try/catch with InputMismatchException creates infinite loop

Question

So I'm building a program which takes ints from user input. I have what seems to be a very straightforward try/catch block which, if the user doesn't enter an int, should repeat the block until they do. Here's the relevant part of the code:

import java.util.InputMismatchException;
import java.util.Scanner;


public class Except {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        boolean bError = true;
        int n1 = 0, n2 = 0, nQuotient = 0;

        do {
            try {
                System.out.println("Enter first num: ");
                n1 = input.nextInt();
                System.out.println("Enter second num: ");
                n2 = input.nextInt();
                nQuotient = n1/n2;
                bError = false;
            } 
            catch (Exception e) {
                System.out.println("Error!");
            }
        } while (bError);

        System.out.printf("%d/%d = %d",n1,n2, nQuotient);
    }
}

If I enter a 0 for the second integer, then the try/catch does exactly what it's supposed to and makes me put it in again. But, if I have an InputMismatchException like by entering 5.5 for one of the numbers, it just shows my error message in an infinite loop. Why is this happening, and what can I do about it? (By the way, I have tried explicitly typing InputMismatchException as the argument to catch, but it didn't fix the problem.

Amit Deshpande · Accepted Answer · 2012-10-04T05:17:03.953

You need to call next(); when you get the error. Also it is advisable to use hasNextInt()

       catch (Exception e) {
            System.out.println("Error!");
           input.next();// Move to next other wise exception
        }

Before reading integer value you need to make sure scanner has one. And you will not need exception handling like that.

    Scanner scanner = new Scanner(System.in);
    int n1 = 0, n2 = 0;
    boolean bError = true;
    while (bError) {
        if (scanner.hasNextInt())
            n1 = scanner.nextInt();
        else {
            scanner.next();
            continue;
        }
        if (scanner.hasNextInt())
            n2 = scanner.nextInt();
        else {
            scanner.next();
            continue;
        }
        bError = false;
    }
    System.out.println(n1);
    System.out.println(n2);

Javadoc of Scanner

When a scanner throws an InputMismatchException, the scanner will not pass the token that caused the exception, so that it may be retrieved or skipped via some other method.

This is really great, thanks! Could you explain in a little more detail why you need the `next()` command? I have some idea but it's a bit unclear to me. — limp_chimp, Oct 03 '12 at 18:09

score 4 · Answer 2 · answered Oct 03 '12 at 05:09

YOu can also try the following

   do {
        try {
            System.out.println("Enter first num: ");
            n1 = Integer.parseInt(input.next());

            System.out.println("Enter second num: ");
            n2 = Integer.parseInt(input.next());

            nQuotient = n1/n2;

            bError = false;
        } 
        catch (Exception e) {
            System.out.println("Error!");
            input.reset();
        }
    } while (bError);

score 3 · Answer 3 · edited Nov 19 '16 at 20:20

3

To follow debobroto das's answer you can also put after

input.reset();
input.next();

I had the same problem and when I tried this. It completely fixed it.

edited Nov 19 '16 at 20:20

zx485

28,498
28
50
59

answered Nov 19 '16 at 19:53

YoMama420

31
1

score 2 · Answer 4 · answered Jul 29 '14 at 11:47

2

another option is to define Scanner input = new Scanner(System.in); inside the try block, this will create a new object each time you need to re-enter the values.

answered Jul 29 '14 at 11:47

ron17ro

21
3

score 1 · Answer 5 · answered Oct 03 '12 at 05:02

As the bError = false statement is never reached in the try block, and the statement is struck to the input taken, it keeps printing the error in infinite loop.

Try using it this way by using hasNextInt()

catch (Exception e) {
            System.out.println("Error!");
           input.hasNextInt();         
        }

Or try using nextLine() coupled with Integer.parseInt() for taking input....

Scanner scan = new Scanner(System.in);

int num1 = Integer.parseInt(scan.nextLine());
int num2 = Integer.parseInt(scan.nextLine());

score 1 · Answer 6 · answered Oct 03 '12 at 05:03

To complement the AmitD answer:

Just copy/pasted your program and had this output:

Error!
Enter first num: 
.... infinite times ....

As you can see, the instruction:

n1 = input.nextInt();

Is continuously throwing the Exception when your double number is entered, and that's because your stream is not cleared. To fix it, follow the AmitD answer.

score 1 · Answer 7 · edited May 23 '17 at 12:26

@Limp, your answer is right, just use .nextLine() while reading the input. Sample code:

    do {
        try {
            System.out.println("Enter first num: ");
            n1 = Integer.parseInt(input.nextLine());
            System.out.println("Enter second num: ");
            n2 = Integer.parseInt(input.nextLine());
            nQuotient = n1 / n2;
            bError = false;
        } catch (Exception e) {
            System.out.println("Error!");
        }
    } while (bError);

    System.out.printf("%d/%d = %d", n1, n2, nQuotient);

Read the description of why this problem was caused in the link below. Look for the answer I posted for the detail in this thread. Java Homework user input issue

Ok, I will briefly describe it. When you read input using nextInt(), you just read the number part but the ENDLINE character was still on the stream. That was the main cause. Now look at the code above, all I did is read the whole line and parse it , it still throws the exception and work the way you were expecting it to work. Rest of your code works fine.

try/catch with InputMismatchException creates infinite loop

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