在循環中初始化雙精度會給出錯誤“變量可能尚未初始化”

Question

這個程序應該從用戶那里得到一些郵件的重量並計算它的成本。 100g后，成本增加2.5美元/50g。 但是，當我嘗試運行該程序時，它說“可變成本可能尚未初始化”。 這是因為我在 if 語句中初始化它嗎？ 我知道有些人有錯誤，因為他們在 if 語句中聲明了變量，但我在 if 語句之外聲明了我的變量。 我究竟做錯了什么。

import java.util.Scanner;
class Main {
  public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        double mailWeight;
        double cost;

    System.out.println("How much does your mail weigh (g)");
        mailWeight = in.nextDouble();

        in.close();

        if (mailWeight > 0 && mailWeight <= 30) {
            cost = 4;
        }else if (mailWeight > 30 && mailWeight <= 50) {
            cost = 5.50;
        }else if (mailWeight > 50) {
            double x = mailWeight - 100;
            if (x >= 50) {
                double y = x/50;
                Math.ceil(y);
                double z = y * 2.5;
                cost = z + 7;
            }
        }
        System.out.println(cost);
  }
}

Answer 1

除了variable not initialized的問題，還有一些地方需要改進：

1. 刪除多余的檢查
2. 修正計算超重成本
3. 基於System.in的Scanner無需關閉

Scanner in = new Scanner(System.in);
System.out.println("How much does your mail weigh (g)");

double mailWeight = in.nextDouble();
double cost;

if (mailWeight <= 0) {
    cost = 0;
} else if (mailWeight <= 30) {
    cost = 4;
} else if (mailWeight <= 50) {
    cost = 5.50;
} else {
    cost = 7;

    if (mailWeight > 100) {
        double y = Math.ceil((mailWeight - 100) / 50.0);
        cost += 2.5 * y;
    }
}
System.out.println(cost);

Answer 2

正如錯誤中所說，您尚未初始化這兩個變量“mailWeight”和“cost”

由於它們是局部變量（在方法內），因此您需要在訪問它們之前對其進行初始化。 您通過賦予初始值進行初始化：

double mailWeight=0;
double cost=0;

Answer 3

如果最后一個 else if 語句出現並且 x < 50 或者當沒有任何 if 語句導致 mailWeight 為 true 時，它不會被初始化。 您可以預先初始化成本。 (double cost = 0) 或者您可以添加所有缺少的 else 塊。 在讀取變量之前，您需要在所有可能的情況下初始化變量。

這可能會起作用：

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);

    double mailWeight;
    double cost;

    System.out.println("How much does your mail weigh (g)");
    mailWeight = in.nextDouble();

    in.close();

    if (mailWeight > 0 && mailWeight <= 30) {
        cost = 4;
    } else if (mailWeight > 30 && mailWeight <= 50) {
        cost = 5.50;
    } else if (mailWeight > 50) {
        double x = mailWeight - 100;
        if (x >= 50) {
            double y = x/50;
            Math.ceil(y);
            double z = y * 2.5;
            cost = z + 7;
        } else {
            cost = 0; // Or whatever you want to set it to in this case
        }
    } else {
        cost = 0; // Or whatever you want to set it to in this case
    }
    System.out.println(cost);
}

或預初始化成本值：

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);

    double mailWeight;
    double cost = 0; // Or whatever you want it to be if none of the cases match

    System.out.println("How much does your mail weigh (g)");
    mailWeight = in.nextDouble();

    in.close();

    if (mailWeight > 0 && mailWeight <= 30) {
        cost = 4;
    } else if (mailWeight > 30 && mailWeight <= 50) {
        cost = 5.50;
    } else if (mailWeight > 50) {
        double x = mailWeight - 100;
        if (x >= 50) {
            double y = x/50;
            Math.ceil(y);
            double z = y * 2.5;
            cost = z + 7;
        }
    }
    System.out.println(cost);
}