The
concept of a radio
button in GUI programming comes from pre-electronic car radios with mechanical
buttons: when you push one in, any other button that was pressed pops out. Thus
it allows you to force a single choice among many.
The
AWT does not have a separate class to represent the radio button; instead it
reuses the Checkbox.
However, to put the
Checkbox
in a radio button group (and to change its shape so it’s visually
different from an ordinary
Checkbox)
you must use a special constructor that takes a CheckboxGroup
object as an argument. (You can also call setCheckboxGroup( )
after the
Checkbox
has been created.)
A
CheckboxGroup
has
no constructor argument; its sole reason for existence is to collect some
Checkboxes
into a group of radio buttons. One of the
Checkbox
objects must have its state set to
true
before you try to display the group of radio buttons; otherwise you’ll
get an exception at run time. If you try to set more than one radio button to
true
then only the final one set will be
true.
Here’s
a simple example of the use of radio buttons. Note that you capture radio
button events like all others:
//: RadioButton1.java
// Using radio buttons
import java.awt.*;
import java.applet.*;
public class RadioButton1 extends Applet {
TextField t =
new TextField("Radio button 2", 30);
CheckboxGroup g = new CheckboxGroup();
Checkbox
cb1 = new Checkbox("one", g, false),
cb2 = new Checkbox("two", g, true),
cb3 = new Checkbox("three", g, false);
public void init() {
t.setEditable(false);
add(t);
add(cb1); add(cb2); add(cb3);
}
public boolean action (Event evt, Object arg) {
if(evt.target.equals(cb1))
t.setText("Radio button 1");
else if(evt.target.equals(cb2))
t.setText("Radio button 2");
else if(evt.target.equals(cb3))
t.setText("Radio button 3");
else
return super.action(evt, arg);
return true;
}
} ///:~
To
display the state, an text field is used. This field is set to non-editable
because it’s used only to display data, not to collect it. This is shown
as an alternative to using a
Label.
Notice the text in the field is initialized to “Radio button 2”
since that’s the initial selected radio button.
You
can have any number of
CheckboxGroups
on a form.
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