The Agile Modeling (AM) Method

UML Interfaces: Diagramming Style Guidelines

A UML interface is a collection of operation signatures and/or attribute definitions that ideally defines a cohesive set of behaviours. UML interfaces are implemented, “realized” in UML parlance, by classes and components – to realize an interface, a class or component must implement the operations and attributes defined by the interface. Any given class or component may implement zero or more interfaces, and one or more classes or components can implement the same interface.
Figure 1. The internals of the seminar component.

Figure 2. Class implementing interfaces.

Figure 3. Logical component architecture.


  1. Depict One Interface Per Port. Ports are connection points between a classifier and its environment that are depicted on the side of frames as small rectangles. In Figure 1 each port has exactly one interface, which is logically cohesive and thus does not reveal anything about the internal design of the Seminar component.
  2. Depict One Port Per Realizing Class. Another approach to organizing the interfaces for SeminarComponent of Figure 1 would be to have a single port offering the EnrollmentTranscript, and DataEntity interfaces. This would make for a more compact diagram although contradicts Depict One Interface Per Port: choose an approach and stick to it.
  3. Provided Interfaces on the Left. In Figure 1 the provided interfaces are depicted using “lollipop” notation.
  4. Required Interfaces on the Right. Required interfaces, such as StudentPersistence, and XMLProcessor in Figure 1 are depicted in UML 2 as “sockets”.
  5. Apply Realizes Relationships for Ports. In Figure 1 SeminarComponent realizes the three ports on the left of the frame and the Enrollment class delegates to the Student port.
  6. Reflect Implementation Language Constraints in Interface Definitions. In Figure 2, the interface includes a public attribute named POID and several public operations. Unfortunately, it could not be implemented in Java because this language does not (yet) support instance attributes in the definition of interfaces.
  7. Name Interfaces According to Language Naming Conventions .
  8. Prefer “Lollipop” Notationto Indicate Realization of an Interface. As you can see in Figure 2 there are two ways to indicate that a class or component implements an interface: the lollipop notation used with the Serializable interface and the realization line (the dashed line with a closed arrowhead) used with the PersistentObject interface.
  9. Define Interfaces Separately from Your Classifiers.
  10. Do Not Depict the Operations and Attributes of Interfaces in Your Classes.
  11. One Label Per Interface Connection. In Figure 3 you see that the IPersistence interface is indicated twice, once for the lollipop and once for the socket.
  12. Place Interface Labels Above The Interface.

Recommended Reading

Modeling style: Elements of UML 2.0 Style The Elements of UML 2.0 Style describes a collection of standards, conventions, and guidelines for creating effective UML diagrams. They are sound, proven strategies that lead to diagrams that are easier to understand and work with. These conventions exist as a collection of simple, concise guidelines that if applied consistently, represent an important first step in increasing your productivity as a modeler. This book is oriented towards intermediate to advanced UML modelers. Although there are numerous examples throughout the book it would not be a good way to learn the UML (instead, consider The Object Primer). The book is 188 pages long and is conveniently pocket-sized so it’s easy to carry around.
Choose Your WoW! 2nd Edition This book, Choose Your WoW! A Disciplined Agile Approach to Optimizing Your Way of Working (WoW) – Second Edition, is an indispensable guide for agile coaches and practitioners. It overviews key aspects of the Disciplined Agile® (DA™) tool kit. Hundreds of organizations around the world have already benefited from DA, which is the only comprehensive tool kit available for guidance on building high-performance agile teams and optimizing your WoW. As a hybrid of the leading agile, lean, and traditional approaches, DA provides hundreds of strategies to help you make better decisions within your agile teams, balancing self-organization with the realities and constraints of your unique enterprise context.
The Object Primer 3rd Edition: Agile Model Driven Development (AMDD) with UML 2 The Object Primer 3rd Edition: Agile Model Driven Development with UML 2 is an important reference book for agile modelers, describing how to develop 35 types of agile models including all 13 UML 2 diagrams. Furthermore, this book describes the fundamental programming and testing techniques for successful agile solution delivery. The book also shows how to move from your agile models to source code, how to succeed at implementation techniques such as refactoring and test-driven development(TDD). The Object Primer also includes a chapter overviewing the critical database development techniques (database refactoringobject/relational mappinglegacy analysis, and database access coding) from my award-winning Agile Database Techniques book.