LayoutManagerを実装して常に中央に子コンポーネントを配置する

// @see javax.swing.colorchooser.CenterLayout
class CenterLayout implements LayoutManager {
  @Override public void addLayoutComponent(String name, Component comp) {
    /* not needed */
  }

  @Override public void removeLayoutComponent(Component comp) {
    /* not needed */
  }

  @Override public Dimension preferredLayoutSize(Container container) {
    return Optional.ofNullable(container.getComponent(0)).map(c -> {
      Dimension size = c.getPreferredSize();
      Insets insets = container.getInsets();
      size.width += insets.left + insets.right;
      size.height += insets.top + insets.bottom;
      return size;
    }).orElseGet(Dimension::new);
  }

  @Override public Dimension minimumLayoutSize(Container c) {
    return preferredLayoutSize(c);
  }

  @Override public void layoutContainer(Container container) {
    Component c = container.getComponent(0);
    c.setSize(c.getPreferredSize());
    if (container instanceof JComponent) {
      Dimension size = c.getSize();
      Rectangle r = SwingUtilities.calculateInnerArea((JComponent) container, null);
      int x = r.x + (r.width - size.width) / 2;
      int y = r.y + (r.height - size.height) / 2;
      c.setBounds(x, y, size.width, size.height);
    }
  }
}