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(Credit card number validation) Credit card numbers follow certain patterns. A credit card number must have between 14 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 numbers. The algorithm is useful to determine if a card number is entered correctly or if a credit card is scanned correctly by a scanner. Almost all 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 4388576018402625):
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.
2 * 2 = 4
2 * 2 = 4
4 * 2 = 8
1 * 2 = 2
6 * 2 = 12 ( 1 + 2 = 3)
5 * 2 = 10 ( 1 + 0 = 1 )
8 * 2 = 16 ( 1 + 6 = 7 )
4 * 2 = 8
2. Now add all single-digit numbers from Step 1.
4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 37
3. Add all digits in the odd places from right to left in the card number
5 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 37
4. Sum the results from Step2 and Step 3.
37 + 37 = 74
5. If the result from Step 4 is divisible by 10, the card number is valid; otherwise, it is invalid. For example, the number 4388576018402625 is invalid, but the number 4388576018410707 is valid.
Write a program that prompts the user to enter a credit card number as a long integer. Display whether the number is valid. Design your program to use the following methods:

/* Return true if the card number is valid */
public Boolean isValid ( long number)

/* Get the result from step 2 */
public static int sumOfEvenPlace(long number)

/* Return this number if it is a single digit, otherwise, return the sum of the two digits*/
public static int getDigit(int number)

/* Return sum of odd place digits in number */
public static int sumOfOddPlace( long number)

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Last Post by sknake
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You could stun your professor and take this one step further -- submit the credit card to a merchant for payment processing. They will do extensive validation on the card number :)

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