Question

初学JAVA ，求大神指点。JPanel里嵌套了一个JPanel，为什么会重复出现一个JPanel，

package asd;

import javax.swing.*;

public class testPanel{

//程序入口
public static void main(String[] args) {

JFrame frame = new JFrame();

GamePanel gp = new GamePanel();

frame.add(gp);

frame.pack();

frame.setVisible(true);

}
}

package asd;

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

public class GamePanel extends JPanel{

Image img = null;

public GamePanel() {

//这是的绿色的子组件JPanel,
JPanel jp = new JPanel();
jp.setBounds(490,200,140,200);
jp.setBackground(Color.green); //绿色背景
jp.setLayout(null);
// jp.setOpaque(false);

//添加JPanel
add(jp);

setLayout(null);
setVisible(true);

setBackground(Color.blue); //蓝色背景
setPreferredSize(new Dimension(640,482));

//获取背景图片
img = getToolkit().getImage("img/back.png");
}

public void paint(Graphics g) {
//这里如果不写paint方法的话子组件就能显示正常，但不用paint方法又不能显示背景图了
g.drawImage(img,0,0,this);

paintComponents(g);
}

}

Answer 1

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

class GamePanel extends JPanel{

Image img = null;

JPanel jp = new JPanel();

public JPanel getjp()
{ return jp;

}

public GamePanel() {
//绿色组件JPanel,
jp.setBounds(490,200,140,200);
jp.setBackground(Color.green); //绿色背景
jp.setLayout(null);
// jp.setOpaque(false);

//添加JPanel
add(jp);

setLayout(null);
setVisible(true);

setBackground(Color.blue); //蓝色背景
setPreferredSize(new Dimension(640,482));

//获取背景图片
img = getToolkit().getImage("img/back.png");
}

public void paint(Graphics g) {
//写paint组件能显示用paint能显示背景图
g.drawImage(img,0,0,this);

paintComponents(g);
}

}
public class testPanel{

//程序入口
public static void main(String[] args) {

JFrame frame = new JFrame();

GamePanel gp = new GamePanel();

frame.add(gp.getjp());

frame.pack();

frame.setVisible(true);

}
}