计算器的程序补充。。。。急

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。