# Chapter 11 Project

## Chapter 11 Project: Credit Card

Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start with:
■ 4 for Visa cards
■ 5 for Master cards
■ 37 for American Express cards
■ 6 for Discover cards
In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card numbers. The algorithm is useful to determine whether a card number is entered correctly or whether a credit card is scanned correctly by a scanner. Credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check, which can be described as follows (for illustration, consider the card number 4388576018402626):

#### Luhn Check (Mod 10 Check)

1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number.43885760184026262 * 2 = 42 * 2 = 44 * 2 = 81 * 2 = 26 * 2 = 12 (1 + 2 = 3)5 * 2 = 10 (1 + 0 = 1)8 * 2 = 16 (1 + 6 = 7)4 * 2 = 82. Now add all single-digit numbers from Step 1.4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 373. Add all digits in the odd places from right to left in the card number.6 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 384. Sum the results from Step 2 and Step 3.37 + 38 = 755. If the result from Step 4 is divisible by 10, the card number is valid; otherwise, it is invalid. For example, the number 4388576018402626 is invalid, but the number 4388576018410707 is valid.

#### What You'll Write

• Write a program that prompts the user to enter a credit card number as a long integer.
• Display whether the number is valid or invalid.
• Design your program to use the following methods:
• Returns true if the card number is valid`public static boolean isValid(long number)`Gets the result from Step 2.
• `public static int sumOfDoubleEvenPlace(long number)`Returns this number if it is a single digit,
• Otherwise, return the sum of the two digits`public static int getDigit(int number)`
• Returns sum of odd-place digits in number`public static int sumOfOddPlace(long number)`
• Returns true if the digit d is a prefix for number`public static boolean prefixMatched(long number, int d)`
• Returns the number of digits in d`public static int getSize(long d)`
• Returns 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)`

Here are sample runs of the program:
``````// Valid Case
Enter a credit card number: 4388576018410707
4388576018410707 is valid

// Invalid Case
Enter a credit card number: 4388576018402626
4388576018402626 is invalid``````

#### Other Info

Bonus points if you can also print to the user what type of card the number is, and for writing JavaDoc comments.
Note: If you need more card numbers to test, you can look here (or simply search "test credit card numbers" on Google).
Note: The practice template has method stubs written for you. Simply delete the default return values and start coding!

Adapted from Exercise 6.31, Introduction to Java Programming (Comprehensive), 10th ed. by Y. Daniel Liang

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