Set the maximum number of items that can be selected in a group in JCheckBox

• download example.jar
• download src.zip
• repository

Code

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();
      }
    });
  }
}

References

• java - How can I limit the check box selected? - Stack Overflow
• Set the maximum number of items that can be selected in a group in JCheckBox(Japanese)

Click on the already selected JToggleButton and clear all selections in the ButtonGroup

• download example.jar
• download src.zip
• repository

Code

class ToggleButtonGroup extends ButtonGroup {
  private ButtonModel prevModel;
  private boolean isAdjusting;

  @Override public void setSelected(ButtonModel m, boolean b) {
    if (isAdjusting) {
      return;
    }
    if (m.equals(prevModel)) {
      isAdjusting = true;
      clearSelection();
      isAdjusting = false;
    } else {
      super.setSelected(m, b);
    }
    prevModel = getSelection();
  }
}

References

• java - How to deselect already selected JRadioButton by clicking on it - Stack Overflow
• Click on the already selected JToggleButton and clear all selections in the ButtonGroup(Japanese)

The JRadioButton that is currently selected in the ButtonGroup set as the default focus component

• download example.jar
• download src.zip
• repository

Code

buttons.setFocusTraversalPolicyProvider(true);
buttons.setFocusTraversalPolicy(new LayoutFocusTraversalPolicy() {
  @Override public Component getDefaultComponent(Container focusCycleRoot) {
    ButtonModel selection = bg.getSelection();
    for (Component c: focusCycleRoot.getComponents()) {
      JRadioButton r = (JRadioButton) c;
      if (r.getModel().equals(selection)) {
        return r;
      }
    }
    return super.getDefaultComponent(focusCycleRoot);
  }
});

References

• [JDK-8033699] Incorrect radio button behavior - Java Bug System
• [JDK-8154043] Fields not reachable anymore by tab-key, because of new tabbing behaviour of radio button groups. - Java Bug System
• java - Get the focus in a ButtonGroup of JRadioButtons to go to the currently selected item instead of first - Stack Overflow
• The JRadioButton that is currently selected in the ButtonGroup set as the default focus component (Japanese)