JDesktopPaneにJInternalFrameを吸着させる

desktop.setDragMode(JDesktopPane.OUTLINE_DRAG_MODE);
desktop.setDesktopManager(new MagneticDesktopManager());

// ...
class MagneticDesktopManager extends DefaultDesktopManager {
  @Override public void dragFrame(JComponent frame, int x, int y) {
    Container c = SwingUtilities.getAncestorOfClass(JDesktopPane.class, frame);
    if (c instanceof JDesktopPane) {
      JDesktopPane desktop = (JDesktopPane) c;
      int e = x;
      int n = y;
      int w = desktop.getSize().width - frame.getSize().width - e;
      int s = desktop.getSize().height - frame.getSize().height - n;
      if (isNear(e) || isNear(n) || isNear(w) || isNear(s)) {
        super.dragFrame(frame, getX(e, w), getY(n, s));
      } else {
        super.dragFrame(frame, x, y);
      }
    }
  }

  private static int getX(int e, int w) {
    return e < w ? isNear(e) ? 0 : e : isNear(w) ? w + e : e;
  }

  private static int getY(int n, int s) {
    return n < s ? isNear(n) ? 0 : n : isNear(s) ? s + n : n;
  }

  private static boolean isNear(int c) {
    return Math.abs(c) < 10;
  }
}