1. 1 CS 426 Senior Projects Chapter 19: Interfaces and Components [Arlow & Neustadt 2005] February 28, 2008

2. 2 Outline Interfaces and subsystems: Introduction Interfaces Interface realization vs. inheritance Components Subsystems Finding interfaces The layering pattern Advantages and disadvantages of interfaces

3. 3 Introduction Chapter 17 roadmap, Fig. 19.1 [Arlow & Neustadt, 2005]

4. 4 Introduction Designing a subsystem is concerned with breaking a system up into subsystems (as independent as possible) Interactions between subsystems are mediated by interfaces. Fig. 19.2 [Arlow & Neustadt, 2005]

5. 5 Interfaces An interface specifies a named set of public features It defines a contract to be implemented by a classifier In other words, an interface defines a service offered by a class, component, or system It also separates specification from implementation An interface cannot be instantiated Anything that realizes an interface (e.g., a class) must accept and agree by the contract defined by the interface

6. 6 Interfaces Interfaces allow “design to a contract” as compared to “design to an implementation” supported by classes This provides a high degree of flexibility Modern software architectures are based on the concept of “service”, supported by interfaces The attributes and operations of an interface should be fully specified, with: Complete operation signature The semantics of the operation (text or pseudocode) Name and type of the attributes Any operation or attribute stereotypes, constraints, tagged values

7. 7 Interfaces The set of interfaces realized by a classifier is known as provided interfaces, with UML syntax (two styles) shown in Fig. 19.3 [Arlow & Neustadt 2005] Note that the two different notations for the realization relationship

8. 8 Interfaces The set of interfaces needed by a classifier for its operations are called required interfaces, with UML syntax shown in Fig. 19.4 [Arlow & Neustadt 2005] Note that the two different notations for the dependency relationship, with the socket symbol in the right-hand side

9. 9 Interfaces Fig. 19.5 [Arlow & Neustadt 2005] shows an example of an assembled system

10. 10 Interfaces Interfaces in Java: the collection classes, Fig. 19.6 [Arlow & Neustadt 2005]

11. 11 Interface realization vs. inheritance Interface: “realizes contract specified by” Inheritance: “is a” Both can generate polymorphism Fig. 19.7 [Arlow & Neustadt 2005] shows an inheritance-based solution

12. 12 Interface realization vs. inheritance Adding non-borrowable items such as journal needs further modeling Fig. 19.8 [Arlow & Neustadt 2005]

13. 13 Interface realization vs. inheritance A more elegant solution is shown in Fig. 19.9 [Arlow & Neustadt 2005]

14. 14 Interface realization vs. inheritance Still better is to combine inheritance and interfaces, Fig. 19.10 [Arlow & Neustadt 2005]. Advantages: every item in the Library is a LibraryItem; borrowability concept factored out; fewer classes; simpler inheritance hierrachy; fewer compositions/inheritances

15. 15 Interfaces and component-based development Interfaces are key elements for component-based development (CBD) 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

16. 16 Components A component is a modular part of the system that encapsulates its contents and whose manifestation is replaceable within its environment It acts as a black box whose external behavior is completely defined by its interfaces (provided and required); hence, it can be replaced by any other component that supports the same protocol Fig. 19.15 [Arlow & Neustadt 2005] shows the UML notation

17. 17 Components Components may depend on other components To decouple components, always mediate the dependency with interfaces, Fig. 19.17 [Arlow & Neustadt 2005]

18. 18 Components Component stereotypes, Table 19.2 [Arlow & Neustadt 2005]

19. 19 Subsystems A subsystem is a component that acts as unit of decomposition for a larger system Interfaces connect subsystems to create a system architecture Subsystems are used to: Separate design concerns Represent large-grained components Wrap legacy systems A system example is shown in Fig. 19.19 [Arlow & Neustadt 2005]

20. 20 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 Factor out sets of attributes that repeat in classes Look at classes that have similar roles in the system Consider future extensions

21. 21 Architecture and the layering pattern Fig. 19.20 [Arlow & Neustadt 2005]

22. 22 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