CardChecker.java

import java.util.Scanner;

public class CardChecker {
    /** Main Method */
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in); // Create a Scanner

        // Prompt the user to enter a credit card number as a long integer
        System.out.print("Enter a credit card number as a long integer: ");
        long cardNumber = input.nextLong();

        System.out.println(
                cardNumber + " is " + (isValid(cardNumber) ? "valid" : "invalid"));
    }

    /** Return true if the card number is valid */
    public static boolean isValid(long number) {

        if(getSize(number) >= 13 && getSize(number) <= 16) {
            if(prefixMatched(number, 4) || prefixMatched(number, 5) ||
                    prefixMatched(number, 37) || prefixMatched(number, 6)) {

                int sumOfDouble = sumOfDoubleEvenPlace(number);
                int sumOfOdd = sumOfOddPlace(number);
                // if the sum is divisible by 10
                if((sumOfDouble + sumOfOdd) % 10 == 0) {
                    return true;
                }
            }
        }
        return false;
    }

    /** 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 >= 10)
            return number / 10 + number % 10;
        else
            return number;
    }

    /** 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;
    }
}