Datasheet

Part I ✦ An Introduction to UML

is not complete, modelers will not be able to use it without customization. If it is

exhaustive — well, it just can’t be. Instead, the OMG adopted the second goal.

Goal 2: Furnish extensibility and specialization mechanisms to extend

the core concepts

In overly simplified terms, the core concepts should represent the old 80/20 rule.

We should be able to build 80 percent of the systems out there with 20 percent of

the conceivable concepts. When these core concepts are not enough, there should

be a way to build on them to get what we need.

Wherever possible a modeler should not have to invent entirely new concepts.

Users should be able to use concepts already defined by UML. There are at least

three ways that UML enables modelers to create new model elements:

✦ The core defines a number of fundamental concepts that may be combined to

create the new concept.

✦ The core provides multiple definitions for a concept.

✦ UML supports the ability to customize a concept by specializing one or more

of its definitions. (To specialize means to use an existing definition and then

override and/or add elements to it.)

A UML-defined solution for wholesale extensibility is a profile. A profile is basically

an implementation of UML for a specific domain, such as a particular technology

platform or a specific line of business. A profile predefines a set of model elements

that are unique or simply common to the target environment. In this manner, pro-

files tailor the modeling elements so that the modeler can represent his environ-

ment more accurately than is possible with generic UML but without losing any of

the semantic clarity of UML concepts.

Goal 3: Support specifications that are independent of particular

programming languages and development processes

One very valuable reason for modeling is to separate the requirements from the

implementation. Tying UML to a particular language automatically alienates every-

one not using that language. An implementation also ties UML to a point in time. For

example, when the programming language changes, UML becomes obsolete until it

can be brought up to date.

However, UML must map to the common object-oriented design constructs defined

in most OO languages. This alignment will support code generation and reverse

engineering, the integration of the modeling and coding environments. But rather

than alter UML to conform to languages, the mapping is accomplished through pro-

files that define the relationships between the model elements and the implementa-

tion constructs. Using a separate mapping layer effectively decouples, or separates,

UML from the implementation languages, allowing both to evolve at their own pace.

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