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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 language

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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In this tutorial, we'll learn to use the OCL language in our UML diagrams and describe its basic syntax. Practical examples included.

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Article has been written for you by David Capka Hartinger
The author is a programmer, who likes web technologies and being the lead/chief article writer at He shares his knowledge with the community and is always looking to improve. He believes that anyone can do what they set their mind to.
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