Chapter 7 GRASP patterns Dr Seham Mostefai CAP 252.

Chapter 7

GRASP patterns

Dr Seham MostefaiCAP 252

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Outline

• Historical background• What is a pattern?• GRASP Patterns

Dr Seham Mostefai CAP 252

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Historical background (1)

ArchitectureChristopher Alexander, professor of architecture: “A pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it

the same way twice.”

C.Larman Slides on GRASP: Designing objects with responsibilitiesDr Seham Mostefai CAP 252

Patterns were used far before software design like in:

Dr Seham Mostefai CAP 252 4


Dress making

C.Larman Slides on GRASP: Designing objects with responsibilities

You can make a dress by specifying the route of a scissors through a piece of cloth Or, by following a pattern.



…and other fields– Mature engineering disciplines have handbooks

describing successful solutions to known problems– Automobile designers don't design cars from

scratch, they reuse standard designs learnt from experience

– Should software engineers make use of patterns? Why?


Patterns in Software design

Developing software from scratch is expensive Patterns support reuse of software design solutions– 1987: Beck & Cunningham: patterns for UI– 1990-200: Gamma, Helm, Johnson and Vlissides: Gof patterns– More recently: C. Larman: GRASP patterns.– UML is a language, how can we apply it?


What is a software pattern?

Alexander: “A pattern is a recurring solution to a standard problem, in a context.”

In Object technology

C. Larman: “ a pattern is a named problem/solution pair that can be applied in new contexts, with advice on how to apply it in novel situations”


Pattern main features

• Must have a significant and suggestive name

• Designed to solve a particular problem

• Facilitates communication and understanding

and furnishes a solution


GRASP Patterns

• GRASP= General Responsibility Assignment Software Pattern

• Name chosen to suggest the importance of grasping fundamental principles to successfully design object-oriented software

• Describe fundamental principles of object design and responsibility expressed as patterns


Five GRASP Patterns

• Information Expert• Creator• Controller• Low Coupling• High Cohesion


Information Expert Pattern(1)

Name: Information ExpertProblem: How to assign responsibilities to objects?Solution: Assign responsibility to the class that has the

information needed to fulfill it?Example: ordering a meal in a restaurantWhich classes have the responsibility to get the total of

an order?Order DishLine DishDescription


Information Expert Pattern (2)

Effect on collaboration diagram

:Invoice :Order :DishLineT:=total() 1*:LT:=LineTotal()

:DishDescription

2: P:=Price()

By Expert

By Expert

By Expert


Information Expert Pattern (3)

• Effect on class diagram

Chapter 7 GRASP patterns Dr Seham Mostefai CAP 252.

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