如何在JPanel中设置背景而不使用paintComponent（）中的超级调用

Question

In this program, I want to draw a series of lines that interact to form a web. Each time the timer ticks, a line is drawn. Therefore, I cannot have the super.paintComponent(g) call in the paintComponent() because I need the previous lines to appear. However, I would like to set the background colour and as far as I've found, the setBackground() method can be called only if the super call is first made. I am not sure if the fillRect method would work either because it would draw a rectangle over the old line each time. I tried having the setBackground() method in the constructor, and it did not work either.

Here is my code:

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

public class prettyWebPanel extends JPanel implements ActionListener {
  Timer time = new Timer(100,this);
  private Color colour1 = Color.black;
  private Color colour2 = Color.white;
  JButton start = new JButton("Start");
  int j = 0;

  public prettyWebPanel() {
    setPreferredSize(new Dimension (550,550));
    this.add(start);
    start.addActionListener(this);
    setBackground(colour1);
  }

  public void paintComponent(Graphics g) {
    setBackground(colour1);
    setForeground(colour2);
    if (j<490) g.drawLine(20, 20+j, 20+j, 500);
  }

  public void actionPerformed(ActionEvent e) {
    if (e.getSource() == start) time.start();
    else if (e.getSource() == time) {
      j+=10;
      repaint();
    }
  }
}

Answer 1

because I need the previous lines to appear.

Then you need to do incremental painting. See Custom Painting Approaches for two common ways to do this:

1. Keep a List of objects to paint and repaint them every time
2. Do the painting onto a BufferedImage .