Chapter 7

GRASP patterns

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Outline

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

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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:

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Historical background (2)

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.

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Historical background (3)

…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?

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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?

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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”

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Pattern main features

• Must have a significant and suggestive name

• Designed to solve a particular problem

• Facilitates communication and understanding

and furnishes a solution

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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

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Five GRASP Patterns

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

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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

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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

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Information Expert Pattern (3)

• Effect on class diagram