Tukukkk

My goal is to have a transparent JFrame with a non-transparent JPanel that is constantly drawing a square at a random place

private static final int alpha = 255;



public static void main(String args) {



    JFrame frame = new JFrame();



    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.setSize(400, 400);

    frame.setLocationRelativeTo(null);



    frame.setUndecorated(true);

    frame.setBackground(new Color(255, 255, 255, alpha));



    CustomPanel panel = new CustomPanel();



    panel.setBackground(new Color(255, 255, 255, 0));



    new Timer().schedule(new TimerTask() {



        @Override

        public void run() {



            panel.revalidate();

            panel.repaint();



        }



    }, 0, 1000);



    frame.add(panel);



    frame.setVisible(true);



}



public static class CustomPanel extends JPanel {



    private static final long serialVersionUID = 1L;



    @Override

    public void paintComponent(Graphics g) {

        super.paintComponent(g);

        g.clearRect(0, 0, getWidth(), getHeight());

        g.setColor(new Color(0, 255, 0));

        g.fillRect((int)(Math.random() * 380), (int)(Math.random() * 320), 20, 20);

    }



}

Currently, the frame is rendered with a white background and the square is drawn correctly.

However, if I change the value of variable alpha to, for example, 31, the clearRect call will only clear the panel with 31 alpha, and the past frame will still be visible

Left: 1 Iteration
Right: 4 Iterations

As you can see, the frame will be drawn 4 times and the previous instance will still be visible.

How can I draw the transparent frame without the old frame being visible?

My OS is Ubuntu 18.04

For a transparent panel don't use a transparent color. Swing does not paint backgrounds with transparency correctly. See Background With Transparency for more information.

However, for full transparency there is a simple solution. Just make the panel transparent:

//panel.setBackground(new Color(255, 255, 255, 0));

panel.setOpaque( false );

Then in the painting code you would use:

super.paintComponent(g);

g.setColor(new Color(0, 255, 0));

g.fillRect((int)(Math.random() * 380), (int)(Math.random() * 320), 20, 20);

Note however you should NOT be using random values in a painting method. You can't control when or how often Swing will repaint a component.

Instead you need a property of the class to be the Color for the Rectangle. Then you need a method like setSquareColor(...) when you want to change the Color of the square.