For the last 20 years, application developers have used structured analysis and design techniques to craft most of today's world-class applications. But those applications were coded using structured programming languages, such as RPG and COBOL. Structured analysis and design techniques are not a good fit for cross-platform applications that are to be written in object-oriented Java. So what's the replacement strategy for structured analysis and design? The simpleton's answer is object-oriented analysis and design (OOAD), but the more accurate answer is Unified Modeling Language (UML). Let me explain.

For a decade, a dozen or so well-known OOAD evangelists each had their own methods. During that period, when you interviewed for a job, you'd be asked questions like, "We at ABC Corporation follow the Rumbaugh methodology. Do you have design experience with Rumbaugh's notation?" Needless to say, there was much confusion in the object-oriented programming (OOP) industry. In 1995, two of the OOAD evangelists, Jim Rumbaugh and Grady Booch, merged their methodologies to form what they called the Unified Method. Then, in 1996, Ivar Jacobson joined the team. The result of their collaborative effort was the Unified Modeling Language. By sheer brute force, the design strategies of UML have become the standard for OOAD.

UML is more than just a set of notations; it is a complete strategy for properly using the notation and diagrams. Today, there are over a dozen UML graphical notations that developers can use to design a system, but I primarily use only three: use cases, class models, and event traces.

Not Just Acting

During the analysis phase of an OO project, the design of use cases is vital. The designers first identify the actors of the application, the people or systems that will be using it. Then the designers flesh out short descriptions of how the actors will be using the application. Those short descriptions are called use cases. A use case is often called a scenario because it describes a typical interaction between an actor and the application. The descriptions are supposed to be atomic. For example, I recently mentored a company's employees in the development of an all-Java application. Even though they had already fleshed out a structured design for the application, I asked them to develop use cases. The first use case they sent to me for review was moderately lengthy.

I told them that it was not one use case but rather a collection of six. They should have broken down their use case for online order processing into several smaller use cases, such as Find Customer, Find Existing Order, and Add Item. The reason why use cases should be atomic is that they are used to drive the programming process. Furthermore, if use cases are properly designed, they promote code reuse.

Use cases are integral to OOAD because they describe the requirements for the system. The domain experts and technical experts form the design team. They continue to develop use cases until everyone agrees that the use cases effectively describe the overall scope of the application. Because the resulting use cases are small, the technical staff can more accurately estimate the time it will take to implement the code for one.

A Model Class

Once enough use cases have been developed to describe the scope of the application, developers use them to drive the object discovery process. The nouns in the use cases suggest requirements for Java classes, and the verbs of use cases suggest class methods. UML has a graphical model appropriately called class model that shows the Java classes of an application, each class's attributes and methods, and how the classes are associated with the other classes of the application.

Once designers have finalized the class model, programming is almost ready to begin. The programming process is driven by use cases. The interaction of objects that flesh out each use case is diagrammed by what are known as sequence diagrams, or event traces. Sequence diagrams are like "pseudocode" but do not include the description of computations. A sequence diagram shows the nested interaction of the Java objects required to perform a single use case.

Got Documentation?

Besides being an integral part of OOAD, use cases make for great higher-level documentation for code maintenance. But whenever you need to make a modification to the system, you should first modify the use case so that the documentation stays accurate. That sounds like a lot of extra work, but wouldn't it be a good idea to have the person or group that requested the change submit it as a change in the use case so everyone can agree up front what the change should be? And isn't it a good idea for the programmers to modify the pseudocode-like sequence diagrams before they begin to change working code, not only to clearly identify what they are about to do but also to document the change for future programmers?

My opinion about UML is that any notation that clarifies thought is worthwhile. That means that if your shop effectively uses a whiteboard, your design strategy works. But if your shop goes into analysis paralysis as developers use the most expensive UML tools available, then your design strategy does not work. My own experience says that you should be somewhere in the middle. Whiteboards work fine for brainstorming, but the models drawn are not retained for later review, modifications, and enhancements. UML, and the software tools that provide a graphical environment for UML, makes it easier to develop quality object-oriented applications.

REFERENCES AND RELATED MATERIALS

• Applying Use Cases: A Practical Guide. Geri Schneider and Jason P. Winters. Reading, Massachusettes: Addison-Wesley, 1998

• UML Distilled: Applying the Standard Object Modeling Language. Martin Fowler and Kendall Scott. Sebastopol, CA: Addison-Wesley, 1997

• UML in a Nutshell. Sinan Si Alhir. Reading, Massachusettes: O'Reilly & Associates, Inc., 1998