java - JButton 未显示在屏幕上

Question

我正在尝试制作一个应用程序，只需单击按钮即可更改交通灯的状态。我的代码:主要

import javax.swing.*;

public class PP416 
{
    public static void main(String[] args)
    {
        JFrame frame = new JFrame("Traffic light");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.getContentPane().add(new TrafficPanel());
        frame.pack();
        frame.setVisible(true);
    }
}

JPanel 类:

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

public class TrafficPanel extends JPanel 
{
    private JButton button;
    private int indicator = 0; // Light is off

    public TrafficPanel()
    {
        button = new JButton("Change");

        this.add(button);
    }

    public void paint(Graphics g)
    {
        if (indicator == 0)
        {
            g.drawOval(30, 40, 30, 30);
            g.drawOval(30, 70, 30, 30);
            g.drawOval(30, 100, 30, 30);
        }
    }

}

按钮没有出现，只有椭圆形。谁能帮我解决这个问题？

Answer 1

不要重写paint，而是重写paintComponent并且最重要，调用super的方法。缺少 super 调用可能会导致 JPanel 无法很好地绘制其子组件。

例如，

@Override
protected void paintComponent(Graphics g) {
   super.paintComponent(g);
   if (indicator == 0) {
       g.drawOval(30, 40, 30, 30);
       g.drawOval(30, 70, 30, 30);
       g.drawOval(30, 100, 30, 30);
   }
}