Code Converts A String That Has All Numbers Into Integer

I am trying to code using error handling and I am a bit confused on how to go about doing it correctly. My code converts a string that has all numbers into an integer and the error handling should recognize that if it isn't a proper number and ask the user to try again or enter 'q' to quit.Do I place a throw new exception in the try block and put conditionals like if charAt(i) is some letter or a symbol then throw new exception?

Java Code:

import java.util.Scanner;
public class practice {
public static void main(String[] args) {
Integer finalValue = null;
boolean validValue = false;
Scanner scan = new Scanner(System.in);
int result = 0;

[code]....