The following is incorrect:

/**
 * This class is used to create a multiple-exclusion scope for
 * a set of buttons. Creating a set of buttons with the
 * same ButtonGroup object means that turning "on" one of those buttons
 * turns off all other buttons in the group. A ButtonGroup can be used with
 * sets of JButton, JRadioButton, or JRadioButtonMenuItem objects.
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           
Any subtype of AbstractButton (including JToggleButton and JCheckBox
and the rest of the menu buttons) can be added. Although typically
JButton and JMenuItem don't make much sense being in a group since they
don't inplement the selected state.