大家帮下忙。。。看看计算器这样写为什么不能运行啊？

Question

import java.awt.*;
import java.awt.event.*;
public class Calculator extends Frame implements ActionListener{
    Button[] btn=new Button[10];
    Button btn_add=new Button("+");
    Button btn_sub=new Button("-");
    Button btn_mul=new Button("*");
    Button btn_div=new Button("÷");
    Button btn_c=new Button("clear");
    Button btn_equ=new Button("=");
    TextField ta=new TextField(10);
    Panel p=new Panel();
    static char c;
    String s;
    double a,b;
    int n=0; //统计运算符个数 
    int i=0; // 1为+；2为-；3为*；4为/
    public Calculator()
    {
     super("我的计算器");
     this.addWindowListener(new WindowAdapter(){
     public void windowClosing(WindowEvent e)
     {
     dispose();System.exit(0);
     }
     });
     for(int i=0;i<10;i++)
     {
     btn[i]=new Button(""+i);btn[i].addActionListener(this);
     }
     btn_add.addActionListener(this);
     btn_sub.addActionListener(this);
     btn_mul.addActionListener(this);
     btn_div.addActionListener(this);
     btn_c.addActionListener(this);
     btn_equ.addActionListener(this);
    
     p.setLayout(new GridLayout(4,4));
     p.add(btn[1]);p.add(btn[2]);p.add(btn[3]);p.add(btn_add);
     p.add(btn[4]);p.add(btn[5]);p.add(btn[6]);p.add(btn_sub);
     p.add(btn[7]);p.add(btn[8]);p.add(btn[9]);p.add(btn_mul);
     p.add(btn[0]);p.add(btn_div);p.add(btn_c);p.add(btn_equ);
    
     this.setLayout(new BorderLayout(10,10));
     this.add(ta,"North");
     this.add(p,"Center");
    
     this.pack();
     this.setVisible(true);
    }


public void actionPerformed(ActionEvent e) {

if(e.getSource()==btn[0]) s=s+"0";
if(e.getSource()==btn[1]) s=s+"1";
if(e.getSource()==btn[2]) s=s+"2";
if(e.getSource()==btn[3]) s=s+"3";
if(e.getSource()==btn[4]) s=s+"4";
if(e.getSource()==btn[5]) s=s+"5";
if(e.getSource()==btn[6]) s=s+"6";
if(e.getSource()==btn[7]) s=s+7;
if(e.getSource()==btn[8]) s=s+8;
if(e.getSource()==btn[9]) s=s+9;
                  if(s=="") ta.setText("");        
                   else ta.setText(s);
                   a=Double.parseDouble(s);
                   if(n<1) b=a;
 if(e.getSource()==btn_add)  
 {
c='+';
s="";
                        n=n+1;
                        if(n>1)
                         {
                           switch(i)
                           {
                        case 1:  b=b+a;break;
case 2:  b=b-a;break;
case 3:  b=b*a;break;
case 4:  b=b/a;break;                            }
                            
                         }
                         i=1;
                           
}
if(e.getSource()==btn_sub)  
{
c='-';
s="";
                        n=n+1;
                        if(n>1)
                         {
                           switch(i)
                           {
                        case 1:  b=b+a;break;
case 2:  b=b-a;break;
case 3:  b=b*a;break;
case 4:  b=b/a;break;                            }
                            
                            }
                            i=2;
                         
}
if(e.getSource()==btn_mul)  
{
c='*';
        s="";
                        n=n+1;
                        if(n>1)
                         {
                           switch(i)
                           {
                        case 1:  b=b+a;break;
case 2:  b=b-a;break;
case 3:  b=b*a;break;
case 4:  b=b/a;break;                            }
                          }
                            i=3;
                         
}
if(e.getSource()==btn_div)  
{
c='/';
s="";
                        n=n+1;
                        if(n>1)
                         {
                           switch(i)
                           {
                        case 1:  b=b+a;break;
case 2:  b=b-a;break;
case 3:  b=b*a;break;
case 4:  b=b/a;break;                            }
                            }
                         i=4;
                         
}
if(e.getSource()==btn_equ)
{


switch(i)
{
case 1:  b=b+a;break;
case 2:  b=b+a;break;
case 3:  b=b+a;break;
case 4:  b=b+a;break;
}
                      ta.setText(Double.toString(b));
}
if(e.getSource()==btn_c)  {ta.setText("");s="";b=0;n=0;i=0;};

} public static void main(String[] args) {
Calculator ca=new Calculator();
}
}
大家帮忙看下，是哪错了还是不可以用这种方法实现？
对编写计算器还有很多不明白的地方，希望给点意见
谢谢

Answer 1

第一，你的String s没有初始化，所以导致始终报空指针，只要String s=""就可以了
第二，我测试了下，逻辑有问题，具体的逻辑自己看吧，如果需要，我可以提供个以前自己写的给你参考

Answer 2

以前学java的时候写的import java.awt.*; 
import java.awt.event.*; 
public class Calculator extends WindowAdapter implements ActionListener { 
    private double result=0,data1=0,radixPointDepth=1; 
    private boolean radixPointIndicate=false,resultIndicate=false; 
    private char prec='+'; 
    private Frame f; 
    private TextField tf; 
    private Button b1,b2,b3,b4,b5,b6,b7,b8,b9,b10,b11,b12,b13,b14,b15,b16,b17; 
    private Panel p; 
    private static String a="";
    static public void main(String args[]) { 
        Calculator de=new Calculator(); 
        de.go(); 
    } 
    public void go() { 
        f=new Frame("傻瓜计算器"); 
        p=new Panel(); 
        p.setLayout(new GridLayout(4,4)); 
        tf=new TextField(30); 
        b1=new Button("7"); 
        b2=new Button("8"); 
        b3=new Button("9"); 
        b4=new Button("+"); 
        b5=new Button("4"); 
        b6=new Button("5"); 
        b7=new Button("6"); 
        b8=new Button("-"); 
        b9=new Button("1"); 
        b10=new Button("2"); 
        b11=new Button("3"); 
        b12=new Button("*"); 
        b13=new Button("0"); 
        b14=new Button("."); 
        b15=new Button("="); 
        b16=new Button("/"); 
        b17=new Button("清零"); 
        f.add(tf,"North"); 
        f.add(p,"Center"); 
        f.add(b17,"South"); 
        p.add(b1); 
        p.add(b2); 
        p.add(b3); 
        p.add(b4); 
        p.add(b5); 
        p.add(b6); 
        p.add(b7); 
        p.add(b8); 
        p.add(b9); 
        p.add(b10); 
        p.add(b11); 
        p.add(b12); 
        p.add(b13); 
        p.add(b14); 
        p.add(b15); 
        p.add(b16); 
        b1.addActionListener(this); 
        b2.addActionListener(this); 
        b3.addActionListener(this); 
        b4.addActionListener(this); 
        b5.addActionListener(this); 
        b6.addActionListener(this); 
        b7.addActionListener(this); 
        b8.addActionListener(this); 
        b9.addActionListener(this); 
        b10.addActionListener(this); 
        b11.addActionListener(this); 
        b12.addActionListener(this); 
        b13.addActionListener(this); 
        b14.addActionListener(this); 
        b15.addActionListener(this); 
        b16.addActionListener(this); 
        b17.addActionListener(this); 
        f.addWindowListener(this); 
        f.setSize(250,190); 
        f.setVisible(true); 
    } 
    public void actionPerformed(ActionEvent e) { 
        String s; 
        s=e.getActionCommand(); 
        switch(s.charAt(0)) { 
            case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9': 
            if(resultIndicate) { 
                result=0; 
                data1=0; 
                prec='+'; 
            } 
            Integer Int1=new Integer(s); 
            if(radixPointIndicate) { 
                radixPointDepth=radixPointDepth/10; 
                data1=data1+(Int1.intValue())*radixPointDepth; 
            }     else { 
                data1=data1*10+(Int1.intValue()); 
            } 
            Double displayNumber=new Double(data1); 
            tf.setText(a+displayNumber.toString()); 
            resultIndicate=false; 
            break; 
            case '+': case '-':case '*':case '/':case '=': 
            if(s.charAt(0)!='='&&resultIndicate) { //不是等于而且真
                prec=s.charAt(0); 
                resultIndicate=false; 
            } else { 
                switch(prec) { 
                    case '+': 
                    result=result+data1; 
                    break; 
                    case '-': 
                    result=result-data1; 
                    break; 
                    case '*': 
                    result=result*data1; 
                    break; 
                    case '/': 
                    result=result/data1; 
                    break; 
                } 
            } 
            radixPointIndicate=false; 
            radixPointDepth=1; 
            displayNumber=new Double(result); 
            a=new String(displayNumber.toString()+s.charAt(0));
            if(s.charAt(0)=='='){
                tf.setText(displayNumber.toString()); 
            }else{
                tf.setText(a);
            }    
            
            if(s.charAt(0)!='=') { //不是等于
                data1=0; 
                prec=s.charAt(0); 
            } else { 
                resultIndicate=true; 
            } 
            break; 
            case '.':
            radixPointIndicate=true; 
            break; 
        } 
        if(s.equals("清零")) { 
            result=0; 
            data1=0; 
            radixPointDepth=1; 
            tf.setText(""); 
            a="";
        } 
    } 
    public void windowClosing(WindowEvent e) { 
        System.exit(0); 
    } 
}