为什么运行不出来？改了很多次了，不知道问题出在哪

Question

import java.awt.*;
import java.applet.*;

public class Calculator extends Applet{
TextField tfAnswer; //显示输入和结果的文本域
Button bPoint,bEqual,bPlus,bMinus,bClear,bMulti,bDivision; //运算符按钮
Button[] b=new Button[10]; //数字按钮

String currentOp,preOp; //当前操作和上一步操作
String foreText,backText; //当前输入和上一次输入
boolean isFloat = false; //运算类型标志

public void init() {
    Panel panel1=new Panel();  //实例化面板
    Panel panel2=new Panel();
    Panel panel3=new Panel();

    currentOp = new String(""); //实例化各组件
    preOp = new String("");
    foreText = new String("");
    backText = new String("");
    tfAnswer = new TextField(8);
    setBackground(Color.lightGray); //设置Applet背景色
    setForeground(Color.blue);   //设置Applet前景色        

    for(int i=9;i>=0;i--){ 
        b[i]=new Button(Integer.toString(i)); //实例化数字按钮
        panel2.add(b[i]); //增加按钮到面板
    }        

    bPoint = new Button("."); //实例化按钮
    bEqual = new Button("=");
    bEqual.setForeground(Color.red);   //设置按钮前景色
    bClear = new Button("清除");
    bClear.setForeground(Color.red);
    bDivision = new Button("/");
    bDivision.setForeground(Color.red);
    bMulti = new Button("*");
    bMulti.setForeground(Color.red);
    bMinus = new Button("-");
    bMinus.setForeground(Color.red);    
    bPlus = new Button("+");
    bPlus.setForeground(Color.red);    

    setLayout(new FlowLayout()); //设置布局管理器
    panel1.setLayout(new FlowLayout());
    panel2.setLayout(new GridLayout(4,3));
    panel3.setLayout(new GridLayout(4,1));
    panel1.add(tfAnswer); //增加组件到面板
    panel1.add(bClear);
    panel2.add(bPoint);
    panel2.add(bEqual);
    panel3.add(bPlus);
    panel3.add(bMinus);
    panel3.add(bMulti);
    panel3.add(bDivision);
    add(panel1); //增加组件到Applet
    add(panel2);
    add(panel3);    
}

public boolean action(Event e, Object o) { //事件处理
    String s = new String("");
    for(int i=0;i<10;i++){
        if(e.target==b[i]||e.target==bPoint){ //按钮事件来自于数字按钮和点按钮
            if(e.target != bPoint) {
                s = (String)o;
                doForeText(s); //处理输入
            }
            if((e.target == bPoint)&&(!isFloat)){ //浮点数输入
                isFloat = true; //设置运算标志
                s = (String)o;
                if(foreText.equals("")){
                    foreText += "0.";  //增加小数点前面0
                }
                else{
                    doForeText(s);
                }
            }
        }
    }      
    if(e.target == bClear) {
        doClear();  //清除输入
    }      
    if((e.target == bMulti)||(e.target == bDivision)|| (e.target == bPlus)||(e.target == bMinus)) { //处理运算
        if(foreText != ""){
            currentOp = ((String)o);
            doOperator();  //处理运算
        }
        else {
            preOp = ((String)o);
        }
    }      
    if(e.target == bEqual) { //等于按钮事件处理
        doOperator(); //处理运算
    }  
    return true;  
}

public void doOperator(){
    double dFore,dBack;
    Double d;

    if(preOp.equals("")) {
        backText = foreText;
        foreText = "";
        tfAnswer.setText(backText); //显示文本
    }
    else {
        dFore = (new Double(foreText)).doubleValue(); //得到第一输入
        dBack = (new Double(backText)).doubleValue(); //得到第二输入
        foreText = "";
        backText = tfAnswer.getText(); 

        if(preOp.equals("+")) { //加运算处理
            d = new Double((dBack + dFore)); //得到运算结果
            tfAnswer.setText(d.toString());  //显示运算结果
            backText = d.toString();Double.parseDouble(
        }
        if(preOp.equals("-")) {
            d = new Double((dBack - dFore));//得到运算结果
            tfAnswer.setText(d.toString()); //显示运算结果
            backText = d.toString();
        }
        if(preOp.equals("*")) {
            d = new Double((dBack * dFore));//得到运算结果
            tfAnswer.setText(d.toString());//显示运算结果
            backText = d.toString();
        }
        if(preOp.equals("/")) {
            if (dFore==0){
                tfAnswer.setText("除数不能为0"); //显示出错信息
                return;
            }
            d = new Double((dBack / dFore));//得到运算结果
            tfAnswer.setText(d.toString());//显示运算结果
            backText = d.toString();
        }
    }Math.sin();
    preOp = currentOp;
}

public void doForeText(String s) {
    foreText += s;
    tfAnswer.setText(foreText); //显示输入
}

public void doBackText(String s){
    backText = foreText;
    foreText = "";
    tfAnswer.setText(foreText); //显示输入
}

public void doClear() { //清除输入
    currentOp = "";
    preOp = "";
    foreText = "";
    backText = "";
    isFloat = false;
    tfAnswer.setText(""); 
}

}