Java 如何退出这个while循环？

这是我的密码。我的程序也做了我需要的一切，但我如何退出while循环？？尝试了不同的选择，但没有任何效果。任何建议或帮助都将不胜感激。非常感谢您抽出时间

另外（我想通过用户输入退出循环），所以当他们键入999时，它会打印出来谢谢并结束

package project02;
import java.util.Scanner;

public class Project02 
{



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

        //Declaring variables for a,b,and c
        double a = 1;
        double b = 8;
        double c = 16;

        //Declaring variables for roots
        double x1 = 0;
        double x2 = 0;

        //Discremenent
        double d = (Math.pow(b, 2) - 4*a*c);

        //Inputs
        System.out.println("Input the values a, b, and c for ax^2+bx+c = 0");
        System.out.println("Input a: ");
        a = scan.nextDouble();

        System.out.println("Input b: ");
        b = scan.nextDouble();

        System.out.println("Input c: ");
        c = scan.nextDouble();

        while (d != 999)
    {   
        if (d > 0)
        {
            x1 = (-b + Math.sqrt(b*b - 4*a*c))/2*a;
            x2 = (-b - Math.sqrt(b*b - 4*a*c))/2*a;

            System.out.println("Root 1 is: " + x1);
            System.out.println("Root 2 is: " + x2);
        }
        else if (d == 0)
        {
            x1 = (-b + Math.sqrt(b*b - 4*a*c))/2*a;

            System.out.println("There is only one real root at x = " + x1);
        }
        else
        {
            System.out.println("There are no real roots");
        }
        System.out.println("\n" + "Input the values a, b, and c for ax^2+bx+c = 0 or enter 999 to      stop.");

        System.out.println("Input a: ");
        a = scan.nextDouble();

        System.out.println("Input b: ");
        b = scan.nextDouble();

        System.out.println("Input c: ");
        c = scan.nextDouble();

    }   
        System.out.println("Thank you!!!");
    }
}

---

为了退出while循环，需要将d设置为999。我在你的代码中看不到这一点。另一个选项是使用“break”。例如：

System.out.println("Input a: "); 
a = scan.nextDouble();
if(a == 999) {
    break;
}

或

您从未设置过“d”变量，因此这意味着它从未设置过。

---

您需要读取用户的输入并在循环内设置d变量

在您的情况下，当a=999时，您需要从while循环中中断

像这样：

System.out.println("Input a: ");
a = scan.nextDouble();

if(a == 999) break;

至于整个程序，我会使用

do while

循环

像这样：

Scanner scan = new Scanner(System.in);

//Variable for continue.
String response = "n";

//Declaring variables for a,b,and c
double a = 1;
double b = 8;
double c = 16;

//Declaring variables for roots
double x1 = 0;
double x2 = 0;

//Discremenent
double d = (Math.pow(b, 2) - 4*a*c);



do
{   
    //Inputs
    System.out.println("Input the values a, b, and c for ax^2+bx+c = 0");
    System.out.println("Input a: ");
    a = scan.nextDouble();

    System.out.println("Input b: ");
    b = scan.nextDouble();

    System.out.println("Input c: ");
    c = scan.nextDouble();

    if (d > 0)
    {
        x1 = (-b + Math.sqrt(b*b - 4*a*c))/2*a;
        x2 = (-b - Math.sqrt(b*b - 4*a*c))/2*a;

        System.out.println("Root 1 is: " + x1);
        System.out.println("Root 2 is: " + x2);
    }
    else if (d == 0)
    {
        x1 = (-b + Math.sqrt(b*b - 4*a*c))/2*a;

        System.out.println("There is only one real root at x = " + x1);
    }
    else
    {
        System.out.println("There are no real roots");
    }
    System.out.println("Continue?(y/n)");
    response = scan.next();

} while(response.equalsIgnoreCase("y");

System.out.println("Thank you!!!");

---

希望这有帮助

我返回并尝试了您的方式，但这部分代码是：System.out.println（“Continue？（y/n）”；response=scan.nextChar（）；我得到一个scan.nextChar（）的错误；出于某种原因…我编辑了代码。我把Java和另一种语言混淆了。nextChar（）不存在。而是使用字符串作为变量。我还将while（）中的比较改为更干净的。最后，它开始工作了。谢谢你的帮助，我的人。不用担心，谢谢！

Scanner scan = new Scanner(System.in);

//Variable for continue.
String response = "n";

//Declaring variables for a,b,and c
double a = 1;
double b = 8;
double c = 16;

//Declaring variables for roots
double x1 = 0;
double x2 = 0;

//Discremenent
double d = (Math.pow(b, 2) - 4*a*c);



do
{   
    //Inputs
    System.out.println("Input the values a, b, and c for ax^2+bx+c = 0");
    System.out.println("Input a: ");
    a = scan.nextDouble();

    System.out.println("Input b: ");
    b = scan.nextDouble();

    System.out.println("Input c: ");
    c = scan.nextDouble();

    if (d > 0)
    {
        x1 = (-b + Math.sqrt(b*b - 4*a*c))/2*a;
        x2 = (-b - Math.sqrt(b*b - 4*a*c))/2*a;

        System.out.println("Root 1 is: " + x1);
        System.out.println("Root 2 is: " + x2);
    }
    else if (d == 0)
    {
        x1 = (-b + Math.sqrt(b*b - 4*a*c))/2*a;

        System.out.println("There is only one real root at x = " + x1);
    }
    else
    {
        System.out.println("There are no real roots");
    }
    System.out.println("Continue?(y/n)");
    response = scan.next();

} while(response.equalsIgnoreCase("y");

System.out.println("Thank you!!!");