while 语句中的无限循环

Question

    import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        System.out.println("\nThe sum of the numbers is: " + getSumOfInput());
    }

    public static int getSumOfInput () {
        int counter = 0;
        int sumOfNums = 0;

        Scanner userInput = new Scanner(System.in);

        while(counter <= 10) {
            System.out.print("Enter the number " + counter + ": ");

            boolean checkValidity = userInput.hasNextInt();

            if(checkValidity) {
                int userNum = userInput.nextInt();
                userInput.nextLine();

                System.out.println("Number " + userNum + " added to the total sum.");
                sumOfNums += userNum;
                counter++;

            } else {
                System.out.println("Invalid input. Please, enter a number.");
            }

        }

        userInput.close();

        return sumOfNums;
    }

}

Hello everybody!

I just started java and I learned about control flow and now I moved on to user input, so I don't know much. The problem is this code. Works just fine if you enter valid input as I tested, nothing to get worried about. The problem is that I want to check for wrong input from user, for example when they enter a string like "asdew". I want to display the error from else statement and to move on back to asking the user for another input, but after such an input the program will enter in an infinite loop displaying "Enter the number X: Invalid input. Please, enter a number.".

Can you tell me what's wrong? Please, mind the fact that I have few notions when it comes to what java can offer, so your range of solutions it's a little bit limited.

Answer 1

The issue is, that once you enter intput, which can not be interpreted as an int , userInput.hasNextInt() will return false (as expected). But this call will not clear the input, so for every loop iteration the condition doesn't change. So you get an infinite loop.

From Scanner#hasNextInt() :

Returns true if the next token in this scanner's input can be interpreted as an int value in the default radix using the nextInt() method. The scanner does not advance past any input.

The fix is to clear the input if you came across invalid input. For example:

    } else {
        System.out.println("Invalid input. Please, enter a number.");
        userInput.nextLine();
    }

Another approach you could take, which requires less input reads from the scanner, is to always take the next line regardless and then handle the incorrect input while parsing.

public static int getSumOfInput() {
    int counter = 0;
    int sumOfNums = 0;

    Scanner userInput = new Scanner(System.in);

    while (counter <= 10) {
        System.out.print("Enter the number " + counter + ": ");

        String input = userInput.nextLine();
        try {
            int convertedInput = Integer.parseInt(input);
            System.out.println("Number " + convertedInput + " added to the total sum.");
            sumOfNums += convertedInput;
            counter++;
        } catch (NumberFormatException e) {
            System.out.println("Invalid input. Please, enter a number.");
        }
    }

    return sumOfNums;
}

Answer 2

Call userInput.nextLine(); just after while :

...
while(counter <= 10) {
  System.out.print("Enter the number " + counter + ": ");
  userInput.nextLine();
...