Initializing doubles in a loop giving error "variable might not have been initialized"

Question

This program is supposed to get the weight of some mail from the user and calculate how much it costs. After 100g, the cost increases by $2.5/50g. However, when I try to run the program, it says "variable cost might not have been initialized". Is this because I am innitalizing it in an if statement or something? I know some people have had errors because they declare the variable in the if statment, but I have declared my variable outside the if statement. What am I doing wrong.

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

It does not get initialized if the last else if statment occures and x < 50 or when none of the if statements result in true for mailWeight. You could preinitialize cost. (double cost = 0) Or you could add all missing else blocks. You need to initialize your variable in all possible cases before reading it.

This would probably work:

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);
}

Or preinitializing the cost value:

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);
}