Java使用do-while和for循环为用户输入错误创建新方法

Question

Alright, so I have this beauty of a code that I want to squish into a new method. So far I figured obviously the error reports will be inside the method. However, it's not as simple as that because while the grade count is an int the grade itself is a double and calculated under a for loop. Many ways I've tried do not even accept an input from the user.

I've tried looking around for a better example than a simple method format, but I have yet to find something similar to my problem. Is there a way to put both grade count and actual grade within the method for it to work? I've tried condensing code within a method before, but it was with a much simpler code than this one.

I get the idea of methods, but actually doing them is confusing. Help is super appreciated!!!!

public static void main(String[] args) {
    // create a console object from the Scanner class
    Scanner input = new Scanner(System.in);

    System.out.println("Welcome to average grade calculation service!");

    String userChoice = null;

    // Use a loop to repeat the average grade calculation
    do {
        // Use a loop to read the number and check the error
        boolean isCorrectNumber = false;

        // First, get the number of grades
        int numberOfGrades = 0;

        // Loop until the user enters a numeric number of grades
        do {
            // Prompt the user to enter the number of grades
            System.out.print("\nEnter the number of grades: ");

            // Check if the number of grades is not numeric 
            if (!input.hasNextInt()) {
                // Display number input error
                System.out.println("Error: The number of grades must be a number.");
                input.nextLine(); // flush out the input
            } else {
                // Read the number of grades
                numberOfGrades = input.nextInt();

                // Check if the number of grade is negative
                if (numberOfGrades < 0) {
                    // Display the non positive number error
                    System.out.println("Error: The number of grades must be positive.");
                } else
                    isCorrectNumber = true; // End the data validation loop
            } 

        } while (!isCorrectNumber); // continue until getting a correct data

        //*** Second, get all these grades to compute the sum of the grades

        // Declare variables for the sum of the grade and grade
        double sumOfGrades = 0;
        double aGrade = 0;

        // Use a loop to read each grade
        for (int i = 1; i <= numberOfGrades; i++) {

            // Use a loop to read and validate each grade
            isCorrectNumber = false;
            do {
                // Prompt the user to enter a grade
                System.out.print("Enter grade " + i + ": ");

                // Check if the grade is not numeric
                if (!input.hasNextDouble()) {
                    // Display grade input error
                    System.out.println("Error: The grade must be a number.");
                    input.nextLine(); // flush out the input
                } else {
                    // Read the grade
                    aGrade = input.nextDouble();

                    // Check if the grade is negative
                    if (aGrade < 0)
                        // Display negative grade input error
                        System.out.println("Error: The grade must be zero or positive.");
                    else
                        isCorrectNumber = true; // End the grade validation loop
                }
            } while (!isCorrectNumber); // continue until getting a correct grade

            // Add the grade to the sum and continue the loop
            sumOfGrades += aGrade;
        }
        input.nextLine();

        // Compute the average grade only if the sum is greater than 0
        double averageGrade = (sumOfGrades > 0)? sumOfGrades / numberOfGrades : 0;

        // Display the average grade
        System.out.printf("\nThe average grade is %4.2f\n",  averageGrade);

        // Prompt if the user want to continue the calculation
        System.out.print("\nDo you want to continue (yes/no)? ");
        userChoice = input.nextLine();
        System.out.println("");

        // Continue the loop if the user wants to continue the calculation
    } while (userChoice.equalsIgnoreCase("yes")); 

    // Display an acknowledgement message
    System.out.println("Thank you and have a nice day!");

    input.close();      // release the console object
}

Answer 1

I'd break your method into few simpler methods. Method should have a specific function. It'll make it easier to define them and also to debug. Also we can easily replace/modify the internal details of a method and it's users won't be affected. In the following example I can modify how getGradeAvg method calculate the average and none of the other method needs to be changed.

import java.util.Scanner;

public class Test {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        System.out.println("Welcome to average grade calculation service!");
        String userChoice = null;

        do {
                int numberOfGrades = getNumberOfGrades(input);
                double averageGrade = getGradeAvg(numberOfGrades, input);
                System.out.printf("\nThe average grade is %4.2f\n",  averageGrade);
                System.out.print("\nDo you want to continue (yes/no)? ");
                userChoice = input.nextLine();
                System.out.println("");

            } while (userChoice.equalsIgnoreCase("yes")); 

        System.out.println("Thank you and have a nice day!");
        input.close();
    }

    //this method is responsible of getting the number of grades
    private static int getNumberOfGrades(Scanner input) {

        int numberOfGrades = 0;
        boolean isCorrectNumber;

        do {
            isCorrectNumber = false;

            System.out.print("\nEnter the number of grades: ");

            if (!input.hasNextInt()) {
                System.out.println("Error: The number of grades must be a number.");
                input.nextLine();
            } else {
                numberOfGrades = input.nextInt();
                if (numberOfGrades < 0) {
                    System.out.println("Error: The number of grades must be positive.");
                } else
                    isCorrectNumber = true;
            } 
        } while (!isCorrectNumber);

        return numberOfGrades;
    }

    /*
     * this method takes the number of grades as a parameter
     * then asks the user to input grade value of each grade
     * then it calculate the average and returns the value
     */
    private static double getGradeAvg(int numberOfGrades, Scanner input) {

        double sumOfGrades = 0;
        boolean isCorrectNumber;
        double aGrade = 0;

        for (int i = 1; i <= numberOfGrades; i++) {

            isCorrectNumber = false;

            do {
                System.out.print("Enter grade " + i + ": ");

                if (!input.hasNextDouble()) {

                    System.out.println("Error: The grade must be a number.");
                    input.nextLine();
                } else {
                    aGrade = input.nextDouble();

                    if (aGrade < 0) {
                        System.out.println("Error: The grade must be zero or positive.");
                    } else {
                        isCorrectNumber = true;
                    }
                }
            } while (!isCorrectNumber);

            sumOfGrades += aGrade;
        }

        double averageGrade = (sumOfGrades > 0)? sumOfGrades / numberOfGrades : 0;
        return averageGrade;
    }
}