object oreinted design

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Chapter 21 -- Assistance -- Magy Seif El-NasrFebruary 1, 1998 R. A. Volz 1

Object-Oriented Design


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Outline

From Analysis toDesign

Design Issues

System Design

Object Design

Design patterns&

Conclusion


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Object-Oriented Designx Transforms OOA model into blueprint for

software construction

x Builds upon four essential design concepts Abstraction

Information hiding

Functional independence

Modularity


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Mapping OOA to OOD


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Outline


Design Issues

System Design

Object Design

Design patterns&

Conclusion


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Comparison of Conventional & OOD

x Representation of module hierarchiesx Specification of data definitionsx Specification of procedural logicx

Indication of end-to-end processing flowx Representation of object states and transitionsx Definition of classes and hierarchiesx Assignment of operations to classes

x Detailed definition of operationsx Specification of message connectionsx Identification of exclusive services


7/39Chapter 21 -- Assistance -- Magy Seif El-NasrFebruary 1, 1998 R. A. Volz 7

Design Issues for Modularity

x Decomposabilityx Composabilityx Understandability

x Continuityx Protection

x Linguistic modular units

x Few interfacesx Small interfaces (weak coupling)x Explicit interfacesx Information hiding (no global data)



OOD Methodologiesx The Booch Method

x

The Coad and Yourdon Methodx The Jacobson Method

x The Rumbaugh Method

x The Wirfs-Brock Method



Booch Methodx Architectural planning

Cluster similar objects in separate architectural

partitions. Layer objects by level of abstraction.

Identify relevant scenarios.

Create a design prototype. Validate the design prototype by applying it to

usage scenarios.



Booch Methodx Tactical design

Define domain-independent policies.

Define domain specific policies.

Develop a scenario that describes the semantics

of each policy.

Create a prototype of each policy. Instrument and refine the prototype.

Review each policy.



Booch Methodx Release planning

Organize scenarios developed during OOA by

priority. Allocate corresponding architectural releases to the

scenarios.

Design and construct each release incrementally.

Adjust goals and schedule of incremental release as

required.



The Rumbaugh OMT

x Perform system designx Conduct object designx

Implement Control mechanismsx Adjust class structure to strengthen inheritancex Design messaging to implement the object

relationships

x Package classes and associations into modules



OutlineFrom Analysis to

Design

Design Issues

System Design

Object Design

Design patterns&

Conclusion



OOD Process Flow



System Design

x Partition the analysis model into subsystems A subsystem is a package (collection) of classes

and associations, events and constraints that are

interrelated and have a small interface. Usually identified by the services it provides.

x Identify concurrenciesx Allocate subsystems to processors and tasksx Choose a basic strategy for data management



System Design

x Identify global resources and access control

mechanismsx Design control mechanism for systemx Determine how to handle boundary

conditions.x Review & modify if necessary



System Design

Partition the Analysis Modelx Small number of subsystems,

Partition subsystems to reduce complexity

x Well-defined interface, servicesx Intra- (use) and inter- (minimize)

communicationx

Achieve high Cohesion within a subsystemx Communication between subsystems: client/

server (one way) or peer-to-peer (two way)



Communication Models



System Design

Cohesion

Goingtow

ards

Hi

ghCoh

esio

n

x Coincidental

x Logical

x Temporal

x Communication

x Sequential

x Functional

x Data



System Design

Concurrency and subsystem Allocationx Identify active objects (threads of control)x Where to allocate subsystems

same processor? -- need concurrency control independent processor ? -- may still need

concurrency control

x Allocation and design issues:

performance requirements. costs

overheads and efficiency



Concurrency Example

Database

DB access

DB access



Concurrency

x Ada -- Use tasks

task type DB_access is end;

type dba is access DB_access;

a, b: dba;

a := new DB_access;

b:= new DB_access;


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Concurrency

x Java -- Use threads

class dbAccess extends thread{

public void run () { }

new dbAccess.start();

new dbAccess.start();

}


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Some Concurrency Issues

x Mutual exclusion of shared objects

Two objects should not be able to write to the same

shared object at the same time.

x Communication

Allow messages to be sent between objects. When

is an object ready to receive a message?

x Synchronization Ensure that two or more objects are at known

points at the same time.


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Data Managementx Generally refers to shared objectsx Concurrency controls essentialx

Often refers to data to have a permanencebeyond scope of program

x Issues Infrastructure for access and storage

Management of datax Often look for interfaces to existing support

subsystems


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System design

Resource Managementx Resources: External entities vs. abstractions

E.g., disk, processor, comm line

Databases, objects, interfacesx Guardian object:

Keeper of the resource

Controlling access to the resource Moderating conflicts for requests

x Language support can vary widely


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Intersubsystem Communicationx Create a table for each contract


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Subsystem Collaboration Graphx List each request made by a collaborator


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Outline


Design Issues

System Design

Object Design

Design patterns&

Conclusion


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Object Design


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Key Object Characteristics

(Available from Analysis)x Object name (or name of class of objects)x Descriptionx

Services provided or functionsx How createdx How invokedx Shared resources

x Communication With whom?

Interfaces


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Object Design

Object Internalsx Protocol description

List each message type the object can receive, e.g.,

MESS(motion sensor): read RET id, statusx Implementation description

Spec. of objects name and reference to class

Spec. of private data encapsulated Procedural description of each operation

Specification of control structures


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Use a PDL to Describe Object

PACKAGEprogram_component_nameIS

type specification of data objects

Proc specifications of related operationsPRIVATE

data structure details for object types

PACKAGE BODYprogram_component_nameIS

PROC operation: (interface specification) IS

PROC operation: (interface specification) IS

END program_component_name


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High Level Sensor PDL

PACKAGE sensor ISTYPE sensor_status IS (ON | OFF)TYPE sensor_id, alarm_charPROC read, set, test

PRIVATEsensor_id, IS STRING LENGTH (8)alarm_char DEFINEDthreshold, sig_type, sig_level IS INTEGER

PACKAGE BODY sensor ISTYPE update_rate IS INTEGERPROC read (sensor_id, sensor_status: OUT)...


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Object Design

Steps done in object design

x Detail object attributes and operations

x Review object-relationship model

x Describe operations and transitions usingPDLs.


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Outline


Design Issues

System Design

Object Design

Design patterns&

Conclusion


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Design Patterns

x An abstraction that conveys meaning about a classsapplicability.

xpattern template Name : (applicability and intent)

Problem description: (env. and conditions) Characteristics

Consequences

xNaming -- choose to aid searchx Two different mechanisms

is-a (inheritance) -- a basis for new subclasses has-a (composition) - ensemble


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What next ?

x From Design to implementationx Success of complex projects relies more on the

design architecture rather than the

implementation. So SW Eng. stresses OOA andOOD

x If you have a good and complete design,

implementation should be straightforwardxNevertheless, language choice does have an

influence.

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