菜鸟求助。。补充计算器的程序

我们老师给了个计算器的程序，，要求补充完好它。老师只给了加法的。。其他的都没。
只要加上减法。乘法，除法，清零，小数点。。
偶不会啊急求。。明天早上要交了。拜托。，我是超级小菜。所以问题比较低级。
下面是老师给的东西import java.awt.*;
import java.awt.event.*;
import javax.swing.*;public class Calculator 
{
        public static void main(String args[])
        {
                MyFrame m=new MyFrame("计算器");
                Toolkit theKit = m.getToolkit();
                Dimension wndSize = theKit.getScreenSize();
                
                m.setBounds(wndSize.width/4, wndSize.height/4, wndSize.width/2, wndSize.height/2);
                m.setVisible(true);
                m.pack();
        }
}class MyFrame extends JFrame implements ActionListener
{
        JTextField jtf;
        JPanel jp;
        JButton[] jb;
        JButton jb1,jb2;
        int num1,num2;        public MyFrame(String title)
        {
                super(title);   
                getContentPane().setLayout(new BorderLayout(0,0));
                setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);                jtf=new JTextField();
                
                jp=new JPanel();
                jp.setLayout(new GridLayout(4,3));      
                jb=new JButton[10];
                jb1=new JButton("+");
                jb2=new JButton("=");
                
                for(int i=0;i<jb.length;i++)
                {
                        jb[i]=new JButton(String.valueOf(i));
                        jb[i].addActionListener(this);
                        jp.add(jb[i]);
                }
                
                jb1.addActionListener(this);
                jb2.addActionListener(this);
                jp.add(jb1);
                jp.add(jb2);
                
                getContentPane().add(jtf,BorderLayout.NORTH);
                getContentPane().add(jp,BorderLayout.CENTER);
        }
        
        public void actionPerformed(ActionEvent e)
        {
                JButton be=(JButton)e.getSource();
                String bt=be.getText();
                
                if(bt.equals("+"))
                {
                        num1=Integer.parseInt(jtf.getText());
                        jtf.setText("");
                }
                else if(bt.equals("="))
                {
                        num2=Integer.parseInt(jtf.getText());
                        jtf.setText(String.valueOf(num1+num2));
                }
                else
                {
                        jtf.setText(jtf.getText()+bt);
                }
        }
}
先谢谢了。