RGB based random color setter for JPanel manipulation doesn't work, the color is always black

I wrote code to generate random color using RGB and fill polygons with it, so that every polygon is different color using "util.Random". However, every time I run this code, all 8 polygons just get filled with black color. xxx is variable name for JPanel.

public void randcol(){
        Graphics g = xxx.getGraphics();
        Random r = new Random();
        int rand1 = r.nextInt(256);
        int rand2 = r.nextInt(256);
        int rand3 = r.nextInt(256);
        g.setColor(new Color(rand1, rand2, rand3));
    }
    public void colorize(){
        Graphics g = xxx.getGraphics();
        randcol(); 
        g.fillPolygon(tr1);
        randcol();
        g.fillPolygon(tr2);
        randcol();
        g.fillPolygon(tr3);
        randcol();
        g.fillPolygon(tr4);
        randcol();
        g.fillPolygon(tr5);
        randcol();
        g.fillPolygon(tr6);
        randcol();
        g.fillPolygon(tr7);
        randcol();
        g.fillPolygon(tr8);
    }

public void randcol(){
    Graphics g = xxx.getGraphics();
    ...
}

public void colorize(){
    Graphics g = xxx.getGraphics();
    ...

When you invoke getGraphics() you get a different instance of the Graphics object each time. So you can't set the Color of the Graphics object in a different method.

Instead you randcol() method should return a Color object. The colorize() method will then invoke this method to set the color of the Graphics object before painting the polygon.

However this is still the wrong approach, because the Swing component can be painted multiple times and each time it is repainted your current logic will randomize the color again.

So the proper solution is to create a custom object that contains:

1. the Polygon to be painted
2. the Color of the Polygon.

This object should be generated once at the start of your class. Then you keep all these objects in an ArrayList and your painting method iterates through the ArrayList to paint each object.

See:Custom Painting Approaches for a working example of this approach.

You can write a function which gives you a random color

public Color getRandomColor(){
    Random r = new Random();
    int rand1 = r.nextInt(256);
    int rand2 = r.nextInt(256);
    int rand3 = r.nextInt(256);
    return new Color(rand1, rand2, rand3);
}

public void colorize(){
   Graphics g = xxx.getGraphics();
   g.setColor(getRandomColor());
   g.fillPolygon(tr1);
   g.setColor(getRandomColor());
   g.fillPolygon(tr2);
   g.setColor(getRandomColor());
   //...
}