Why am I getting an input Invalid Infinite Loop?

Question

Why does this cause me to get stuck in an endless loop when the initial choice is invalid?

while (true) {
    System.out.print("Choice:\t");
    try {
        int choice = scanner.nextInt();
        break;
    } catch (InputMismatchException e) {
        System.out.println("Invalid Input!");
    }
}

Output:

Choice: df
Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!
Choice: Invalid Input!

A `break` in the `catch` clause would do. But what is the purpose of the `while (true)` loop, then? — David Conrad, Mar 14 '14 at 22:21

score 9 · Accepted Answer · answered Mar 14 '14 at 22:21

9

From the Javadoc:

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.

So the "df" string is still in the Scanner. You have to clear it somehow, by calling next() or some other means.

answered Mar 14 '14 at 22:21

durron597

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+1 for official JavaDoc reference and explaining *why*. – asteri Mar 14 '14 at 22:22
Thanks this has been bothering me for some time. – jshbrntt Mar 14 '14 at 22:27

score 4 · Answer 2 · answered Mar 14 '14 at 22:22

Scanner is trying to parse and return the same token over and over again (and it's not integer, so it's throwing an exception). You could fix it by discarding invalid token:

while (true) {
    System.out.print("Choice:\t");
    try {
        int choice = scanner.nextInt();
        break;
    } catch (InputMismatchException e) {
        System.out.println("Invalid Input!");
        scanner.next();                         // here, discard invalid token.
    }
}

score 1 · Answer 3 · answered Mar 14 '14 at 22:22

You break while statement in only in try clause. When your while loop enters in catch, it just doesn't break the loop.

while (true) {
  System.out.print("Choice:\t");
  try {
    int choice = scanner.nextInt();
  } catch (InputMismatchException e) {
    System.out.println("Invalid Input!");
  }
  finally {
   break;
  }
}

Why am I getting an input Invalid Infinite Loop?

3 Answers3