1. Interfaces and Components
CS 425/625
Software Engineering
Interfaces and Components
Chapter 19 [Arlow and Neustadt, 2005]
University of Nevada, Reno
Department of Computer Science & Engineering

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

3. 1 Introduction
Chapter 19 roadmap, Fig [Arlow & Neustadt, 2005]

4. 1 Introduction
Designing large software applications is concerned with breaking a system up into subsystems (as independent as possible)
Interactions between subsystems are mediated by interfaces
Fig [Arlow & Neustadt, 2005]

5. Interfaces and Components
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
Interfaces and Components

6. Interfaces and Components
Interface features that need to be realized, Table 19.1 [Arlow & Neustadt 2005]
Interfaces and Components

7. Interfaces and Components
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
Interfaces and Components

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

9. Interfaces and Components
The set of interfaces needed by a classifier for its operations are called required interfaces, as shown in Fig [Arlow & Neustadt 2005]
Note that the two different notations for the dependency relationship, with the socket symbol in the right-hand side
March 9, 2010
Interfaces and Components

10. Interfaces and Components
Fig [Arlow & Neustadt 2005] shows an example of an assembled system
Interfaces and Components

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

12. 3 Interface realization vs. inheritance
Interface: “realizes contract specified by”
Inheritance: “is a”
Both can generate polymorphism
Fig [Arlow & Neustadt 2005] shows an inheritance-based solution
Interfaces and Components

13. 3 Interface realization vs. inheritance
Adding non-borrowable items such as journal needs further modeling
Fig [Arlow & Neustadt 2005]
Interfaces and Components

14. 3 Interface realization vs. inheritance
A more elegant solution is shown in Fig [Arlow & Neustadt 2005]
Interfaces and Components

15. 3 Interface realization vs. inheritance
Still better is to combine inheritance and interfaces, Fig [Arlow & Neustadt 2005] . Advantages: every item in the Library is a LibraryItem; borrowability concept factored out; fewer classes; simpler inheritance hierrachy; fewer compositions and inheritances
Interfaces and Components

16. 4 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
Interfaces and Components

17. Interfaces and 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 behaviour is completely defined by its interfaces (provided and required); hence, it can be replaced by any other component that supports the same protocol
Fig [Arlow & Neustadt 2005] shows the UML notation
Interfaces and Components

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

19. 4 Components
Component stereotypes, Table 19.2 [Arlow & Neustadt 2005]

20. Interfaces and Components
5 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 [Arlow & Neustadt 2005]
Interfaces and Components

21. Interfaces and Components
6 Finding Interfaces
Techniques for finding interfaces in a 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
Interfaces and Components

22. 7 Architecture and the layering pattern

23. 7 Architecture and the layering pattern: HEDC

24. 8 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
Interfaces and Components