1. 1 CS 691z / 791z Topics on Software Engineering Chapter 17: Interfaces and Subsystems [Arlow & Neustadt, 2002] March 6, 2007

2. 2 Outline Interfaces and subsystems: Introduction Interfaces and component-based development Finding interfaces Subsystems and interfaces Advantages and disadvantages of interfaces

3. 3 Interfaces: Introduction… Chapter 17 roadmap, Fig. 17.1 [Arlow & Neustadt, 2002]

4. 4 Interfaces:.Introduction.. An interface specifies a named set of operations It defines a contract to be implemented by a classifier It also separates specification from implementation If the classifier that realizes (implements) the interface is physically packaged in a subsystem or component and the interface is public in this subsystem or component, then the subsystem or component also realizes the interface

5. 5 Interfaces:..Introduction. Interfaces allow “design to contract” as compared to “design to implementation” supported by classes Each operation in an interface must have a complete operation signature (name, types of all parameters, and return type), semantics (recorded as text or pseudo- code) and, optionally, a stereotype and set of constraints Interfaces may not have: Attributes Operation implementations Relationships navigable from the interface

6. 6 Interfaces: …Introduction UML interface syntax, Fig. 17.2 [Arlow & Neustadt, 2002]

7. 7 Interfaces and Component-based Development…. Interfaces are key elements for component- based development They allow addition of “plug-in” parts (with varied implementations) without changing the specification Both with components and subsystems, interfaces support low coupling and provide high architectural flexibility

8. 8.Interfaces and Component-based Development… Example of interface, Fig. 17.3 [Arlow & Neustadt, 2002]

9. 9..Interfaces and Component-based Development.. Example of interfaces and subsystems Fig. 17.4 [Arlow & Neustadt,2002]

10. 10 …Interfaces and Component-based Development. Another example of interface, Fig. 17.5 [Arlow & Neustadt, 2002]

11. 11 …. Interfaces and Component-based Development Interfaces in Java: the collection classes Fig. 17.6 [Arlow & Neustadt,2002]

12. 12 Finding Interfaces Techniques for finding interfaces in a designed system or subsystem: Challenge each association Challenge each message sent Factor out groups of operations reusable elsewhere Factor out sets of operations that repeat in classes Look at classes that have similar roles in the system Consider future extensions

13. 13 Subsystems.. Subsystems are packages stereotyped > They are used both in design and implementation Design subsystems contain: Design classes and interfaces Use case realizations Other subsystems Specification elements such as use cases Subsystems are used to: Separate design concerns Represent large-grained components Wrap legacy systems

14. 14.Subsystems. Alternatives for drawing subsystems Fig. 17.7 [Arlow & Neustadt, 2002]

15. 15..Subsystems More detailed representation of a subsystem Fig. 17.8 [Arlow & Neustadt, 2002]

16. 16 Subsystems and Interfaces.. By introducing public interfaces in subsystems many of the classes in the subsystems could be hidden. As such subsystems could act as “black boxes”. Using interfaces in a subsystem is different from listing subsystem operations (see Figure 17.8). In the latter case, public classes must realize the operations. When a class of a subsystem realizes an interface, the subsystem realizes that interface A subsystem that provides a set of services by realizing one or more interfaces can be seen as a component

17. 17.Subsystems and Interfaces. Example of subsystems that realize interfaces Fig. 17.9 [Arlow & Neustadt, 2002]

18. 18..Subsystems and Interfaces Physical architecture and the layering pattern, Fig. 17.10 [Arlow & Neustadt, 2002]

19. 19 Advantages and disadvantages of interfaces Designing with interfaces increases flexibility and extensibility Also, using interfaces supports low coupling by reducing the number of dependencies between classes, subsystems and components With interfaces, a model can be neatly separated in cohesive subsystems Drawbacks of interfaces relate to added complexity and increased performance costs As a guideline, use interfaces for the more “fluid” parts of the system and dispense of them for the more stable parts of the system