Takes the input of two integers and outputs all the Kaprekar numbers in that range

KaprekarNumbers.java

/*
 * twitter.com/acidflip
 * KaprekarNumbers
 * December 10th, 2016
 * 
 * Takes the input of two integers and outputs all the Kaprekar numbers in that range
 * 
 * In mathematics, a Kaprekar number for a given base is a non-negative integer, the 
 * representation of whose square in that base can be split into two parts that add up 
 * to the original number again. For instance, 45 is a Kaprekar number, 
 * because 45^2 = 2025 and 20+25 = 45. The Kaprekar numbers are named after D. R. Kaprekar.
 */

import java.util.Scanner;

public class KaprekarNumbers {
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        
        System.out.println("Enter the first integer");
        int firstNum = Math.abs(keyboard.nextInt());
        System.out.println("Enter the second integer");
        int secondNum = Math.abs(keyboard.nextInt());
        
        System.out.println("\nThe Kaprekar numbers between " + firstNum + " and " + secondNum + " are...");
        for (int i = firstNum; i < secondNum; i++) {
            int resultNum = kapNum(i);
            
            if (resultNum != -1) {
                System.out.println(resultNum);
            }
        }
    }
    
    public static int kapNum(int number) {
        int square = number * number;
        String result = Integer.toString(square);
        
        int length = result.length();
        if (length == 1) {
            return -1;
        } else {
            for (int index = 1; index < length; index++) {
                int num1 = Integer.parseInt(result.substring(0, index));
                int num2 = Integer.parseInt(result.substring(index, length));
                
                if (num1 == 0 || num2 == 0) {
                    continue;
                }
                
                if (num1 + num2 == number) {
                    return number;
                }
            }
            return -1;
        }
    }
}