JSplitPaneの展開・収納ボタンをDividerサイズの中央に配置する

JSplitPane splitPane = new JSplitPane();
splitPane.setTopComponent(new JScrollPane(new JTable(8, 3)));
splitPane.setBottomComponent(new JScrollPane(new JTree()));
splitPane.setOneTouchExpandable(true);
splitPane.setDividerSize(32);

String key = "SplitPane.centerOneTouchButtons";
JCheckBox check = new JCheckBox(key, UIManager.getBoolean(key)) {
  @Override public void updateUI() {
    super.updateUI();
    EventQueue.invokeLater(() -> {
      boolean b = UIManager.getLookAndFeelDefaults().getBoolean(key);
      setSelected(b);
      UIManager.put(key, b);
      SwingUtilities.updateComponentTreeUI(splitPane);
    });
  }
};
check.setOpaque(false);
check.addActionListener(e -> {
  JCheckBox cb = (JCheckBox) e.getSource();
  UIManager.put(key, cb.isSelected());
  SwingUtilities.updateComponentTreeUI(splitPane);
});