import javax.swing.*;
import java.awt.event.*;
import java.awt.Graphics;
import java.awt.image.BufferedImage;

public class ShowRainbowColors extends JComponent {
  BufferedImage bufferedImage;

  public void initialize() {
    int wd= getSize().width;
    int ht = getSize().height;
    int[] data = new int[wd * ht];
    int index = 0;
    for (int j = 0; j < ht; j++) {
      int red = (j * 255) / (ht - 1);
      for (int k = 0; k < wd; k++) {
        int green = (k* 255) / (wd - 1);
        int blue = 128;
        data[index++] = (red << 16) | (green << 8) | blue;
      }
    }
    bufferedImage = new BufferedImage(wd, ht, BufferedImage.
     TYPE_INT_RGB);
    bufferedImage.setRGB(0, 0, wd, ht, data, 0, wd);
  }
    public void paint(Graphics g) {
    if (bufferedImage == null)
      initialize();
    g.drawImage(bufferedImage, 0, 0, this);
  }
public static void main(String[] args) {
    JFrame frame = new JFrame("Show Rainbow Colors");
    frame.getContentPane().add(new ShowRainbowColors());
    frame.setSize(450, 300);
    frame.setLocation(100, 100);
    frame.addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent event) {
        System.exit(0);
      }
    });
    frame.setVisible(true);
  }
}