Question

 1 // This program reads in an item's cost and some coupons' information,
 2 // and then determines which is the best coupon to use and the amount 
 3 // to pay.
 4 
 5 import java.util.*;
 6 
 7 
 8 public class Redeem {
 9         
 10     public static void main(String[] args) {
 11         Scanner sc = new Scanner(System.in);
 12         double price = sc.nextDouble();
 13         int size = sc.nextInt();
 14         int i=0;
 15         double amtPay = price;
 16         double negativeAmt= -99999.99;
 17         
 18         ArrayList<Coupon>coupons = new ArrayList<Coupon>(size);
 19         
 20         for(i=0; i<size; i++)   {
 21             Coupon newCoupon = new Coupon(sc.next(), sc.nextDouble());
 22             coupons.add(i, newCoupon);
 23             sc.nextLine();
 24         }   
 25             
 26         for(i=0; i<size; i++)   {
 27             double temp = (coupons.get(i)).payment(price);
 28             if (temp < 0)   {   
 29                 if (temp > negativeAmt) {
 30                     negativeAmt = temp;
 31                     Coupon bestCoupon = new Coupon(coupons.get(i));
 32                 }   
 33                 amtPay = 0.00;
 34             }   
 35             else    {
 36                 if (amtPay != 0)    {
 37                     if (temp < amtPay)  {
 38                         amtPay = temp;
 39                     }
 40                     Coupon bestCoupon = new Coupon(coupons.get(i));
 41                 }   
 42             }       
 43         }       
 44         System.out.println("Best choice: " + bestCoupon);
 45         System.out.printf("You need to pay $%.2f\n", amtPay);
 46     }
 47 }

我不断得到的错误是

Redeem.java:44: error: cannot find symbol
                System.out.println("Best choice: " + bestCoupon);
                                                     ^
  symbol:   variable bestCoupon
  location: class Redeem
1 error

为什么它没有看到它是优惠券类的对象？我在此之前编译了我的Coupon类（使用toString方法，因此println应该检测到一个字符串）并且仍然有这个问题:( helppp

Answer 1

因为在if的范围内宣布，所以它只在那里知道：

if(temp > negativeAmt) { negativeAmt = temp; Coupon bestCoupon = new Coupon(coupons.get(i)); //bestCoupon is known here } //But not here

更一般的例子：

if(something) { int a; if(somethingElse) { int b; a = b; //OK } b = a; //Error, b is not known here }

Answer 2

范围很重要。

您在if旁边限制了该变量的范围。如果该变量不可用，那就在那边。

将您的声明Coupon bestCoupon移至for循环顶部。

然后在循环的一边

 bestCoupon = new Coupon(coupons.get(i));

示例：

Coupon bestCoupon= null;
    for{
      if(){
        bestCoupon=...
       }
    }
//Access here.

Answer 3

 Coupon bestCoupon =null;
 for(i=0; i<size; i++)   {
    ...
    bestCoupon = new Coupon(coupons.get(i)); //not Coupon bestCoupon = new Coupon(coupons.get(i));
    ...
 }
 System.out.println("Best choice: " + bestCoupon);

Answer 4

更改优惠券bestCoupon的范围，如 -

    public class Redeem {

        public static void main(String[] args) {
            Scanner sc = new Scanner(System.in);
            double price = sc.nextDouble();
            int size = sc.nextInt();
            int i=0;
            double amtPay = price;
            double negativeAmt= -99999.99;
            Coupon bestCoupon  =null; //<-- Declaration of Coupon
            ArrayList<Coupon>coupons = new ArrayList<Coupon>(size);

            for(i=0; i<size; i++)   {
                Coupon newCoupon = new Coupon(sc.next(), sc.nextDouble());
                coupons.add(i, newCoupon);
                sc.nextLine();
            }   

            for(i=0; i<size; i++)   {
                double temp = (coupons.get(i)).payment(price);
                if (temp < 0)   {   
                    if (temp > negativeAmt) {
                        negativeAmt = temp;
                        bestCoupon = new Coupon(coupons.get(i));
                    }   
                    amtPay = 0.00;
                }   
                else    {
                    if (amtPay != 0)    {
                        if (temp < amtPay)  {
                            amtPay = temp;
                        }
                        bestCoupon = new Coupon(coupons.get(i));
                    }   
                }       
            }       
            System.out.println("Best choice: " + bestCoupon);
            System.out.printf("You need to pay $%.2f\n", amtPay);
        }
    }

初始化和定义对象时，无法找到对象的符号错误

4 个答案: