CPSC 120B Programming Assignment #4

Credit Cards

Due Wednesday, December 11 by noon

In this assignment you will write a class that models a credit card and a driver program to use the class.

Your CreditCard class must have the following instance data:

the credit card number -- a String
the current balance on the card (the total charges) -- a double
the credit limit -- a double
the monthly interest rate -- a double
a boolean variable to indicate whether or not the bill was paid in full the last month

The class must have the following public instance methods:

a constructor with three parameters -- a credit card number (String), a credit limit (double), and an annual interest rate (double). The constructor should use the parameters to initialize the object. Assume the initial balance is 0 and that the bill was paid in full. The constructor should convert the annual interest rate (the parameter) to a monthly rate to store in the instance variable. Assume the rates are in decimal form (so an annual rate of 22.9% would be 0.229 which is a monthly rate of 0.0190833333...).
boolean charge (double amt) -- a method that represents charging the amt to the account. The method should check to make sure that the charge is ok -- that is, that it does not cause the balance to exceed the credit limit. The method should return a boolean representing whether or not the charge was ok (true if the charge was ok; false if not).
String prepareBill() -- a method that computes the interest for the month and adds it to the balance. The interest is 0 if the bill was paid in full the last month; otherwise it is the monthly rate times the balance. The method should return a string that contains billing information. For example, the string may look something like the following:
```
             Charges:      $340.00
             Interest:     $ 68.00
             New Balance:  $408.00
```
Note that your string can contain the escape sequence for new line (\n).
void makePayment(double amt) -- adds amt to the current balance. If the resulting new balance is greater than 0 (the bill was not paid in full), the boolean instance variable should be set to false.
double getBalance() -- returns the current balance on the credit card.

The CreditCard class should also have the following static method:

boolean validNumber (String num) -- this method takes a string as a parameter and determines if the parameter is a valid credit card number. Credit card companies use various "check digit" schemes to make sure that credit card numbers are valid (to prevent people from just making numbers up and to catch errors in data entry). Your method should use the following algorithm for determining validity: First, the number must be exactly 16 digits long. Then, if it is 16 digits test its validity as follows:
1. Find the sum of the digits in the positions with even numbered indices (assuming the indices are 0-based -- they start with 0 as Java String indices do).
2. Find the number of digits greater than 4 with even numbered indices.
3. Find the sum of the digits in the positions with odd numbered indices.
4. Compute: sum of digits in even positions times 2, plus the number of digits greater than 4 in even positions, plus the sum of digits in odd positions.
5. If the result is divisible by 10, the number is valid; otherwise it is not.
Some examples:
1. The number 3125600196430012 is valid because:
```
Sum of digits in even positions:  3 + 2 + 6 + 0 + 9 + 4 + 0 + 1 = 25
Number of digits > 4 in even positons:  2   (the 6 and 9)
Sum of digits in odd positions: 1 + 5 + 0 + 1 + 6 + 3 + 0 + 2 = 18

25 * 2 + 2 + 18 = 70

70 is divisible by 10 so the number is valid.
```
2. The number 4128001234567893 is not valid because:
```
 
Sum of digits in even positions: 4 + 2 + 0 + 1 + 3 + 5 + 7 + 9 = 31
Number of digits > 4 in even positions: 3  (the 5, 7, and 9)
Sum of digits in odd positions: 1 + 8 + 0 + 2 + 4 + 6 + 8 + 3 = 32

31 * 2 + 3 + 32 = 97 which is not divisible by 10 so the number is not valid
```
NOTE: You will need to use methods in the String class and wrapper classes in this method. You could break this method up into some private support methods.

The Driver Program Your driver program will read in a credit card number, instantiate a credit card, let the user charge things to the credit card (as long as the user wishes or until the credit limit is reached), display the bill and let the user make a payment. The user can then go back and charge more and so on. All input/output in the program will be done using dialog boxes (look at the last exercise in Lab 11). The main program should use the three support methods listed below. Here is a rough outline of the main method:

         get the credit card number (call the method validCCNum to do this)
         if the number is not valid
            display a message telling the user he/she is suspected of credit 
            card fraud (or something equally scary)
         else  // the number is valid
         {
            read in the credit limit and annual interest rate
            instantiate a CreditCard object
            
            do
            {
                let the user charge things (call the method makeCharges to do this)
                let the user make a payment (call the method makePayment to do this)
            }
            while the user wants to continue
          }

The three support methods used by main are as follows:

String validCCNum () -- this method asks the user to enter a credit card number (using an input dialog box), then it sends that number (the String object returned by the showInputDialog method) to the CreditCard class (invokes the static method validNumber) to check it out. If it is valid the number is returned. If it isn't valid the user is allowed to try again. The user gets at most 3 chances to enter a valid number (use a loop!). The method should return a valid number or the empty string ("") if a valid number has not been entered after 3 tries.
void makeCharges(CreditCard card) -- this method contains a loop that lets the user keep charging to the credit card as long as he/she wants. For each charge, the amount must be input then sent to the appropriate method of the CreditCard class to be charged to the account. A message should be displayed if a charge cannot be okayed because it would exceed the credit limit.
void payBill(CreditCard card) -- this method calls the appropriate method in the CreditCard class to compute the bill and displays the bill returned by the method (display it in an input dialog box). The method then gets the amount of the payment from the user and calls the appropriate method in the CreditCard class to make the payment. The new balance on the credit card should then be displayed.

NOTE: As usual use good programming style and good programming habits!!! Do not try to type everything in at once -- do the program incrementally. Use good variable names and proper indentation and white space. Be sure to include brief documentation for each method you use!

Hand In: A printout of each file (CreditCard.java and class that uses it) and tar your directory containing the program and email it to me.