如何接受用户输入并将其放入 while 条件

Question

This is my code, the while loop does not have an input and the rep variable does not accept an input:

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

Either just add one more keyboard.nextLine() before rep = keyboard.nextLine(); ( in order to clear the newline character ), or read your double gpa value with:

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

Important point to understand here (especially if you're a novice Java developer), about why your code doesn't work, is, that you invoke nextDouble() as a last method on your Scanner instance, and it doesn't move the cursor to the next line .

A bit more details:

All the methods patterned nextX() (like nextDouble() , nextInt() , etc.), except nextLine() , read next token you enter, but if the token isn't a new line character, then the cursor isn't moved to the next line. When you enter double value and hit Enter , you actually give to the input stream two tokens: a double value , and a new line character , the double value is initialized into the variable, and the new line character stays into input stream. The next time you invoke nextLine() , that very new line character is read, and that's what gives you an empty string.

Answer 2

You need to skip blank lines.

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 3

Here's the same code using a while loop instead of do-while. It works the way you want it to.

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 4

Use nextLine instead of 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();
}