java程序，想问问为什么窗口内没有内容

java程序，想问问为什么窗口内没有内容

package exp_08;

public class Ques9_2 {
    public static void main(String[] args) {
        WindowFrameCal cal=new   WindowFrameCal();
        cal.setTitle("计算");
        cal.setBounds(100,100,500,300);
    }
}

package exp_08;

import javax.swing.*;
import java.awt.*;
public class WindowFrameCal extends JFrame {
    Box box;
    JTextField input1,input2,output;
    JButton[]buttons = new JButton[4];
    Cal ca;
    JPanel panel;
    JLabel label;
    WindowFrameCal()
    {
        init();
        setVisible(true);
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    }
    void init()
    {

        box = Box.createVerticalBox();
        panel =new JPanel();
        input1=new JTextField(10);
        input2=new JTextField(10);
        output=new JTextField(10);
        buttons[0] = new JButton("加");
        buttons[1] = new JButton("减");
        buttons[2] = new JButton("乘");
        buttons[3] = new JButton("除");
        Font f = new Font("宋体",Font.BOLD,28);
        SetFont.setFont
                (f,input1,input2,output,buttons[0],buttons[1],buttons[2],buttons[3]);
        panel.add(input1);
        panel.add(input2);
        panel.add(output);
        panel.add(box);
        for(int i=0;i<buttons.length;i++)
        {
            buttons[i].addActionListener(ca);
            box.add(buttons[i]);
        }
    }
}

package exp_08;

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

public class Cal implements ActionListener {

    WindowFrameCal view;
    public void setView(WindowFrameCal view)
    {
        this.view=view;
    }

    public void actionPerformed(ActionEvent e) {
        if(e.getSource()==view.buttons[0]) {
            try {
                double a = Double.parseDouble(view.input1.getText());
                double b = Double.parseDouble(view.input2.getText());
                double ans = a+b;
                view.output.setText("" + ans);
            } catch (NumberFormatException ex) {
                System.out.println("输入非法字符 " + ex.getMessage());
            }
        }
        else if(e.getSource()==view.buttons[1])
        {
            try {
                double a = Double.parseDouble(view.input1.getText());
                double b = Double.parseDouble(view.input2.getText());
                double ans = a-b;
                view.output.setText("" + ans);
            } catch (NumberFormatException ex) {
                System.out.println("输入非法字符 " + ex.getMessage());
            }
        }
        else if(e.getSource()==view.buttons[2])
        {
            try {
                double a = Double.parseDouble(view.input1.getText());
                double b = Double.parseDouble(view.input2.getText());
                double ans = a*b;
                view.output.setText("" + ans);
                }
            catch (NumberFormatException ex) {
                System.out.println("输入非法字符 " + ex.getMessage());
            }

        }
        else if(e.getSource()==view.buttons[3])
        {
            try {
                double a = Double.parseDouble(view.input1.getText());
                double b = Double.parseDouble(view.input2.getText());
                double ans = a/b;
                view.output.setText("" + ans);
            }
            catch (NumberFormatException ex) {
                System.out.println("输入非法字符 " + ex.getMessage());
            }

        }




    }
}

在WindowFrameCal类下的init方法底部添加一行代码：add(panel)；

 void init()
    {
        box = Box.createVerticalBox();
        panel =new JPanel();
        input1=new JTextField(10);
        input2=new JTextField(10);
        output=new JTextField(10);
        buttons[0] = new JButton("加");
        buttons[1] = new JButton("减");
        buttons[2] = new JButton("乘");
        buttons[3] = new JButton("除");
        Font f = new Font("宋体",Font.BOLD,28);
       // SetFont.setFont
             //   (f,input1,input2,output,buttons[0],buttons[1],buttons[2],buttons[3]);
        panel.add(input1);
        panel.add(input2);
        panel.add(output);
        panel.add(box);
        for(int i=0;i<buttons.length;i++)
        {
            buttons[i].addActionListener(ca);
            box.add(buttons[i]);
        }
        // 加入此行代码即可显示
        add(panel);
    }

研究不易，望采纳！