkbox( )
Checkbox(String str)
Checkbox(String str, boolean on)
Checkbox(String str, boolean on, CheckboxGroup cbGroup)
Checkbox(String str, CheckboxGroup cbGroup, boolean on)
The first form creates a check box whose label is initially blank. The state of the check
box is unchecked. The second form creates a check box whose label is specified by str.
The state of the check box is unchecked. The third form allows you to set the initial
state of the check box. If on is true, the check box is initially checked; otherwise, it is
cleared. The fourth and fifth forms create a check box whose label is specified by str
and whose group is specified by cbGroup. If this check box is not part of a group, then
cbGroup must be null. (Check box groups are described in the next section.) The value
of on determines the initial state of the check box.
To retrieve the current state of a check box, call getState( ). To set its state, call
setState( ). You can obtain the current label associated with a check box by calling
getLabel( ). To set the label, call setLabel( ). These methods are as follows:
boolean getState( )
void setState(boolean on)
String getLabel( )
void setLabel(String str)
Here, if on is true, the box is checked. If it is false, the box is cleared. The string passed
in str becomes the new label associated with the invoking check box.
Handling Check Boxes
Each time a check box is selected or deselected, an item event is generated. This is sent to
any listeners that previously registered an interest in receiving item event notifications
from that component. Each listener implements the ItemListener interface. That interface defines the itemStateChanged( ) method. An ItemEvent object is supplied as the argument
to this method. It contains information about the event (for example, whether it was
a selection or deselection).
The following program creates four check boxes. The initial state of the first box is
checked. The status of each check box is displayed. Each time you change the state of
a check box, the status display is updated.
// Demonstrate check boxes.
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
/*


*/
public class CheckboxDemo extends Applet implements ItemListener {
String msg = "";
Checkbox Win98, winNT, solaris, mac;
public void init() {
Win98 = new Checkbox("Windows 98/XP", null, true);
winNT = new Checkbox("Windows NT/2000");
solaris = new Checkbox("Solaris");
mac = new Checkbox("MacOS");
add(Win98);
add(winNT);
add(solaris);
add(mac);
Win98.addItemListener(this);
winNT.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 state: ";
g.drawString(msg, 6, 80);
msg = " Windows 98/XP: " + Win98.getState();
g.drawString(msg, 6, 100);
msg = " Windows NT/2000: " + winNT.getState();
g.drawString(msg, 6, 120);
msg = " Solaris: " + solaris.getState();
g.drawString(msg, 6, 140);
msg = " MacOS: " + mac.getState();
g.drawString(msg, 6, 160);
}
}