一个比较傻瓜的问题

Question

要求编写一个模拟计算器的Applet,使用面板和网格布局，添加一个文本框，10个数字按钮（0~9），4个加减乘除按钮，一个等号按钮，一个清除按钮。要求将计算公式和结果显示在文本框当中。 
其中数字按钮和清除按钮的事件好解决，但是4个加减乘除按钮，一个等号按钮的事件处理我不会，请高手指点，不胜感激。

Answer 1

public class Calculator extends Applet implements ActionListener
{
         private final String[] KEYS={"7","8","9","/",
                                        "4","5","6","*",
                                         "1","2","3","-",
                                         ".","0","=","+"};
          private Button keys[]=new Button[KEYS.length];
          private Button reset=new Button("CE");
          private TextField display=new TextField("0");
     
          private void setup()
        {
            String s=new String();
            Panel calkeys=new Panel();
            calkeys.setLayout(new GridLayout(4,4));
            for(int i=0; i<KEYS.length;i++)
                {
                keys[i]=new Button(KEYS[i]);
                calkeys.add(keys[i]);
                }            Panel top=new Panel();
            top.setLayout(new BorderLayout());
            top.add("Center",display);
            top.add("East",reset);
            setLayout(new BorderLayout());
            add("North",top);
            add("Center",calkeys);        }
          public void init()
                   {
                       setup();
                       for(int i=0;i<KEYS.length;i++)
                           keys[i].addActionListener(this);
                       reset.addActionListener(this);
                       display.addActionListener(this);
        
                   }
                   public void actionPerformed(ActionEvent e)
                   {
                       Object target=e.getSource();
                       String label=e.getActionCommand();
                       if(target==reset)
                           handleReset();
                       else if ("0123456789.".indexOf(label)>=0)
                           handleNumber(label);
                       else
                           handleOperator(label);
                   }
        
                 private boolean firstDigit=true;
                   private void handleNumber(String key)
                   {
                       if(firstDigit)
                           display.setText(key);
                       else if((key.equals("."))&&(display.getText().indexOf(".")<0))
                           display.setText(display.getText()+".");
                       else if (!key.equals("."))
                           display.setText(display.getText()+key);
                       firstDigit=false;
                   }
        
                   private String operator="=";
                   private double number=0.0;
                    private void handleOperator(String key)
                   {
                    if(operator.equals("+"))
                        number+=getNumberFromDisplay();
                    else if(operator.equals("-"))
                        number-=getNumberFromDisplay();
                    else if(operator.equals("*"))
                        number-=getNumberFromDisplay();
                    else if(operator.equals("/"))
                        number/=getNumberFromDisplay();
        
                    else if(operator.equals("="))
                        number=getNumberFromDisplay();
                    display.setText(String.valueOf(number));
                    operator=key; 
                    firstDigit=true;
                }
                private void handleReset()
                   {
                      display.setText("0");
                      firstDigit=true;
                      operator="=";
                   }
                public double getNumberFromDisplay()
                  { 
                      return  Double.valueOf(display.getText()).doubleValue(); 
          }