在循环中初始化双精度会给出错误“变量可能尚未初始化”

Question

这个程序应该从用户那里得到一些邮件的重量并计算它的成本。 100g后，成本增加2.5美元/50g。 但是，当我尝试运行该程序时，它说“可变成本可能尚未初始化”。 这是因为我在 if 语句中初始化它吗？ 我知道有些人有错误，因为他们在 if 语句中声明了变量，但我在 if 语句之外声明了我的变量。 我究竟做错了什么。

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的问题，还有一些地方需要改进：

1. 删除多余的检查
2. 修正计算超重成本
3. 基于System.in的Scanner无需关闭

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

正如错误中所说，您尚未初始化这两个变量“mailWeight”和“cost”

由于它们是局部变量（在方法内），因此您需要在访问它们之前对其进行初始化。 您通过赋予初始值进行初始化：

double mailWeight=0;
double cost=0;

Answer 3

如果最后一个 else if 语句出现并且 x < 50 或者当没有任何 if 语句导致 mailWeight 为 true 时，它不会被初始化。 您可以预先初始化成本。 (double cost = 0) 或者您可以添加所有缺少的 else 块。 在读取变量之前，您需要在所有可能的情况下初始化变量。

这可能会起作用：

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);
}

或预初始化成本值：

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);
}