import javax.swing.*;
import java.awt.*;

class Flag{

  private static void drawFlag(Graphics2D g){
    g.setColor(Color.blue);
    g.fillRect(0,0,400,200);
    g.setStroke(new BasicStroke(40));
    g.setPaint(new Color(255,255,255));
    g.drawLine(0,0,400,200);
    g.drawLine(400,0,0,200);
  }

  public static void main(String args[]) {
    JFrame mainWin = new JFrame("Main Window");
    JPanel panel = new JPanel()
    {
      public void paint(Graphics g){
        super.paint(g);
        drawFlag((Graphics2D)g);
      }
    };
    panel.setPreferredSize(new Dimension(400,200));
    mainWin.getContentPane().add(panel);
    mainWin.setDefaultCloseOperation(
      JFrame.EXIT_ON_CLOSE
    );
    mainWin.pack();
    mainWin.setVisible(true);
  }
}