Toggle Buttons - The Definitive Guide to Java Swing - page 114

Although the add() method is typically the only ButtonGroup method you'll ever need, the
following class definition shows that it's not the only method of ButtonGroup in existence:

public class ButtonGroup implements Serializable {
// Constructor
public ButtonGroup();
// Properties
public int getButtonCount();
public Enumeration getElements();
public ButtonModel getSelection();
// Other methods
public void add(AbstractButton aButton);
public boolean isSelected(ButtonModel theModel) ;
public void remove(AbstractButton aButton);
public void setSelected(ButtonModel theModel, boolean newValue);
}

One interesting thing the class definition shows is that given a ButtonGroup , you cannot
directly find out the selected AbstractButton . You can directly ask only which ButtonModel is
selected. However, getElements() returns an Enumeration of all the AbstractButton elements in
the group. You can then loop through all the buttons to find the selected one (or ones) by using
code similar to the following:

Enumeration elements = group.getElements();
while (elements.hasMoreElements()) {
AbstractButton button = (AbstractButton)elements.nextElement();
if (button.isSelected()) {
System.out.println("The winner is: " + button.getText());
break; // Don't break if sharing models -- could show multiple buttons selected
}
}

The other interesting method of ButtonGroup is setSelected() . The two arguments of the
method are a ButtonModel and a boolean . If the boolean value is false , the selection request is
ignored. If the ButtonModel isn't the model for a button in the ButtonGroup , then the ButtonGroup
deselects the currently selected model, causing no buttons in the group to be selected. The
proper usage of the method is to call the method with a model of a component in the group and
a new state of true . For example, if aButton is an AbstractButton and aGroup is the ButtonGroup ,
then the method call would look like aGroup.setSelected(aButton.getModel(), true) .

■ Note If you add a selected button to a ButtonGroup that already has a previously selected button, the

previous button retains its state and the newly added button loses its selection.

Now, let's look at the various components whose data model is the ToggleButtonModel .