Swing/ButtonGroupEmptySelection のバックアップの現在との差分(No.1)

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ButtonGroup bg = new ToggleButtonGroup();
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JPanel p = new JPanel();
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Stream.of("A", "B", "C").map(JToggleButton::new).forEach(r -> {
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  r.setActionCommand(r.getText());
  p.add(r);
  bg.add(r);
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});
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JLabel label = new JLabel();
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JButton button = new JButton("check");
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button.addActionListener(e -> {
  String txt = Optional.ofNullable(bg.getSelection())
    .map(b -> String.format("\"%s\" isSelected.", b.getActionCommand()))
    .orElse("Please select one of the option above.");
  label.setText(txt);
  // ButtonModel bm = bg.getSelection();
  // if (bm != null) {
  //   label.setText(String.format("\"%s\" isSelected.", bm.getActionCommand()));
  // } else {
  //   label.setText("Please select one of the option above.");
  // }
});