CMPU 125 Assignment 3: Checksum Validation

Due on Friday, September 26th, by 5pm.

Problem Specification:

A common requirement for software applications supporting inventory management is to validate the items in stock. For example, suppose a store manager wants to add a new item to the store's inventory database. To do so, the manager would need to insert a new entry into the database that gives the name and price of the item along with its code number. Correctly entering all three pieces of data is important, since an improperly entered code would make it impossible for price scanners to recognize the item.

One way to increase the likelihood of correct data entry is to use checksum software to test whether the digits of a potential code number have some particular property.

For example, there is a verification scheme for the Universal Product Code (UPC) numbers. The familiar UPC in bar code form is shown below.

Fig. 1: UPC bar code 128016691675.

A valid UPC number is a 12-digit value that meets the following criteria:

Let m be the sum of the 2nd, 4th, 6th, 8th, and 10th digits (where the numbering of digits is from left to right, starting at 1.)
Let n be the sum of the 1st, 3rd, 5th, 7th, 9th, and 11th digits.
Let r = 10 - (( m + 3 * n ) mod 10).

A number meets the UPC encoding criteria if its 12th digit equals r.

Consider the number 780070121354.

For this number,

 m = 8 + 0 + 0 + 2 + 3 = 13
 n = 7 + 0 + 7 + 1 + 1 + 5 = 21
 r = 10 - ((13 + 3 * 21) mod 10) 
 = 10 - (76 mod 10)
 = 10 - 6 
 = 4

Because r = 4, the 12th digit in the UPC code, the number corresponds to a possible UPC code.

Now consider the number 123456789011.

For this number,

 m = 2 + 4 + 6 + 8 + 0 = 20
 n = 1 + 3 + 5 + 7 + 9 + 1 = 26
 r = 10 - ((20 + 3 * 26) mod 10) 
 = 10 - (98 mod 10)
 = 10 - 8 
 = 2

Because r is not equal to 1, the 12th digit, the number does not correspond to a possible UPC code.

Programming Assignment:

Write a driver program (i.e., a program with a main method) called TestUPC that does the following:

 0. Display the function of the program to the user, i.e.,
    let them know what they should enter and what to expect
    for a result
 1. While true 
 2.    Prompt the user to enter a 12-digit number (read this in as a String)
 3.    While the String entered has the wrong number of digits, or if the String contains non-digits
 4.       Report an invalid entry and prompt the user to re-enter the number
 5.    Create a new UniversalProductCode object called upc 
 6.    If upc is valid
 7.       Tell the user that the number is a feasible UPC code
 8.    Else
 9.       Tell the user that the number is not a feasible UPC code
 10.    If the user does not want to enter another UPC code, break
 11. Exit the program

To make steps 1-4 easier, there is a String method that returns the number of characters in a String. In the Character class, there is a static method that will convert a char to the equivalent integer and a static method that determines whether a given char is a digit (look these methods up in the Java API).
The UniversalProductCode class should include a private instance variable of type String and a boolean instance variable called valid.

A UML (Unified Modeling Language) diagram for this assignment is shown below. Try to make your classes follow the guidelines shown.

The method evenValues in the UniversalProductCode class returns the sum of the digits in even positions of the upcCode and the oddValues method returns the sum of the digits in odd positions. The checkValidity method takes the resulting sums and an array of digits and returns true if the upcCode String is valid and false otherwise. There is a getter method for the private boolean variable called valid.

Make your program modular. Your solution does not have to match the UML diagram shown exactly, but it should include at least 2 classes. You can add more methods than the ones shown in the diagram above. Remember, no method should be longer than a page and every method should have one well-defined task. Put some time in thinking about how your program will be designed before starting to code.

What you should hand in:

When your program compiles and runs correctly, submit your hw3 folder. If you have problems submitting the folder from home, e-mail the program to your instructor at walter@cs.vassar.edu.
Your program should include a descriptive header comment and should have either intuitively named variables or should contain comments explaining obscure parts of the code. The lines in your code should be well indented and span no more than 80 columns.
Provide some sample test cases in the program header for full credit. You can find UPC test cases on everyday items like a box of Kleenex or a notebook.

If you have questions, please send e-mail to either soroberts@vassar.edu or walter@cs.vassar.edu. Also, note the coaching hours that are posted on the course web page.