Emulate Object-Oriented Programming in ILE RPG

RPG
Typography
  • Smaller Small Medium Big Bigger
  • Default Helvetica Segoe Georgia Times

What do you get when you combine nested data structures with prototyped subprocedures?

 

According to Wikipedia, although the concepts behind object-oriented (OO) programming have been around since the 1960s, they weren't part of commonly used languages until the 1990s. At its most basic level, OO programming allows a programmer to work with objects in a hierarchical structure, so anything associated with a given "object" can be found within that object's structure. This article explores how to combine nested qualified data structures with prototyped subprocedures to achieve OO-like functionality in ILE RPG.

Qualified Nested Data Structures

Qualified data structures, first introduced with V5R1, allow you to refer to the subfields of a given data structure by using a dotted notation similar to that used within languages like C++ and Java. Below is an example of a qualified data structure.

D objectData  Ds                  Qualified

D   Height                   5  0

D   Width                    5  0

D   Color                   15

   

Note that our data structure named objectData contains three subfields: Height, Width, and Color. When referencing any of these fields within a program, we reference the subfields using the data structure name as a qualifying prefix:

   

 /Free

      Dsply objectData.Color;

      Area = objectData.Height * objectData.Width;

 /End-free

 

While your initial reaction may be that this is just additional typing that you'll have to do every time you reference those fields, there is a huge benefit to using qualified data structures. Not least of these is that there will be no ambiguity about what field you are referring to. The real power of qualified data structures comes when you utilize multiple copies of the same data structure within a program via the LIKEDS keyword. This keyword gives you the ability to create a data structure that is a "copy" of another data structure.

 

D object2     Ds                  likeds(objectData)

This new data structure contains its own copy of all of the subfields as defined on the original data structure. In V5R2, the LIKEDS keyword also gave us the ability to create nested data structures, so now a single data structure can actually encompass multiple levels.

   

D ingred_DS   Ds                  Qualified

D   Ingredient              30   

D   Amount                   5  0

D   UnitOfMeasure            5

D nutrit_DS   Ds                  Qualified

D   Calories                 5  0

D   Carbs                    5  0

D Recipe      Ds                  Qualified

D   Name                   128A 

D   Servings                 5  0

D   Serving_size             5  0

D   Instructions           512A 

D   Ingredients                   likeds(ingred_DS) dim(50)

D   Nutritional                   likeds(nutrit_DS)

   

This multi-level approach allows us to define an object structure similar to that used in languages like Java. Note that in our example above, within our Recipe data structure, we identify that the ingred_DS ­­data structure is an array with 50 elements. When referenced within our application, we can refer to individual items within the Recipe object as shown here:

 

/Free

      Recipe.Name = 'Green Eggs and Ham';

      Recipe.Ingredients(1).Ingredient = 'Eggs';

      Recipe.Ingredients(1).Amount = 2;

      Recipe.Ingredients(2).Ingredient = 'Green Food Coloring';

      Recipe.Ingredients(2).Amount = 1;

      Recipe.Ingredients(2).UnitOfMeasure = 'tsp';

      Recipe.Ingredients(3).Ingredient = 'Ham';

      Recipe.Ingredients(3).Amount = 0.5;

      Recipe.Ingredients(3).UnitOfMeasure = 'cups';

      Recipe.Nutritional.Calories = 250;

      Recepe.Nutritional.Carbs = 0;

      Receipt.Servings = 1;

/End-free

 

Notice how we are able to refer to individual subfields of each element of our Ingredients collection, as well as refer to subfields of the Nutritional data structure along with the simple value for the Servings property. If we take this concept to the next level, we can one again use the LIKEDS keyword to assign the value of the whole Recipe object to another data structure altogether:

D RecipeCard  Ds                  likeds(Recipe)

...

/Free

      RecipeCard = Recipe;

/End-free

 

After execution of this command, all of the subfields within the Recipe object will be transferred to the same subfield within the ReceipeCard object. This simple transfer allows us to build our data in a "work" object and transfer it to another object after completion. To extend this concept, we can create an arrayed version of our object to store multiple similar objects, as shown here:

 

D RecipeBox   Ds                  likeds(Recipe) dim(255)

...

/Free

      cardNumber +=1;

      RecipeBox(cardNumber) = Recipe;

/End-free

 

This example defines our RecipeBox object as an arrayed data structure with 255 elements. Each element has the same structure as our Recipe data structure. Figure 1 below illustrates how this structure works:

                               
090308Faustfig01.jpg

Figure 1: This illustrates how our array object is organized.  (Click image to enlarge.)

 

As this example shows, what we end up with is something similar to a "collection" within other object-oriented languages (VB, Java, etc.).

 

While this may not seem very useful within two data structures of a single program, it can become extremely valuable when combined with prototyped procedures within a service program. Next time, we'll explore that concept as well as work with a few useful subprocedure examples that make use of this concept.

Mike Faust

Mike Faust is a senior consultant/analyst for Retail Technologies Corporation in Orlando, Florida. Mike is also the author of the books Active Server Pages Primer, The iSeries and AS/400 Programmer's Guide to Cool Things, JavaScript for the Business Developer, and SQL Built-in Functions and Stored Procedures. You can contact Mike at This email address is being protected from spambots. You need JavaScript enabled to view it..


MC Press books written by Mike Faust available now on the MC Press Bookstore.

Active Server Pages Primer Active Server Pages Primer
Learn how to make the most of ASP while creating a fully functional ASP "shopping cart" application.
List Price $79.00

Now On Sale

JavaScript for the Business Developer JavaScript for the Business Developer
Learn how JavaScript can help you create dynamic business applications with Web browser interfaces.
List Price $44.95

Now On Sale

SQL Built-in Functions and Stored Procedures SQL Built-in Functions and Stored Procedures
Unleash the full power of SQL with these highly useful tools.
List Price $49.95

Now On Sale

BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

$

Book Reviews

Resource Center

  •  

  • LANSA Business users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.

  • The MC Resource Centers bring you the widest selection of white papers, trial software, and on-demand webcasts for you to choose from. >> Review the list of White Papers, Trial Software or On-Demand Webcast at the MC Press Resource Center. >> Add the items to yru Cart and complet he checkout process and submit

  • SB Profound WC 5536Join us for this hour-long webcast that will explore:

  • Fortra IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn: