The LA Fox Developer Newsletter

April 1997

Realizing the Power of

Business Objects Through

•

Polymorphism

by Fred P Haeckl

Polymorphism is one of the greatest assets that OOP brings to

the programming community. Far too often, it is underutilized,

creating a great deal of unnecessary procedural style code.

Furthermore, this code is hard to maintain, and usually cannot

be a candidate for reuse.

Current class design is limited to visible objects such as Forms,

Buttons, TextBoxes, etc. ‘Business Objects’ have been left

alone for the most part. Those that do exist, serve as mere data

table navigators, with methods such as First, Previous, Next,

Last, and other table routines such as New, Save, and Revert.

This type of class is needed - as a navigation class. But these

classes should work in conjunction with business classes. In

constructing true ‘Business Objects’, those that mimic every

day business events, with the power of polymorphism, reuse

becomes the norm, and flexibility with unlimited growth potential

becomes the reward.

One such example is a Payroll Class. Every company needs to

calculate payroll. The problem in programming has been trying

to calculate all of the different ways that there are to calculate

payroll. You havA hourly employees, salary employees, com-

missioned employees, etc. If you have multiple states that you

do business in or you are a software company trying to market

to multiple states, you need to worry additionally about the

different state payroll tax calculations.

The typical procedural code loops through the employees and

calculates each employee through a series of IF. ..THEN.. .ELSE

statements or CASE statements to handle the various sce-

narios mentioned above. This is fine for a simple company that

only has a couple of types of employees and that resides in one

state. But how flexible is this code? What is involved in chang-

ing code to accommodate additional types of employees as well

as additional states? You can see how this style of coding

becomes unruly very quickly.

Enter OOP. Now we have a Payroll object that hopefully calcu-

lates our payroll for us. However, not much changes. The

methods replace old style procedures, but generally, the same

IF.. .THEN. . . ELSE structure applies in handling the various types

of employees, and states. Many developers create separate

methods for each variable, but essentially, it’s not much better

than the old procedural code.

With polymorphism, and creative class design, payroll can be

done, not with a Payroll object, but rather an Employee object,

that knows how to calculates his/her own salary.

Employee Business Object Calculating Payroll:

(Con’t, page 5)

Book Review

Instant SQL Programming

by Bill Anderson

I received an unexpected package in the mail a few months ago.

The package turned out to be a book from Wrox Press titled

“Instant SQL Programming” by Joe Celko (ISBN# 1-874416-50-

8). According to a letter inside the package I was one of five

people to win a free book from Wrox Press. It’s certainly kind of

Wrox to send me this book although I don’t know how I’m on

their mailing list. Nevertheless, I didn’t read this book right away

— I was in the middle of digesting the Client Server Survival

Guide. After sitting on my shelf for two or three months I just got

around to reading this book.

The book is aimed at beginning to intermediate level SQL

application developers. The book is clearly written and the

examples provided complement the text nicely. Also, the

runtime version of the Watcom SQL engine is provided on

diskette. This allows the reader to type in the examples (or

experiment) and return results to the screen.

The first nine chapters cover the SQL basics — what is SQL,

building a database, SQL arithmetic, logic and nulls, referential

integrity and keys, the SELECT statement, JOINs, aggregate

functions and using subqueries. The final three chapters cover

more complex material such as views, embedded SQL and

performance tips.

The difference in the basic SELECT syntax supported by the

Watcom SQL engine versus the Visual FoxPro 5.0 engine is

really rather minor. I only noted a lack of support for the key-

words CROSS JOIN and IS [NOT] TRUE I NULL I FALSE (not

critical at all since VFP 5.0 can handle these keywords in a

different fashion), support for more than one level of nested

subqueries and support for scalar subqueries (don’t worry about

it). So it turns out this book is an excellent reference for last

month’s L. A. Fox presentation on VFP 5.0 SQL.

Grade: Varies based on the level of prior SQL knowledge

Beginners:

A-

Experts:

C+

MicroEndeavors Announces

Pre-Conference Event

MicroEndeavors, in cooperation with Microsoft, has announced

that they will be holding a two-day pre-conference seminar on

May 15-16, 1997. The purpose of this seminarwill beto show

FoxProiWindows developers how to leverage their knowledge of

FPW and bring it into the world of Visual FoxPro. Cost of the

seminar, to be held at the LAX Westin, will be $595. For further

details, visit their Website at http://www.microendeavors.com/

foxsem97.htm.

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