How to Avoid Infinite Loops When Handling InputMismatchException in Java?

try/catch with InputMismatchException: Eliminating Infinite Loop

Problem Encountered

In a program that reads integer inputs from the user, utilizing a try/catch block to handle invalid entries, an infinite loop occurs when an InputMismatchException is thrown (e.g., when the user enters a non-integer character).

Cause of the Infinite Loop

The infinite loop is caused by two issues:

1. Missing Input Cleanup: When an InputMismatchException is thrown, the entered token is not consumed from the input stream. It remains in the stream, causing subsequent read attempts to repeatedly throw the exception.
2. General Exception Handling: The catch block of the try/catch is currently set to catch (Exception e). This broad handling includes all exceptions, which is not specific enough for the InputMismatchException we need to address.

Solution

To fix the infinite loop and ensure correct handling of InputMismatchException, several steps can be taken:

1. Consume the Erroneous Input: After catching the InputMismatchException, explicitly consume the input token to prevent it from causing repeated exceptions. This can be done by calling input.next(); within the catch block.
2. Use Specific Exception Handling: Replace the catch (Exception e) with catch (InputMismatchException e) to handle only InputMismatchExceptions specifically.
3. Implement Validation: Additionally, it is advisable to use the scanner.hasNextInt() method to verify if the input is an integer before attempting to read it. This can help avoid exceptions altogether.

Here's the modified code with the suggested improvements:

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 (InputMismatchException e) {
                System.out.println("Error!");
                input.next(); // Consume the erroneous input
            }
        } while (bError);

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

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