如何接受用户输入并将其放入 while 条件

Question

这是我的代码，while 循环没有输入，rep 变量不接受输入：

import java.util.Scanner;

public class MixedData {
    public static void main(String[] args) {
        String rep = "";
        do {
            Scanner keyboard = new Scanner(System.in);
            System.out.print("Enter your full name");
            String name = keyboard.nextLine();

            System.out.print("Enter your GPA: ");
            double gpa = keyboard.nextDouble();

            System.out.println("Name: " + name + ", GPA: " + gpa);
            System.out.println("Do you want to enter the data for another student?(y/n)");
            rep = keyboard.nextLine();
        } // This does not accept input
        while (rep.equals("y"));
    }
}

Answer 1

要么在rep = keyboard.nextLine(); 之前再添加一个keyboard.nextLine()（为了清除换行符），要么读取您的 double gpa 值：

double gpa = Double.parseDouble(keyboard.nextLine());

重要点要在这里理解（尤其是如果您是 Java 开发新手），关于为什么您的代码不起作用，是您调用 nextDouble() 作为您的最后一个方法扫描仪实例，它不会将光标移动到下一行。

更多细节：

nextX() 模式下的所有方法（如nextDouble()、nextInt() 等），nextLine() 除外，读取您输入的下一个令牌，但如果令牌不是换行字符，则光标不会移动到下一行。当您输入双精度值并点击Enter 时，您实际上给输入流两个标记：一个双精度值和一个换行符，双精度值被初始化到变量中，换行字符留在输入流中。下次您调用 nextLine() 时，会读取那个非常新的行字符，这就是给您一个空字符串的原因。

Answer 2

这是使用 while 循环而不是 do-while 的相同代码。它按照您想要的方式工作。

import java.util.Scanner;

public class MixedData {
    public static void main(String[] args) {
        String rep = "y";

        while (!rep.equals("n")) {
            Scanner keyboard = new Scanner(System.in);
            System.out.print("Enter your full name: ");
            String name = keyboard.nextLine();

            System.out.print("Enter your GPA: ");
            double gpa = keyboard.nextDouble();

            System.out.println("Name: " + name + ",GPA: " + gpa);
            System.out.println("Do you want to enter the data for another student?(y/n)");
            rep = keyboard.next();
        }
    }
}

Answer 3

你需要跳过空白行。

public static void main(String[] args) {
    String rep;
    Scanner keyboard = new Scanner(System.in);
    do {
        System.out.print("Enter your full name");
        String name = keyboard.nextLine();

        System.out.print("Enter your GPA: ");
        double gpa = keyboard.nextDouble();

        System.out.println("Name: " + name + ", GPA: " + gpa);
        System.out.println("Do you want to enter the data for another student?(y/n)");
        rep = keyboard.next();
        keyboard.skip("\r\n"); // to skip blank lines
    }
    while (rep.equalsIgnoreCase("y"));
    keyboard.close();
}

Answer 4

使用nextLine 代替nextDouble：

public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);
    String rep = "";
    do {
        System.out.println("Enter your full name:");
        String name = keyboard.nextLine();

        System.out.println("Enter your GPA:");
//        double gpa = keyboard.nextDouble();
        double gpa = Double.parseDouble(keyboard.nextLine());

        System.out.println("Name: " + name + ", GPA: " + gpa);

        System.out.println("Do you want to enter the data for another student?(y/n)");
        rep = keyboard.nextLine();
    } while (rep.equals("y"));
    keyboard.close();
}