java - How to handle infinite loop caused by invalid input (InputMismatchException) using Scanner

Question

So, I'm getting stuck with this piece of code:

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

public class ConsoleReader {

    Scanner reader;

    public ConsoleReader() {
        reader = new Scanner(System.in);
        //reader.useDelimiter(System.getProperty("line.separator"));
    }

    public int readInt(String msg) {
        int num = 0;
        boolean loop = true;

        while (loop) {
            try {
                System.out.println(msg);
                num = reader.nextInt();

                loop = false;
            } catch (InputMismatchException e) {
                System.out.println("Invalid value!");
            } 
        }
        return num;
    }
}

and here is my output:

Insert a integer number:
Invalid value!
Insert a integer number:
Invalid value!
...

question from:https://stackoverflow.com/questions/3572160/how-to-handle-infinite-loop-caused-by-invalid-input-inputmismatchexception-usi

Answer 1

As per the javadoc for 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.

That means that if the next token is not an int, it throws the InputMismatchException, but the token stays there. So on the next iteration of the loop, reader.nextInt() reads the same token again and throws the exception again. What you need is to use it up. Add a reader.next() inside your catch to consume the token, which is invalid and needs to be discarded.

...
} catch (InputMismatchException e) {
    System.out.println("Invalid value!");
    reader.next(); // this consumes the invalid token
}