Infinite While Loop When InputMidmatchException is

问题:

This question already has an answer here:

• try/catch with InputMismatchException creates infinite loop 7 answers

I keep getting my code caught in an infinite while loop.

It is nothing to advanced, but i can not figure it out for the life of me!

Someone Please help

I have purplosely just re created the specific error without all of the if statements i have in my actual program.

    package bs;

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

    public class bs {

        public static void main(String[] args) {

            Scanner sc = new Scanner(System.in);

            boolean continueVar = true;

            while (continueVar) {

                try {
                    System.out.println("Enter Something");
                    int input = sc.nextInt();

                } catch (InputMismatchException i) {
                    System.out.println("What the f***?");
                    continueVar = true;
                }
            }


        }
    }

The infinite loop occurs when the Input mismatch exception is caught. I would think that it would atleast ask the user to re enter their input but instead of doing that it just continues in the loop as so:

    run:
    Enter Something
    df
    What the f***?
    Enter Something
    What the f***?
    Enter Something
    What the f***?

It acts like it is just ignoring the scanner object sc?!

回答1:

No the scanner is not skipped, it's just starting at the beginning of the input. From the JavaDoc:

If the translation is successful, the scanner advances past the input that matched.

This means if the conversion isn't successfull the scanner won't advance. You'd thus have to manually skip the incorrect input using just next().

Edit: you might want to check for hasNextInt() before trying to read the input.

回答2:

you loop while continueVar is true, but you never set to to false, so the loop never exits.

I think you want to set continueVar to false in the exception handler.

回答3:

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.

The token that caused the mismatch is still in the scanner's buffer. You need to clear it before trying to scan again.

You can do that by calling next() in your catch block, like this:

catch (InputMismatchException i) {
    System.out.println("What the f***?");
     sc.next();           
}

Also, you don't need to set continueVar to true again. You never set it to false, so it will stay true. Guessing that's an artifact of you removing this into a mini program.

回答4:

Scanner does not advance when bad token is found. Look at Scanner.java, lines 2095-2096:

catch (NumberFormatException nfe) {
    position = matcher.start(); // don't skip bad token