https://github.com/BigMarker/deskshare-public/blob/master/jnlp/src/main/java/com/bigmarker/client/deskshare/ScreenCap.java にあった showOnScreen という関数を利用させてもらう。
private static void showOnScreen(int screen, JFrame frame) {
GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
GraphicsDevice[] gd = ge.getScreenDevices();
if (screen > -1 && screen < gd.length) {
frame.setLocation(gd[screen].getDefaultConfiguration().getBounds().x, frame.getY());
} else if (gd.length > 0) {
frame.setLocation(gd[0].getDefaultConfiguration().getBounds().x, frame.getY());
} else {
throw new RuntimeException("No Screens Found");
}
}