java - 为什么 JButton 不显示在 JFrame 上？

Question

当我运行程序时，窗口及其所有属性都正确，但按钮不显示，知道我做错了什么吗？

我有两个类 window 和 TimeTable0:

这是窗口:

package timetable0;

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

public class window extends JFrame {

    JButton bt1,bt2,bt3,bt4 = new JButton();
    JPanel panel = new JPanel();

    public void ventana() {

        setResizable(false);
        setTitle("Time Table");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400,550);
        setVisible(true);

        bt1.setText("Show Grades");
        bt2.setText("Show Time Table");
        bt3.setText("");
        bt4.setText("");

        panel.add(bt1);
        panel.add(bt2);
        panel.add(bt3);
        panel.add(bt4);

    }

    public void actions (){

        bt1.addActionListener((ActionEvent e) -> {
            System.out.println("");
               });
        bt2.addActionListener((ActionEvent e) -> {
            System.out.println("");
               });
        bt3.addActionListener((ActionEvent e) -> {
            System.out.println("");
               });
        bt4.addActionListener((ActionEvent e) -> {
            System.out.println("");
               });

    }

}

这是 TimeTable0:

package timetable0;

public class TimeTable0 {
    public static void main(String[] args) {
        window menu = new window();

        menu.ventana();
        menu.actions();
    }
}

Answer 1

1. 您没有将 JPanel 添加到框架中。

   panel.add(bt1);
   panel.add(bt2);
   panel.add(bt3);
   panel.add(bt4);
   setContentPane(panel);
   

2. 您没有在 Event Dispatch Thread 上运行该程序:

   public static void main(String[] args) {
       EventQueue.invokeLater(new Runnable() {
           @Override
           public void run() {
               window menu = new window();
               menu.ventana();
               menu.actions();
           }
       });
   }
   

3. 您并没有创建所有四个按钮:

   JButton bt1 = new JButton();
   JButton bt2 = new JButton();
   JButton bt3 = new JButton();
   JButton bt4 = new JButton();