JCheckBoxのグループ内で選択可能な上限を設定する

class GroupCheckBox extends JCheckBox {
  protected GroupCheckBox(String title) {
    super(title);
  }

  @Override public void updateUI() {
    super.updateUI();
    setModel(new ToggleButtonModel() {
      private static final int GROUP_SIZE = 3;

      @Override public void setSelected(boolean selected) {
        if (selected) {
          if (getSelectedObjects().length == GROUP_SIZE) {
            UIManager.getLookAndFeel()
                .provideErrorFeedback(GroupCheckBox.this);
          } else {
            super.setSelected(true);
          }
        } else {
          super.setSelected(false);
        }
      }

      @Override public Object[] getSelectedObjects() {
        Container parent = getParent();
        return Arrays.stream(parent.getComponents())
            .filter(AbstractButton.class::isInstance)
            .map(AbstractButton.class::cast)
            .filter(AbstractButton::isSelected)
            .toArray();
      }
    });
  }
}