Luhn algorithm

gistfile1.txt

public class CreditCardValidation {

	public static void main (String[] args) {
		if (args.length < 1) {
			System.out.println("usage: CreditCardValidation number");
		} else {
			String cardNumber = args[0];	
			int checksum = 0; 
			int length = cardNumber.length();
			for (int i = 0; i < length; i++) {
				int num = Character.getNumericValue(cardNumber.charAt(length - (i+1)));
				if (i % 2 == 1) {
					int addedValue = num * 2;
					if (addedValue > 10) {
						int realAddedValue = (addedValue / 10 + addedValue % 10);	
						checksum += realAddedValue;
					} else {
						checksum += addedValue;
					}
				} else {
					checksum += num; 	
				}
			}
		
			System.out.println("value: " + checksum); 
			System.out.println("multiply 9 value: " + checksum * 9); 
			int realValue = (checksum * 9) % 10; 
			System.out.println("realValue: " + realValue);

			int realValue2 = 0; 	
			if (checksum % 10 == 0) {
				realValue2 = 0;
			} else {
				realValue2 = ((checksum / 10 + 1) * 10 - checksum); 
			}	
					
			System.out.println("realValue2: " + realValue2);
			if (realValue == realValue2) {
				System.out.println("valid...");
			} else {
				System.out.println("invalid...");
			}

		}	
			
	}
}