java - 输入特定数字后停止的 while 循环

Question

我正在尝试编写一个程序，该程序从用户处获取成绩数字，并在输入 -1 时停止。但是，每次我输入-1时，while循环都会继续运行，如何阻止它运行？

import java.util.Scanner;

public class getLetterGrade {
    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        int count = 0;
        while (count >= 0)
            getGrade();
        count++;
        System.out.println("You entered" + " " + count + " " + "Student(s)");
    }

    public static void getGrade() {
        Scanner reader = new Scanner(System.in);
        double grade;

        System.out.println("Welcome to the grade calculator. \n Please enter a numeric grade. \n After the last student in the class, enter a grade of -1.");
        grade = reader.nextDouble();

        if (grade >= 90)
            System.out.println("That's an A");
        else if (grade >= 80 && grade < 90)
            System.out.println("That's an B");
        else if (grade >= 70 && grade < 80)
            System.out.println("That's an C");
        else if (grade >= 60 && grade < 70)
            System.out.println("That's an D");
        else if (grade <= 50 && grade > 0)
            System.out.println("That's an F");
        if (grade == -1)
            System.out.println("");
    }
}

Answer 1

需要更改一些内容...首先，您的循环不断循环，因为您从未将 count 设置为小于 0，因此条件始终为 true。您还创建了 2 个 Scanner 对象，但只使用第二个对象(在 getGrade() 中)。

尝试此代码，并查看注释以查看更改的所有内容:

    public class getLetterGrade  {

    static Scanner reader;      //Add reader as member variable

    public static void main(String[] args) {
        //Scanner reader = new Scanner(System.in);  //Remove this unused Scanner
        int count = 0;
        reader = new Scanner(System.in);

        while (getGrade())      //Change the condition here so the loop will continue while getGrade() returns true.
            count++;

        reader.close();     //Close the Scanner
        System.out.println("You entered" + " " + count + " " + "Student(s)");
    }

    public static boolean getGrade() {      //getGrade() returns a boolean...


        System.out.println("Welcome to the grade calculator. \n Please enter a numeric grade. \n After the last student in the class, enter a grade of -1.");
        double grade = reader.nextDouble();  //Consider using a try/catch block here.


        if (grade >= 90)
            System.out.println("That's an A");
        else if (grade >= 80 && grade < 90)
            System.out.println("That's an B");
        else if (grade >= 70 && grade < 80)
            System.out.println("That's an C");
        else if (grade >= 60 && grade < 70)
            System.out.println("That's an D");
        else if (grade <= 50 && grade > 0)
            System.out.println("That's an F");
        else if(grade != -1)
            System.out.println("Invaled input!"); //Print if 'grade' is something other then the above statements.
        if (grade == -1) {                      //Add brackets
            System.out.println("");
            return false;                       //return false if -1 was entered...
        }
        return true;                            //else return true
    }
}