!7

p {
  JFrame f = new JFrame();
  f.setUndecorated(true);
  f.setBackground(new Color(0,255,0,0));
  f.setSize(512, 512);
  f.add(new JPanel() {
      @Override
      protected void paintComponent(Graphics g) {
          g.setColor(Color.RED);
          g.drawRect(0, 0, 511, 511);
      }
  });
  f.setLocationRelativeTo(null);
  f.setVisible(true);
}