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					          CS 426/CPE 426
           Senior Projects
Chapter 19: Interfaces and Components

         [Arlow and Neustadt, 2005]

    University of Nevada, Reno
    Department of Computer Science & Engineering

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

                    Interfaces and Components     2
1 Introduction

 Chapter 19
  roadmap, Fig.
  19.1 [Arlow &
  Neustadt, 2005]

 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. 19.2 [Arlow & Neustadt, 2005]

2 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

                       Interfaces and Components             5
     2 Interfaces
 Interface features
  that need to be
  realized, Table 19.1
  [Arlow & Neustadt

                         Interfaces and Components   6
2 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

                            Interfaces and Components                      7
    2 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

                                Interfaces and Components                      8
   2 Interfaces
 The set of interfaces needed by a classifier for its operations are called
  required interfaces, as 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

                               Interfaces and Components                       9
  2 Interfaces

 Fig. 19.5 [Arlow & Neustadt 2005] shows an example of an assembled

                            Interfaces and Components                  10
 2 Interfaces

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

                                Interfaces and Components   11
3 Interface realization vs. inheritance

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

                              Interfaces and Components   12
3 Interface realization vs. inheritance

 Adding non-borrowable items such as journal needs further modeling
  Fig. 19.8 [Arlow & Neustadt 2005]

                            Interfaces and Components                  13
3 Interface realization vs. inheritance

 A more elegant solution is shown in Fig. 19.9 [Arlow & Neustadt 2005]

                             Interfaces and Components                    14
 3 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 and inheritances

                                 Interfaces and Components                     15
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

                      Interfaces and Components            16
4 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. 19.15 [Arlow & Neustadt 2005] shows the UML notation

                            Interfaces and Components                   17
4 Components

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

                          Interfaces and Components            18
4 Components

Component stereotypes, Table 19.2 [Arlow & Neustadt 2005]

5 Subsystems

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

                             Interfaces and Components   20
6 Finding Interfaces

 Techniques for finding interfaces in a system or
     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          21
7 Architecture and the layering pattern

7 Architecture and the layering pattern: HEDC

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
  With interfaces, a model can be neatly separated in cohesive
  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

                             Interfaces and Components                 24

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