CreditCardValidation.java

// Exercise 6.31 (Financial: credit card number validation) on page 241

import java.util.Scanner;
public class CreditCardValidation {
	public static void main(String[] args) {
		
		// Create Scanner and prompt user for input
		Scanner input = new Scanner(System.in);
		System.out.print("Enter a credit card number as a long integer: ");
		long creditCard = input.nextLong();
		
		if (isValid(creditCard)) {
			System.out.print(creditCard + " is valid\n");
		}
		else
		{
			System.out.print(creditCard + " is invalid\n");
		}
	}
			
		/** Return true if the card number is valid */
		public static boolean isValid(long number) {
			boolean valid = (getSize(number) >= 13 && getSize(number) <= 16) && (prefixMatched(number, 4) || prefixMatched(number, 5) || 
					prefixMatched(number, 37) || prefixMatched(number, 6)) && ((sumOfDoubleEvenPlace(number) + sumOfOddPlace(number)) % 10 == 0);
            return valid;
		}
			
		/** Get the result from Step 2 */
		public static int sumOfDoubleEvenPlace(long number) {
			int sum = 0;
            String num = number + "";
            for (int i = getSize(number) - 2; i >= 0; i -= 2) {
                    sum += getDigit(Integer.parseInt(num.charAt(i) + "") * 2);
            }
            return sum;
		}
		
		/** Return this number if it is a single digit, otherwise, return the sum of the two digits */
		public static int getDigit(int number) {
			if (number < 9)
				return number;
			else
				return number / 10 + number % 10;
		}
		
		/** Return sum of odd-place digits in number */
		public static int sumOfOddPlace(long number) {
            int sum = 0;
            String num = number + "";
            for (int i = getSize(number) - 1; i >= 0; i -= 2) {
                    sum += Integer.parseInt(num.charAt(i) + "");
            }
            return sum;
		}
		
		
		/** Return true if the digit d is a prefix for number */
		public static boolean prefixMatched(long number, int d) {
			return getPrefix(number, getSize(d)) == d;
			
		}
		
		/** Return the number of digits in d */
		public static int getSize(long d) {
			String num = d + "";
			return num.length();
		}
		
		/** Return the first k number of digits from number. If the number of digits in number is less than k, return number. */
		public static long getPrefix(long number, int k) {
            if (getSize(number) > k)  {
                String num = number + "";
                return  Long.parseLong(num.substring(0, k));
        }
        return number;
		}
}