Question

This code should create a black window, and then add a line and polygon over it.

public class gui extends JFrame {

 JPanel pane = new JPanel();
 gui(String title){
    super(title);
    setBounds(100,100,500,500);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    Container con = this.getContentPane();
    pane.setBackground(new Color(0,0,0));
    con.add(pane);
    setVisible(true);
 }

 public void paint(Graphics g){

    g.drawLine(100, 100, 400, 400);

    Point p1 = new Point(400, 100);
    Point p2 = new Point(100, 300);
    Point p3 = new Point(200, 400);

    int[] xs = { p1.x, p2.x, p3.x };
    int[] ys = { p1.y, p2.y, p3.y };
    Polygon triangle = new Polygon(xs, ys, xs.length);

    g.setColor(new Color(250,0,0));
    g.fillPolygon(triangle);
 }
}

When I delete the paint() method, a plain black GUI is created, as expected.

However, when the paint() method is in place, you get the line and polygon over a white background, not a black background.

How can I make the black background show through?

Answer 1

You need to call

super.paint(g);

in your paint method.

In Swing the preferred approach is to override paintComponent although since JFrame is not actually a JComponent this method will not be called. To use this approach, the functionality could be moved to a custom JComponent instead.