Lesson 14 - UML - Object Constraint Language - OCL
In the previous lesson, UML - UML under the hood and Profile diagram, we showed how UML is defined internally and we learned how to extend its syntax using Profile Diagrams. In today's last UML tutorial, we're gonna describe the OCL language that extends UML and adds a formal way of writing constraints (conditions).
OCL is an abbreviation of Object Constraint
Language. It's defined in the document of the same name and was
developed by IBM. Unlike the other UML parts, OCL is not a graphic language but
a language which is textual and formalized (meaning it has a strict form). We
can simply say that OCL is the standard way how to write conditions into
diagrams. So far, we have written guards, constraints, and other conditions to
our diagrams just how it felt right (such as
[a > 5] or
[Invoice is valid]). For most diagrams, this was all right.
However, some diagrams may require higher accuracy and hence the a unified
notation of these conditions to avoid misinterpretation.
OCL is often compared to the SQL language. It's relatively simple and we'll now describe the basics of its syntax. It's also a typed language, but it's not a programming language. Its purpose is really just to define conditions and we can't write application logic in it as we are used to from imperative programming languages.
It's all about constraints in OCL, the rules. Whether a rule commands, restricts or prohibits anything, it does not affect the
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