CreditCardNumberGenerator.java

import java.util.Random;

/**
 * A credit card number generator.
 * 
 * @author Josef Galea
 */
public class CreditCardNumberGenerator {

  private Random random = new Random(System.currentTimeMillis());

  /**
   * Generates a random valid credit card number. For more information about
   * the credit card number generation algorithms and credit card numbers
   * refer to <a href="http://www.merriampark.com/anatomycc.htm">Anatomy of
   * Credit Card Numbers</a>, <a
   * href="http://euro.ecom.cmu.edu/resources/elibrary/everycc.htm">Everything
   * you ever wanted to know about CC's</a>, <a
   * href="http://www.darkcoding.net/credit-card/">Graham King's blog</a>, and
   * <a href="http://codytaylor.org/2009/11/this-is-how-credit-card-numbers-are-generated.html"
   * >This is How Credit Card Numbers Are Generated</a>
   * 
   * @param bin 
   *            The bank identification number, a set digits at the start of the credit card 
   *            number, used to identify the bank that is issuing the credit card.
   * @param length 
   *            The total length (i.e. including the BIN) of the credit card number.
   * @return 
   *            A randomly generated, valid, credit card number.
   */
  public String generateRandomCreditCardNumber(String bin, int length) {

    // The number of random digits that we need to generate is equal to the
    // total length of the card number minus the start digits given by the
    // user, minus the check digit at the end.
    int randomNumberLength = length - (bin.length() + 1);
    
    StringBuffer buffer = new StringBuffer(bin);
    for (int i = 0; i < randomNumberLength; i++) {  
      int digit = this.random.nextInt(10);
      buffer.append(digit);
    }
    
    // Do the Luhn algorithm to generate the check digit.
    int checkDigit = this.getCheckDigit(buffer.toString());
    buffer.append(checkDigit);
    
    return buffer.toString();
  }

  /**
   * Generates the check digit required to make the given credit card number
   * valid (i.e. pass the Luhn check)
   * 
   * @param number
   *            The credit card number for which to generate the check digit.
   * @return The check digit required to make the given credit card number
   *         valid.
   */
  private int getCheckDigit(String number) {

    // Get the sum of all the digits, however we need to replace the value
    // of every other digit with the same digit multiplied by 2. If this
    // multiplication yields a number greater than 9, then add the two
    // digits together to get a single digit number.
    //
    // The digits we need to replace will be those in an even position for
    // card numbers whose length is an even number, or those is an odd
    // position for card numbers whose length is an odd number. This is
    // because the Luhn algorithm reverses the card number, and doubles
    // every other number starting from the second number from the last
    // position.
    int sum = 0;
    int remainder = (number.length() + 1) % 2;
    for (int i = 0; i < number.length(); i++) {
  
      // Get the digit at the current position.
      int digit = Integer.parseInt(number.substring(i, (i + 1)));
  
      if ((i % 2) == remainder) {
        digit = digit * 2;
        if (digit > 9) {
          digit = (digit / 10) + (digit % 10);
        }
      }
      sum += digit;
    }

    // The check digit is the number required to make the sum a multiple of
    // 10.
    int mod = sum % 10;
    int checkDigit = ((mod == 0) ? 0 : 10 - mod);

    return checkDigit;
  }
}