Looping in exceptions

[Carlos Obregon]

I'm trying to catch a number from a Scanner. I thought that I could catch the InputMismatchException and re-ask the user to submit another real number.

I use this but if I raise the exception it keeps in an infinite loop:

public class Exc {

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		boolean done = false;
		while(!done) {
			try {
				System.out.println("Input a number");
				in.nextInt();
				done = true;
			} catch(InputMismatchException e) {
				System.out.println("You did not input a number");
			}
		}
	}

}

Thanks!

[Campbell Ritchie]

Because of the exception, the done variable remains false for ever. The Scanner continues to look at the token which wasn't a number, so consume that token with a call to in.next(); inside the catch. No need to do anything with that token.

Then you can enter a new token, and the loop can go round once more.

[Rob Spoor]

You shouldn't use exceptions this way if there are valid checking methods available.
An improved version:

public class Exc {   
  
    public static void main(String[] args) {   
        Scanner in = new Scanner(System.in);   
        boolean done = false;   
        while(!done) {   
            System.out.println("Input a number");   
            if (in.hasNextInt()) {
                in.nextInt();
                done = true;
            }
            else {
                // consume the non-int
                in.next();
            }
        }   
    }   
}