import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.awt.image.WritableRaster;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Demo extends JFrame {
   public static void main(String[] args) {
      new Demo();
   }

   public Demo() {
      add(new MyPanel());
      pack();
      setVisible(true);
   }

   private static class MyPanel extends JPanel {
      public Dimension getPreferredSize() {
         return new Dimension (400, 400);
      }

      public void paint(Graphics g) {
         Random r = new Random(0);
         int[] pixels = new int[400 * 400 * 4];
         for (int i = 0; i < 400 * 400 * 4; ++i) {
            pixels[i] = r.nextInt();
         }
         BufferedImage buf = new BufferedImage(400, 400, BufferedImage.TYPE_INT_RGB);
         WritableRaster raster = (WritableRaster) buf.getData();
         raster.setPixels(0, 0, 400, 400, pixels);
         g.drawImage(buf, 0, 0, 400, 400, null);
      }
   }

}