GRASP Principles

Boutique product development company
It is amazing what you can accomplish when you have a client-centric team to deliver outstanding products.

GRASP
PRINCIPLES
Boutique product development company
It is amazing what you can accomplish when you have a client-centric team to deliver outstanding products.
Raheel Arif | Software Engineer

Understanding responsibilities is key to object-oriented
design - Martin Fowler
GRASP
o GRASP = General Responsibility Assignment Software
Patterns
o A set of principles for assigning responsibilities to classes –
the key skill in OO software design

GRASP PRINCIPLES
9 GRASP principles:
o Information Expert
o Creator
o Low Coupling
o Controller
o High Cohesion
o Polymorphism
o Pure Fabrication
o Indirection
o Protected Variations

GRASP Principles
Information Expert
Problem: What is a general principle for assigning
responsibilities to objects?
Solution: Assign a responsibility to the information expert, that
is, the class that has the information necessary to fulfill the
responsibility.
Example: E.g., Board information needed to get a Square

GRASP Principles
Pros and Cons
• Facilitates information encapsulation: why?
o Classes use their own info to fulfill tasks
• Encourages cohesive, lightweight class definitions
But:
• Information expert may contradict patterns of
Low Coupling and High Cohesion

GRASP Principles
Creator
Problem: Who creates an A?
Solution: Assign class B the responsibility to create an
instance of class A if one of these is true (the more the
better):
• B "contains" or compositely aggregates A.
• B records instances of A
• B closely uses A
• B has the initializing data for A.

GRASP Principles
Who creates the Squares?

GRASP Principles

GRASP Principles
Low Coupling
Problem: How to support low dependency, low change
impact, increased reuse?
Solution: Assign a responsibility so coupling is low.
Coupling – a measure of how strongly one element is
connected to, has knowledge of, or relies on other elements

GRASP Principles
Low Coupling
A class with high coupling relies on many other classes –
leads to problems:
• Changes in related classes forces local changes
• Harder to understand in isolation
• Harder to reuse

GRASP Principles
Example

GRASP Principles
Benefits
• Understandability: Classes are easier to understand in
isolation
• Maintainability: Classes aren’t affected by changes in
other components
• Reusability: easier to grab hold of classes

GRASP Principles
Controller
Problem: Who should be responsible for handling a system
event? (Or, what object receives and coordinates a system
operation?)
Solution: Assign the responsibility for receiving and/or
handling a system event to one of following choices:
• Represent the overall system, device or subsystem
(façade controller)
• Represent a use case scenario within which the system
event occurs (a <UseCase>Handler)

GRASP Principles
High Cohesion
Problem: How to keep classes focused and manageable?
Solution: Assign responsibility so that cohesion remains
high.
Cohesion measures how strongly related and focused are
the responsibilities of an element

GRASP Principles
High Cohesion
Problems from low cohesion (does many unrelated things or
does too much work):
• Hard to understand/comprehend
• Hard to reuse
• Hard to maintain
Brittle – easily affected by change

GRASP Principles
High Cohesion

GRASP Principles
High Cohesion
• Very low cohesion – a class is responsible for many things
in different functional areas
• High cohesion – a class has moderate responsibilities in
one functional area and collaborates with other classes to
fulfill tasks

GRASP Principles
High Cohesion
Typically high cohesion => few methods with highly related
functionality
Benefits of high cohesion:
• Easy to maintain
• Easy to understand
• Easy to reuse

GRASP Principles
Polymorphism
Problem: How to handle alternatives based on type?
How to create pluggable software components
Solution: When related alternatives or behaviors vary by
type (class), assign responsibilities for the behavior—using
polymorphic operations—to the
types for which the behavior varies.
• Polymorphic operations are those that operate on
differing classes
• Don’t test for the type of the object and use conditional
logic to perform varying statements based on type.

GRASP Principles
Monopoly Problem: How to Design for Different Square Actions?

GRASP Principles
Polymorphism

GRASP Principles
Pure Fabrication
Problem: What object should have the responsibility, when
you do not want to violate High Cohesion and Low Coupling,
or other goals, but solutions offered by Expert (for example)
are not appropriate?
Solution: Assign a highly cohesive set of responsibilities to
an artificial or convenience class that does not represent a
problem domain concept something made up, to support
high cohesion, low coupling, and reuse.

GRASP Principles
Pure Fabrication- Example
Who should be responsible for saving Sale instances in a
relational database?
- by Information Expert ??
- leads to low cohesion
- and high coupling
- better: create (“fabricate”) a new class that has this
responsibility

GRASP Principles
Indirection
Sometimes objects must interact with other objects or external
systems, which may change (or replaced) in future. Direct coupling to
such objects or systems may result in modification in our objects
Problem: Where to assign a responsibility, to avoid direct coupling
between two (or more) things? How to de-couple objects so that low
coupling is supported and reuse potential remains higher?
Solution:
Assign the responsibility to an intermediate object to mediate
between other components or services so that they are not directly
coupled. The intermediary creates an indirection between other
components.

GRASP Principles
Indirection--A Simple Example
Consider a CreditAuthorizationService class that needs to
use a Modem
Bad approach:
Put low-level calls to the Modem API directly in the methods of the
CreditAuthorizationClass
Better approach:
Add an intermediateModemclass that insulates
CreditAuthorizationClass from the Modem API.

GRASP Principles
Indirection--A Simple Example
Problems:
In a Sales System, there are multiple external third-party tax calculators
that must be supported.
The system needs to be able to integrate with different calculators
according to some conditions.
For example; if total is above 500TL it uses the external "Tax Master"
program, otherwise "Good As Gold" program.
Each tax calculator has a different interface .
One product may support a raw TCP socket protocol, another may offer a
SOAP interface, and a third may offer a Java RMI interface.
In the future, a new calculator program may be integrated into the system
or an existing calculator may be removed
Actually Sale class is responsible to calculate the total and therefore needs
the tax.
However, we want to keep our system (Sale) independent from the varying
external tax calculators.

GRASP Principles
Example: Third-Party (External) Tax Calculators in the NextGen System

GRASP Principles
Indirection - Notes
● The GoF Proxy, Bridge, and Mediator patterns utilize indirection.
● For that matter, classes created for Indirection are usually also Pure
Fabrications, thus exemplifying two patterns for the price of one. :-)
● Lower coupling between components
● Indirection is pervasive in computer science:
"Most problems in computer science can be solved by another
level of indirection."- David Wheeler
..but:
"Most problems in performance can be solved by removing
another layer of indirection." - anonymous

GRASP Principles
Protected Variations
Problem: How to design objects, subsystems, and systems so that
the variations or instability in these elements does not have an
undesirable impact on other elements?
Solution: Identify points of predicted variation or instability; assign
responsibilities to create a stable interface around them.
(The term "interface" is used in the broadest sense of an access
view; it does not literally only mean something like a Java interface.

GRASP Principles
Protected Variations - Example
Problem: a client explained that the logistical support application
used by an airline was a maintenance headache. There was frequent
modification of the business logic to support the logistics. How do you
protect the system from variations at this point?

GRASP Principles
Protected Variations - Example
Solution: A rules engine was added to the system, and an external
rule editor let the subject matter experts update the rules without
requiring changes to the source code of the system.

GRASP Principles
Thank you for your time
If you have any questions
then please ask
Waleed Bin Dawood | Software Engineer

GRASP Principles

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Ähnlich wie GRASP Principles

Ähnlich wie GRASP Principles (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

GRASP Principles