How to implement int validation, so the input can only be an int and between a min and max?

Question

I'm wondering how do I go about validating this code so the input can only be an int and between a min and max? So far I can only stop the input being less than 1 and whatever maximum is used. But I cant seem to create a scenario where if the user inputs anything other than between the max and min (e.g. "AAA") it loops. I keep getting an Input Mismatch error. Any help would be greatly appreciated!

private static int getUserOption(String prompt, int max) { 
    int h;
    do {
        Scanner sc = new Scanner(System.in);
        System.out.print(prompt);
        h=sc.nextInt();
        if(!sc.hasNextInt()) {
                System.out.println("Invalid option. Try again!:");
        }
    } while(h<1 || h>max);
    return h;    
}

score 1 · Answer 1 · answered Nov 20 '13 at 11:51

1

The loop is breaking because the nextInt() method is throwing an exception, which terminates the method early.

One option would be to use a try / catch block to trap the exception:

private static int getUserOption(String prompt, int max) { 
    int h = 0;
    do {
        Scanner sc = new Scanner(System.in);
        System.out.print(prompt);
        try {
            h=sc.nextInt();
        } catch (InputMismatchException e) {
            System.out.println("Invalid option. Try again!:");
        }
    }while(h<1 || h>max);
    return h;
}

answered Nov 20 '13 at 11:51

Rich Smith

1,675
12
24

Thanks, so this is only possible with a try/ catch? I have never used it before and was hoping to become proficient in do/while, for and if/else before moving on. If so thanks very much for your help! – user2988501 Nov 20 '13 at 12:18
The `sc.nextInt()` method expects an integer so is always going to throw an exception if you enter a string. Your other option would be to call `sc.next()` which will return any input in the form of a string; you could then check to see if you could convert this to an integer and if not print an error message. Arguably more complicated :) – Rich Smith Nov 20 '13 at 13:01

Sionnach733 · Answer 2 · 2013-11-20T12:04:54.240

a few points:

1. to check if the input is less than a min value just add int min to the method signature.

2. to check if input is an int, catch InputMismatchException.

The revised code would be:

private static int getUserOption(String prompt, int max, int min) { 
    int h;
    do {
        Scanner sc = new Scanner(System.in);
        System.out.print(prompt);
        try {
            h=sc.nextInt();
        } catch (InputMismatchException e) {
            System.out.println("Invalid option. Try again!:");
        }
    } while(h<min || h>max);
    return h;    
}

score 0 · Answer 3 · answered Nov 20 '13 at 12:22

I couldn't understand if you want to keep asking until the user types a valid answer or just ask once. The while block suggests to keep asking until is a valid input. Here is a small snippet that what you request. I suggest that you read some books, there a plenty of suggestions at SO.

public static class InvalidUserException extends Exception {

    public InvalidUserException(String message) {
        super(message);
    }

    public InvalidUserException(String message, Throwable cause) {
        super(message, cause);
    }

}

private static int getIntFromScanner(int max) throws InvalidUserException {
    int nextInt = 0;

    Scanner sc = new Scanner(System.in);
    try {
        nextInt = sc.nextInt();

    } catch (InputMismatchException e) {
        throw new InvalidUserException("Input must be a valid Integer", e);
    }

    if (nextInt > max) {
        throw new InvalidUserException(
                "Input is bigger than allowed! Max: " + max + " Input: "
                        + nextInt);
    }
    return nextInt;
}

public static int getUserOption(String prompt, int max) {
    System.out.println(prompt);
    do {
        try {
            return getIntFromScanner(max);
        } catch (InvalidUserException e) {
            System.out.println("Invalid option. Try again ("
                    + e.getMessage() + ")");
        }
    } while (true);
}

public static void main(String[] args) {
    int userOption = getUserOption("Gimme less than or equal 6!", 6);
    System.out.println("You gave me " + userOption);
}

score 0 · Accepted Answer · edited May 23 '17 at 11:56

You Can try this Code !

Scanner sc = new Scanner(System.in);
int number;
do {
System.out.println("Please enter a valid number: ");
while (!sc.hasNextInt()) {
   System.out.println("Error. Please enter a valid number: ");
   sc.next(); 
}
number = sc.nextInt();
} while (!checkChoice(number));

private static boolean checkChoice(int choice){
if (choice <MIN || choice > MAX) {     //Where MIN = 0 and MAX = 20
    System.out.print("Error. ");
    return false;
 }
 return true;
 }

Ref. Validating input with while loop and scanner

How to implement int validation, so the input can only be an int and between a min and max?

4 Answers4