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CS 691z / 791zTopics on Software

EngineeringChapter 17: Interfaces and

Subsystems [Arlow & Neustadt, 2002]

March 6, 2007

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Outline

Interfaces and subsystems:IntroductionInterfaces and component-based development

Finding interfaces Subsystems and interfacesAdvantages and disadvantages of interfaces

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Interfaces: Introduction…

Chapter 17 roadmap, Fig. 17.1 [Arlow & Neustadt, 2002]

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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

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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

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Interfaces: …Introduction

UML interface syntax, Fig. 17.2 [Arlow & Neustadt, 2002]

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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

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.Interfaces and Component-based Development…

Example of interface, Fig. 17.3 [Arlow & Neustadt, 2002]

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..Interfaces and Component-based Development..

Example of interfaces

and subsystems Fig. 17.4 [Arlow & Neustadt,2002]

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…Interfaces and Component-based Development.

Another example of interface, Fig. 17.5 [Arlow & Neustadt, 2002]

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…. Interfaces and Component-based Development

Interfaces in Java: the

collection classesFig. 17.6 [Arlow &

Neustadt,2002]

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Finding Interfaces

Techniques for finding interfaces in a designed system or subsystem:Challenge each associationChallenge each message sentFactor 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

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Subsystems..

Subsystems are packages stereotyped <<subsystem>> 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

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.Subsystems.

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

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..Subsystems

More detailed representation of a subsystemFig. 17.8 [Arlow & Neustadt, 2002]

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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

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.Subsystems and Interfaces.

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

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

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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