Java, try-catch with Scanner

Question

I am creating a small algorithm and this is a part of it.

If the user enters non integer values, I want to output a message and let the user enter a number again:

boolean wenttocatch;

do 
{
    try 
    {
        wenttocatch = false;
        number_of_rigons = sc.nextInt(); // sc is an object of scanner class 
    } 
    catch (Exception e) 
    {
        wenttocatch=true;
        System.out.println("xx");
    }
} while (wenttocatch==true);

I am getting a never ending loop and I can't figure out why.

How can I identify if the user enters some non integer number?
If the user enters a non integer number, how can I ask the user to enter again?

Update
When I am printing the exception I get 'InputMismatchException', what should I do?

Answer 1

You dont have to do a try catch. This code will do the trick for you :

public static void main(String[] args) {
    boolean wenttocatch = false;
    Scanner scan = new Scanner(System.in);
    int number_of_rigons = 0;
    do{
        System.out.print("Enter a number : ");
        if(scan.hasNextInt()){
            number_of_rigons = scan.nextInt();
            wenttocatch = true;
        }else{
            scan.nextLine();
            System.out.println("Enter a valid Integer value");
        }
    }while(!wenttocatch);
}

Answer 2

The Scanner does not advance until the item is being read. This is mentioned in Scanner JavaDoc. Hence, you may just read the value off using .next() method or check if hasInt() before reading int value.

boolean wenttocatch;
int number_of_rigons = 0;
Scanner sc = new Scanner(System.in);

do {
    try {
        wenttocatch = false;
        number_of_rigons = sc.nextInt(); // sc is an object of scanner class
    } catch (InputMismatchException e) {
        sc.next();
        wenttocatch = true;
        System.out.println("xx");
    }
} while (wenttocatch == true);