Question

java gui编程，jpanel在添加到主界面后无法显示

代码如下，在没有布局的时候能实现
package car;

import javax.swing.*;
import javax.swing.border.Border;

import java.awt.*;
import java.awt.event.*;

public class Car extends JFrame {

public static void main(String[] args) {
Car frame = new Car();
frame.setTitle("car");
frame.setSize(700,600);
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
frame.setVisible(true);
}

int x1,x2,x3,x4;

public Car()
{

JLabel jl1=new JLabel("car 1");
JLabel jl2=new JLabel("car 2");
JLabel jl3=new JLabel("car 3");
JLabel jl4=new JLabel("car 4");
JTextField tf1=new JTextField(2);
JTextField tf2=new JTextField(2);
JTextField tf3=new JTextField(2);
JTextField tf4=new JTextField(2);
MakeCar car1=new MakeCar();
MakeCar car2=new MakeCar();
MakeCar car3=new MakeCar();
MakeCar car4=new MakeCar();
setLayout(new BorderLayout());
JPanel p1=new JPanel();
p1.setBounds(0, 0, 300, 20);
p1.setLayout(new GridLayout(1,8));
p1.add(jl1);
p1.add(tf1);
p1.add(jl2);
p1.add(tf2);
p1.add(jl3);
p1.add(tf3);
p1.add(jl4);
p1.add(tf4);
add(p1,BorderLayout.NORTH);
JPanel p2=new JPanel();
p2.setLayout(new GridLayout(1,4));
p2.add(car1);
p2.add(car2);
p2.add(car3);
p2.add(car4);
add(p2,BorderLayout.CENTER);
// car1.setBounds(0, 100, 200, 100);
//car2.setBounds(0,120, 200, 100);
//car3.setBounds(0, 250,200, 100);
//car4.setBounds(0,350, 200, 100);
//add(p1);

//inputListener ipListener=new inputListener();
//tf4.addActionListener(ipListener);

}

static public class MakeCar extends JPanel {
private int y = getHeight()-50;

private int x = 50;
public MakeCar() {

Timer timer = new Timer(50,new TimerListener());
timer.start();
}
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Polygon trapezia = new Polygon(); // trapezia 梯形
trapezia.addPoint(x + 10, y -20);
trapezia.addPoint(x + 40, y -20);
trapezia.addPoint(x + 30, y -30);
trapezia.addPoint(x + 20, y -30);
g.setColor(Color.BLACK);
g.fillOval(x + 30, y - 10, 10, 10);
g.fillOval(x + 10, y - 10, 10, 10);
g.setColor(Color.gray);
g.fillRect(x, y - 20, 50, 10);
g.setColor(Color.red);
g.fillPolygon(trapezia);

}

class TimerListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
//if(x >250)
//x = 50;
x = x + 10;
repaint();

}
}
}

//class inputListener implements ActionListener{
// public void actionPerformed(ActionEvent e)
// {
// x1=Integer.parseInt(tf1.getText());
// x2=Integer.parseInt(tf2.getText());
// x3=Integer.parseInt(tf3.getText());
// x4=Integer.parseInt(tf4.getText());
//
// }
// }
}

Answer 1

这边有一个重点要告诉你：在建立MakeCar的时候，Jpanel他是没有高度宽度的，也就是getHeight() = 0，所以你的车子实际上有画出来，不过是在-50的地方(出界)。放在动画区的话，你重新移动窗口大小，车子也会随之上下移。

追问

求问这个程序如何改，我之前把y设置成50的时候也无法显示，并且求问如何画边框？？？谢谢啦

追答

1. 你要变成直的话，把GridLayout的参数改一下就好

   原：p2.setLayout(new GridLayout(1,4));

   现：p2.setLayout(new GridLayout(4,1));

   
   

   
   

    2. 画边框需要BorderFactory：