System.out.println("购买物品\t" + "单价\t" + "个数\t" + "金额");
Scanner in = new Scanner(System.in);
String String1 ;

    int a = 0;
    int b = 0;
    int c = 0;
    double sum=0;

    do {
        System.out.println("请选择你购买的物品");

        String aString = in.next();

        if (aString.equals("上衣")) {
            System.out.println("请选择购买的数量：");
            int num1 = in.nextInt();
            a = num1 * 245;
            System.out.println("上衣的金额为" + a);

        } else if (aString.equals("网球鞋")) {
            System.out.println("请选择购买的数量：");
            int num2 = in.nextInt();
            b = 570 * num2;
            System.out.println("网球鞋的金额为：" + b);

        } else if (aString.equals("网球拍")) {
            System.out.println("请选择购买的数量：");
            int num3 = in.nextInt();
            c = 320 * num3;
            System.out.println("网球拍的金额为：" + c);

        } else {
            System.out.println("输入错误，请重新输入。");
        }

        System.out.println("是否继续输入Y/N");
         String1 = in.next();

    } while (String1.equals("Y"));
    sum=a+b+c;
    System.out.println("总金额为：" +sum );
}

}

int a = 0;
int b = 0;
int c = 0;

这一部分可以简化为int a=0；

再把sum=sum+a;放进do-while循环体里面，就可以自动计算总价了。