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Radio buttons

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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