JAVA编程小题目，求大神帮忙解答看看，谢谢！

import java.util.Scanner;
public class test{
public static void main(String [] args){
System.out.println("***************");
System.out.println("请选择购买的商品编号：");
System.out.println("1.T 恤 2.网球鞋 3.网球拍");
System.out.println("***************");
double total=0;

Scanner in=new Scanner(System.in);
boolean b=true;

do{
int num1, num2;
double temp;
System.out.println();
System.out.print("请输入商品编号：");
num1=in.nextInt();
System.out.print("请输入购买数量：");
num2=in.nextInt();
switch(num1){
case 1: temp = num2*245.0;
total += temp;

System.out.println("T 恤 245.0 数量"+num2+" 合计"temp);
break:
case 2: temp = num2*570.0;
total += temp;

System.out.println("网球鞋 570.0 数量"+num2+" 合计"temp);
break:
case 3: temp = num2*1000.0;
total += temp;

System.out.println("T 恤 245.0 数量"+num2+" 合计"temp);
break:

}
while(true)(

System.out.print("是否继续(y/n)");
String str = in.next();
if(str=="n"){
b=false;

break;

}else if(str=="y"){}
else{System.out.print("输入错误!请重新输入")})

}while(b)
System.out.println("折扣：0.8");
System.out.println("应付金额："+total);
System.out.print("实付金额：");
double x;
x=in.nextDouble();
double a=x-total

System.out.print("找钱："+a);
}

}

如果没什么问题而且没要求用类的话应该是这样，