INFS 6225 – Object-Oriented INFS 6225 – Object-Oriented Systems Analysis & DesignSystems Analysis & Design

Chapter 8: Chapter 8: Class & Method DesignClass & Method Design

Class & Method DesignClass & Method Design

Criteria, activities, & techniques used to design classes & methodsCriteria, activities, & techniques used to design classes & methods

Part of Design Phase where work is actually accomplishedPart of Design Phase where work is actually accomplished

Refinement of UML models to create the Refinement of UML models to create the mostmost efficientefficient system system possible that still meets the users’ requirementspossible that still meets the users’ requirements

The steps The steps justjust beforebefore the actual coding of the system the actual coding of the system

Review of TermsReview of Terms

ObjectObject – basic building block of a system; instances of classes – basic building block of a system; instances of classes

ClassClass – defines both data & processes that each object contains – defines both data & processes that each object contains

AttributesAttributes – describe data about the object – describe data about the object

MethodsMethods – specify processes that objects can perform – specify processes that objects can perform

MessageMessage – procedural call from one object to another – procedural call from one object to another

More TermsMore Terms

InheritanceInheritance – superior classes pass attributes & methods to – superior classes pass attributes & methods to subordinate classessubordinate classes

EncapsulationEncapsulation – combining processes & data into a single object – combining processes & data into a single object

Information HidingInformation Hiding – only information required to use an object – only information required to use an object should be available outside of an objectshould be available outside of an object

PolymorphismPolymorphism – same message is interpreted differently by different – same message is interpreted differently by different objectsobjects

Dynamic bindingDynamic binding – interpretation of message by an object takes – interpretation of message by an object takes place at run timeplace at run time

New TermsNew Terms

CohesionCohesion – how specific a module is within a system (i.e., – how specific a module is within a system (i.e., a class or object should represent only one thing)a class or object should represent only one thing)

CouplingCoupling – level of interdependence among modules – level of interdependence among modules (classes, objects, & methods) in a system(classes, objects, & methods) in a system

The higher the degree of coupling, the more likely that changes in The higher the degree of coupling, the more likely that changes in part of a system will require changes in other partspart of a system will require changes in other parts

ConnascenceConnascence – two modules are – two modules are soso intertwined that a change in intertwined that a change in one requires a change in the otherone requires a change in the other

Ideally, a system should have Ideally, a system should have highhigh Cohesion Cohesion and and lowlow (i.e., (i.e., loose)loose) Coupling Coupling

Opportunities for ReuseOpportunities for Reuse

Class LibraryClass Library Set of implemented classes that are designed for reuseSet of implemented classes that are designed for reuse Pre-built “template” classes with Attributes and Methods already definedPre-built “template” classes with Attributes and Methods already defined Can be utilized using Can be utilized using InheritanceInheritance Support Foundation LayerSupport Foundation Layer

ComponentComponent Self-contained piece of software that can be “plugged” into a systemSelf-contained piece of software that can be “plugged” into a system Simplify development of objects on Problem Domain & Human-Simplify development of objects on Problem Domain & Human-

Computer Interaction LayersComputer Interaction Layers

Optimizing the DesignOptimizing the Design

1.1. Shorten/simplify paths between objectsShorten/simplify paths between objects

2.2. Move attributes to provide more direct accessMove attributes to provide more direct access

3.3. Reduce the number of messages (Fan-out) sent by an Reduce the number of messages (Fan-out) sent by an objectobject

4.4. Reorder execution of statementsReorder execution of statements

5.5. Cache (temporarily save) computational resultsCache (temporarily save) computational results

Constraints & ContractsConstraints & Contracts

ContractContract formalizes the interactions between client and server objectsformalizes the interactions between client and server objects Document the message passing between objectsDocument the message passing between objects Contain information necessary for programmer to understand Contain information necessary for programmer to understand

methodmethod

Contracts may contain:Contracts may contain: Pre-conditionsPre-conditions – constraints that must be met for a method to – constraints that must be met for a method to

executeexecute Post-conditionsPost-conditions – constraints that must be met after the method – constraints that must be met after the method

executes (or method is undone)executes (or method is undone)

Contract ExampleContract Example

Method Name: Class Name: ID:

Clients (Consumers):

Associated Use Cases:

Description of Responsibilities:

Arguments Received:

Type of Value Returned:

Pre-Conditions:

Post-Conditions:

Method SpecificationMethod Specification

Method Specifications – written documents that include explicit Method Specifications – written documents that include explicit instructions on how to write the code to implement a methodinstructions on how to write the code to implement a method

No standard syntax for Method SpecificationNo standard syntax for Method Specification

Written in Written in Structured EnglishStructured English – short sentences – short sentences

4 Components:4 Components:1.1. General InformationGeneral Information about Method about Method2.2. EventsEvents – items that – items that triggertrigger method method3.3. Message Passing Message Passing – what is being passed, and returned– what is being passed, and returned4.4. Algorithm Specification Algorithm Specification (calculations, formula, etc.)(calculations, formula, etc.)