import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;

import javax.swing.JPanel;

/**
 * 一個可以動態加載一個圖片做背景的Jpanel
 */
public class ImagePanel extends JPanel{
 Image im;
 //構造函數制定Jpanel的大小
 public ImagePanel(Image im) {
  this.im = im;
  //希望該Panel的大小事自適應的
  int width = Toolkit.getDefaultToolkit().getScreenSize().width;
  int height = Toolkit.getDefaultToolkit().getScreenSize().height;
  this.setSize(width,height);
 }
 //畫出背景
 @Override
 protected void paintComponent(Graphics g) {
  // 清屏
  super.paintComponent(g);
  g.drawImage(im, 0, 0, this.getWidth(), this.getHeight(), this);
 }
}

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

public class Demo extends JFrame
{
    public Demo()
    {
     super("Title");
     NewPanel p = new NewPanel();
     this.getContentPane().add(p); //將面板添加到JFrame上
     this.setSize(596,298); //初始窗口的大小
     this.setLocationRelativeTo(null); //設置窗口居中
     this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     this.setVisible(true);
    }

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

    class NewPanel extends JPanel
   {
     public NewPanel()
     {

     }

     public void paintComponent(Graphics g)
     {
        int x=0,y=0;
        java.net.URL imgURL=getClass().getResource("test.jpg");

        //test.jpg是測試圖片，與Demo.java放在同一目錄下
        ImageIcon icon=new ImageIcon(imgURL);
        g.drawImage(icon.getImage(),x,y,getSize().width,getSize().height,this);
        while(true)
        {
          g.drawImage(icon.getImage(),x,y,this);
          if(x>getSize().width && y>getSize().height)break;
          //這段代碼是為了保證在窗口大于圖片時，圖片仍能覆蓋整個窗口
          if(x>getSize().width)
          {
             x=0;
             y+=icon.getIconHeight();
          }
          else
           x+=icon.getIconWidth();
        }
     }
   }
}