Java Reference
In-Depth Information
12.9 Radio Buttons
Radio buttons are similar to check boxes because they can be either checked or
unchecked. However, there are two major differences. First, unlike check boxes, radio but-
tons are round. Second, radio buttons are usually logically placed inside the same button
group. Only one radio button can be selected at any time inside a button group. When
a radio button is pressed, the last radio button in the button group that was selected is
deselected. To see how radio buttons work, here is a very simple program.
import java .awt . ;
import javax . swing . ;
public class Test {
public static void main(String [] args)
NumbersFrame f = new NumbersFrame () ;
f . setVisible( true );
class NumbersFrame extends JFrame {
public NumbersFrame () {
setSize (400,100) ;
JPanel p = new JPanel () ;
a d d ( p , B o r d e r L a y o u t . NORTH) ;
JRadioButton upButton = new JRadioButton( "Increment" , true );
JRadioButton downButton = new JRadioButton( "Decrement" , false );
p. add(upButton) ;
p . add(downButton) ;
ButtonGroup group = new ButtonGroup () ;
group .add(upButton) ;
group .add(downButton) ;
The program's window is shown in Figure 12.6. Note that the text field in the figure will
be added in the next section. The constructor of the JRadioButton class takes as input the
name of the button and a Boolean variable that specifies if the button is initially selected.
Note that at most one radio button in the group can be initially selected.
There is no J before the name of the ButtonGroup class. The reason is that this is
a logical class and is not part of the Swing library. The add method is used to add radio
buttons to a button group. In every button group, at most one radio button can be selected.
Note that if the button group was not created and the radio buttons added to it, then both
the Increment and Decrement radio buttons could be selected at the same time. If you run
the program, you will see that only one radio button in the group can be selected at any
given time.
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