static Window centerWindow(Component c) {
  Window w = getWindow(c);
  if (w != null)
    w.setLocationRelativeTo(null); // magic trick
  ret w;
}

static Window centerWindow(Component c, int width, int height) {
  setWindowSize(c, width, height);
  ret centerWindow(c);
}

static JWindow centerWindow(JWindow w) {
  centerWindow(getRootPane(w));
  ret w;
}

Began life as a copy of #1001308