This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Actually, in the default Metal LaF at least, it's just one Icon* which reacts to rollover/pressed/selected states by casting the Component passed to its paintIcon method to a JRadioButton and retrieving its model.
* private class javax.swing.plaf.metal.MetalIconFactory.RadioButtonIcon, obtainable via public static Icon getRadioButtonIcon()