package hack.

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

public class WaterMarkTextField extends JTextField{
    BufferedImage img;
    TexturePaint texture;
   
    public WaterMarkTextField(File file) throws IOException{
        super();
        img = ImageIO.read(file);
        
        Rectangle rect = new Rectangle(0, 0,
                img.getWidth(null), img.getHeight(null));
        texture = new TexturePaint(img, rect);
        
      //  setOpaque(false); 这句不要
    }
   
    public void paintComponent(Graphics g) { //方法名错误
        super.paintComponent(g);      
        Graphics2D g2 = (Graphics2D) g;
        g2.setPaint(texture);
        g.fillRect(0,0,getWidth(),getHeight()); //这两句话看不懂。其实觉得直接调用g.drawImage就行了
    }
   
    public static void main(String argv[]) throws Exception{
        JFrame frame = new JFrame("A text field");
        JTextField textfield = new WaterMarkTextField(
                new File("bg.png"));
        textfield.setText("Hello world");
        frame.getContentPane().add(textfield);
        frame.pack();
        frame.setVisible(true);
    }
}

这样的活动蛮好的，知识难度太高了，看不懂，有最简单的么