JSplitPaneの分割割合を維持する

class SplitPaneWrapper extends JPanel {
  private final JSplitPane splitPane;

  protected SplitPaneWrapper(JSplitPane splitPane) {
    super(new BorderLayout());
    this.splitPane = splitPane;
    add(splitPane);
  }

  private static int getOrientedSize(JSplitPane sp) {
    return sp.getOrientation() == JSplitPane.VERTICAL_SPLIT
        ? sp.getHeight() - sp.getDividerSize()
        : sp.getWidth() - sp.getDividerSize();
  }

  @Override public void doLayout() {
    int size = getOrientedSize(splitPane);
    int loc = splitPane.getDividerLocation();
    BigDecimal ratio = BigDecimal.valueOf(loc / (double) size)
        .setScale(2, RoundingMode.HALF_UP);
    super.doLayout();
    if (splitPane.isShowing()) {
      EventQueue.invokeLater(() -> {
        int sz = getOrientedSize(splitPane);
        int iv = (int) (.5 + sz * ratio.doubleValue());
        splitPane.setDividerLocation(iv);
      });
    }
  }
}