Hi Andre,
this is the radiobutton-icon.
Below is a code-snippet.
Hope this helps
Frank

import javax.swing.*;

public class ButtonIconResolver {
 /**
  * extracted from BasicRadioButtonUI
  * @param b
  * @return
  */
 public static Icon getIcon(JRadioButton b) {
  ButtonModel model = b.getModel();
  Icon icon = b.getIcon();
  if (icon != null) {
   if (!model.isEnabled()) {
    if (model.isSelected()) {
     icon = b.getDisabledSelectedIcon();
    } else {
     icon = b.getDisabledIcon();
    }
   } else if (model.isPressed() && model.isArmed()) {
    icon = b.getPressedIcon();
    if (icon == null) {
     icon = b.getSelectedIcon();
    }
   } else if (model.isSelected()) {
    if (b.isRolloverEnabled() && model.isRollover()) {
     icon = b.getRolloverSelectedIcon();
     if (icon == null) {
      icon = b.getSelectedIcon();
     }
    } else {
     icon = b.getSelectedIcon();
    }
   } else if (b.isRolloverEnabled() && model.isRollover()) {
    icon = b.getRolloverIcon();
   }
   if (icon == null) {
    icon = b.getIcon();
   }
   return icon;
  } else {
   return UIManager.getIcon("RadioButton.icon");
  }
 }
}

But why do you want to get that?

Remember that, the native behavior (for example, on the Mac and Windows)
of check boxes and radio buttons is that they are active if the icon OR
the text label is clicked.  This behavior is, of course, highly useful
for user productivity, since it gives a much larger target to hit with
the mouse button.  (See Fitt's law, e.g.,
<http://en.wikipedia.org/wiki/Fitts'_law>,
<http://www.asktog.com/basics/firstPrinciples.html>)

Mac users, in particular, would tend to notice that the behavior isn't
quite what they expected.  But I suppose that this