1 CS 426 Senior Projects Chapter 4: Use Case Modeling [Arlow and Neustadt, 2002] February 8, 2007.
1 CS 426 Senior Projects Chapter 19: Interfaces and Components [Arlow & Neustadt 2005] February 28,...
-
date post
19-Dec-2015 -
Category
Documents
-
view
219 -
download
0
Transcript of 1 CS 426 Senior Projects Chapter 19: Interfaces and Components [Arlow & Neustadt 2005] February 28,...
11
CS 426Senior Projects
Chapter 19: Interfaces and Components
[Arlow & Neustadt 2005]
February 28, 2008
22
Outline
Interfaces and subsystems:IntroductionInterfacesInterface realization vs. inheritance ComponentsSubsystemsFinding interfacesThe layering patternAdvantages and disadvantages of interfaces
44
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]
55
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
66
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
77
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
88
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
1111
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
1212
Interface realization vs. inheritance
Adding non-borrowable items such as journal needs further modelingFig. 19.8 [Arlow & Neustadt 2005]
1313
Interface realization vs. inheritance
A more elegant solution is shown in Fig. 19.9 [Arlow & Neustadt 2005]
1414
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
1515
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
1616
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
1717
Components Components may depend on other components To decouple components, always mediate the
dependency with interfaces, Fig. 19.17 [Arlow & Neustadt 2005]
1919
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]
2020
Finding Interfaces
Techniques for finding interfaces in a designed system or subsystem:Challenge each associationChallenge each message sentFactor out groups of operations reusable elsewhereFactor 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
2222
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