Thursday, December 01, 2005

Comments and Classes

I came across this article entitled “How to Write Comments”. I feel that generally this is really good advice. However, the author intermingles the topics of good commenting and object design, which are not the same thing. These are topics that cannot be fully addressed in a single article, so I will only touch on them.

There seems to be debate as to the “reason” why you comment your code. Personally, I don’t think there should be question as to why you write you comments, you just do it. Code is easier to understand, is more maintainable, and typically is better quality when commented. If I had to give a single reason, I would have to agree with the author that the primary reason is for readability of the code, whether it is for other coders, or for yourself. How many times have you had to revisit some piece of code you wrote a long time ago, and had to ask yourself “What the hell was I thinking?” Imagine that another coder had to figure that out. I try to imagine that everyone’s time is as limited as mine, which means they don’t have time to trace through code to figure out some ridiculous algorithm. While commenting is definitely and important aspect of design, I rank readability higher. I feel that design should be done separately through flow charts, pseudo-code, or UML and transitioned to comments before writing code. This is especially true for larger projects where tasks are broken up among individuals where the program flow is designed in a case tool and the programmers are simply typing it in and debugging.

Unfortunately, modern languages are not as verbose as languages like COBOL. COBOL is a wordy language, with very descriptive syntax. Languages like Java and C make commenting even more necessary for understanding the logic due to their use of symbols and their syntactical flexibility. My rule of thumb is to comment blocks of code accomplishing a single task in the problem solving steps. Some argue for very verbose commenting, some argue for less. I say use as much wording as is necessary to fully describe the task, and if possible, why are you using that approach. For example, lets say we had an object that handles connections to a database. Establishing a connection to the database is a single step in the overall algorithm, however it requires several steps of its own. So a single, descriptive comment telling a reader that you are setting the required properties and establishing the database connection would precede the series of lines. Below are two examples using the fictitious object to establish a database connection demonstrating commenting to describe what is being done.

Ex 1:
     //Establish a database connection using the connect call, passing in the user name
     //password, and database to connect to. This is done to avoid overhead of calling
     //the separate set functions for the three properties and to take advantage of the
     //error handling built in to the overloaded connect method
     data_object->connect(“user”, “password”, “OracleInstance1”);

Ex 2:
     //A database connection is established by setting the user name, password,
     //and database name, then calling the connect method. This is done separately
     //for readability and clarity on the separate steps and to isolate data assignment
     //errors from database connection errors

The above examples explain what the code is doing, and why.

Object design is another issue altogether. Understanding what objects are goes a long way to writing good objects. Objects are “abstractions of real world objects” that “know things” (properties) and “knows how to do things” (methods) (The Object-Oriented Approach: Concepts, Systems Development, and Modeling with UML page 17). This is one of the best definitions I have ever read for the definition of an object. Objects can be a representation in the system of anything, such as an airplane, care, disk drive, incident, or location. Classes are definitions of those objects. Objects are implementations of classes. These terms are used inappropriately used interchangeably sometimes. However if you understand those definitions, you can derive their meaning from the context. Books are written about good object design principles, so I will not go in to depth.

I agree with the author that objects should be describable in one “noun”, I question “verb” since actions are supposed to be reserved for methods, but I will concede that for the time being. Lets look at an example. Here is a simple event class from some sort of event generator. Events have a description, unique id, and a timestamp. These are the properties of the Event object. In the context of our system, the Event class will also need to know how to write the event out to a database, read an event in from a database, and generate the unique ID. These are the methods. The database related methods will be exposed for public use since we want the outside world to interact with those methods. The outside world does not care how the unique ID is generated, so it will be private. We will also need methods to set and return the values in our properties. This is the preferred method for exposing properties since it allows for type checking, error handling, possible logic built into property assignment, and to allow properties to be read-only or write-only. The class definition in C++ would look something like this:

//This class will handle Events from our fictitious event system
class event{
          //variables relating to the event
          string p_unique_id; //the ID for this events
          string p_description; //What is the actual alert message
          time_t p_timestamp; //when did the event occur

          //internal function to generate the ID for a new event
          string generate_unique_id();
          //public methods exposing the properties of the event
          string get_unique_id();
          string get_description();
          string get_timestamp();

          //public methods allowing for assignment of properties
          int set_unique_id(const string &);
          int set_description(const string &);
          int set_timestamp(const string &);

          //methods to work with a database for events
          int retrieve_Event_from_Database(string &);
          int insert_Event_into_Database();

In the above, what the object “knows”, and what it “knows how to do” are clearly defined. As a developer, I can interact with that object through its exposed methods, and the implementation is taken care of behind the scenes. I don’t care how it does it, as long as it does what it is supposed to do. A good class should shield me from the specifics, and do one thing and do it really well. The above class is simple and handles only one simple abstraction.

One of the biggest issues with Object Oriented Analysis and Design comes from choosing an OO language where it is not the appropriate language of choice for a project. OOAD has been touted as “The Way” for so long that people try to kludge it to scenarios where is not appropriate. For a simple program to run a sequence of steps, a scripting language would be more appropriate. For a low-level operating system driver handling a device or a tightly constrained program running on an embedded device, assembler or C would be more appropriate. For a simple piece of office software like a clock in/clock out program, Visual Basic would be more appropriate. For a system representing real world objects and procedures, an OO based language like C++ or Java would definitely fit. Knowing which scenarios to apply OO design concepts is essential to design. OO programming is not the end all answer to all software design issues; it is merely another tool in the developer’s toolkit.

There are several tools for working with OOAD models. The most robust one I have worked with is Rational Rose. However, Rose has a hefty price tag well outside most consumers. My alternative favorite is Umbrello, which is now included in the KDE SDK. I use it both under Linux and under Windows using Cygwin and it works very well, and includes a basic code generator. Visio also has stencils for UML models, although I only use Visio for flowcharts when Dia is unavailable.

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