How do you handle invalid input if you need the user to enter an integer?

Question

I'm trying to write a program that takes user integer inputs and does something with them, and continues to do so until the user enters a non-integer input, at which point the user will no longer be asked for input. This is what I've tried:

import java.io.IOException;

public class Question2 {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        boolean active = true;
        String num_string_positive = "";
        String num_string_negative = "";
        int my_int;
        while (active) {
            try {
                my_int = in.nextInt();
            } catch(IOException e) {
                active = false;
            }
            in.close(); 
        }
    }

}

This doesn't seem to work; there seems to be something wrong with the catch block. When I hover over IOException, Eclipse says "Unreachable catch block for IOException. This exception is never thrown from the try statement body". Shouldn't a non-integer input throw an I/O exception when I call the nextInt() method?

When I replace catch(IOException e) with catch(Exception e), the code does run, but it always terminates after one input, even if the input is an integer.

What am I doing wrong?

Answer 1

First, if the input is not an Integer the exception that will be thrown is InputMismatchException, and you shouldn't close the scanner until you continue with it, try this instead:

import java.util.InputMismatchException;

public class Question2 {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        boolean active = true;
        String num_string_positive = "";
        String num_string_negative = "";
        int my_int;
        while (active) {
            try {
                my_int = in.nextInt();
            } catch(InputMismatchException e) {
                active = false;
                in.close();
            } 
        }
    }

}