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Fall 2009 ACS-3913 Ron McFadyen 1
From the Merriam-Webster’s online dictionary (www.m-w.com):
Main Entry: an·thro·po·mor·phism
Date: 1753
an interpretation of what is not human or personal in terms of human or personal characteristics : HUMANIZATION
Anthropomorphism
Fall 2009 ACS-3913 Ron McFadyen 2
Anthropomorphism
Anthropomorphism: Object-oriented programming
works like human organizations. Each object will
communicate with another one by sending messages. So
the software objects work by just sending those messages.
Fall 2009 ACS-3913 Ron McFadyen 3
Responsibility-Driven Design (RDD)
• Detailed object design is usually done from the point of view of:
– Objects have responsibilities
– Objects collaborate
– Similar to how we conceive of people
• In RDD we do object design such that we will ask questions such as:
– What are the responsibilities of this object?
– Who does it collaborate with?
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Architectural Layers
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Sample Problem 1
• What object should receive this message?
• How should objects interact to fulfill the request?
???
Presentation
ApplicationLogic
Video Store
Record Rental
Video ID
...
...
...
... ...
Clerk
appLogicRequest()
Now what happens?
• How do we justify out decision?
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Sample Problem 2
• How should the objects interact in order for changes in the data to be reflected in the two displays?
Graphic
Display
List
Display
Data
User
• How do we justify our decision?
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•idea was first put forth by Christopher Alexander (1977) in his work on architectural design principles
•a pattern is a named problem/solution pair that can be applied in new contexts
•advice from previous designers to help designers in new situations
•rules of thumb - not new ideas
•There are many books on the subject; examples:•Design Patterns - Erich Gamma et al
•Java Design Patterns: a tutorial - James Cooper
•Design Patterns Java Workbook - Steven John Metsker
•Data Access Patterns - Clifton Nock
•Patterns of Enterprise Application Architecture – Martin Fowler
Patterns
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• Guiding principles to help us assign responsibilities
• GRASP:
– General Responsibility Assignment Software Patterns
– Very very fundamental, simple, basic principles of object design.
– Developed by Craig Larman
GRASP Patterns Fundamental Principles of Object Design
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GRASP Patterns
• Expert
• Creator
• Controller
• Low Coupling
• High Cohesion
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???
Presentation
ApplicationLogic
Video Store
Record Rental
Video ID
...
...
...
... ...
Clerk
appLogicRequest()
What object should this be?
What object in the domain (or application layer) receives requests for work from the UI layer?
Controller
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Basic Principle: Interface objects should not have responsibility for handling system events
Controller: a non-user interface object responsible for receiving or handling a system event.
In the Process Sale Use Case, there are several system events:
makeNewSale, enterItem, endSale, makePayment
The Controller doesn’t do much … it should delegate work to other objects … it receives the request & coordinates fulfillment
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makeNewSale
enterItem
endSale
makePayment
System Operations
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In general there are two candidates:– An object whose name reflects the use case.
• e.g. ProcessSaleHandler
– An object whose name reflects the overall server, business, or large-scale entity.
• A kind of “façade” object• e.g. Register, Store, Cashier
Choose or invent an object in the application layer for this.
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Register, Store, Cashier
Register is chosen in the text
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Allocating System Operations
Register has been chosen to handle all system operations
Ch 20 shows the code for this
If Use Case handlers were chosen
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The Controller pattern promotes reuse
•UI code is not intertwined with system event code
•UI can be replaced
•Multiple UIs could be utilized
When a legal sequence of operations must occur, state information must be kept … a Controller object is an excellent choice for this information
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A controller is class whose job it is to coordinate the system events
The controller sees to it the messages are sent to the correct objects in the model – it delegates
The reason to have a controller is to separate the business model from the visual logic called a view
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Advantage - is that the changes to the model do not affect the GUI (view) logic
Advantage - is that the changes to the GUI (view) do not affect the model logic – could have multiple GUIs – GUI is replaceable
It provides a buffer between the visual (view) and the business logic (model)
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desirable coupling of interface layer to domain layer
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Expert
• Assign a responsibility to the object that has the information necessary to fulfill it.
– “That which has the information, does the work.”
– Not a sophisticated idea - rather, it is common sense
– E.g., What software object calculates grand total?
• What information is needed to do this?
• What object has the majority of this information.
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Expert Example.
In the NextGEN POS application, it is necessary to know the grand total of a sale.
Where should that responsibility be placed?
{We will be assigning a few responsibilities in this example}
Expert suggests that we should look for a class that has the information needed to determine the grand total.
If our design is just beginning, we look at the Domain Model and bring the pertinent conceptual classes into the class model
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What information is needed to determine the grand total?
It is necessary to know all the SalesLineItem instances of a sale and to sum their subtotals. A Sale instance is aware of these … Sale is the Expert choice for having the responsibility of knowing the grand total.
Sale
Sales LineItem Product
Description
contains
Described by
1
1..*
*1
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Consider figure 17.15 …
Expert leads us to place the method getTotal() in Sale
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Consider figure 17.16 …
A line item knows its quantity and the associated Product, so it is the expert … SalesLineItem should determine the line item subtotal
… another method, getSubTotal() required in SalesLineItem
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Consider Figure 17.17 …
Only the ProductDescription knows the price; so ProductDescription needs a method ...
… another method getPrice() required in ProductDescription
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Creator
• What object should have the responsibility to create an X?
– Ignores special-case patterns such as Factory… later GoF
• Choose an object C, such that:
– C contains or compositely aggregates X
– C closely uses X
– C records instances of X objects
– C has the initializing data for X
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Example: Who should be responsible for creating a SalesLineItem? Since Sale “contains” SalesLineItems, Creator suggests Sale as the candidate for this responsiblity
Sale
Sales LineItem Product
Description
contains
Described by
1
1..*
*1
Fall 2009 ACS-3913 Ron McFadyen 28
:Register
:SalesLineItem
makeLineItem(qty)
create(qty)
:Sale
We assign the responsibility of creating a SalesLineItem to Sale – Sale will have a method makeLineItem
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Low CouplingHigh Cohesion
We use the same example, “creating a payment”, for both of these patterns.
Both patterns happen to suggest the same collaboration
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Low Coupling
• When we need to assign a responsibility to a class, we should do so such that coupling remains low.
• Coupling: a measure of how strongly one element is connected to, has knowledge of, or relies on other elements.
• Low coupling: not dependent on too many other elements
• High coupling results in
– classes that are harder to understand in isolation
– changes to related classes force local changes
– classes that are harder to reuse
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Low Coupling
• Example
Assume we need to create a Payment instance … what class should do this?
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Creator pattern suggests Register
Register creates Payment (Collaboration diagram)
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An alternative design
Which of the two designs supports lower coupling?
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High Cohesion
• How do we assign responsibilities so that cohesion remains high?
• Cohesion: a measure of how strongly related and focused the responsibilities of an element are.
– Cohesion is a measure of how single purpose the attributes and behavior within a class are
– It is better that attributes and behaviour in classes be related.
• A class with highly related responsibilities and which does not do excessive amounts of work has high cohesion
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High Cohesion
•Rule of Thumb (ROT): A class with high cohesion has a relatively small number of methods, with highly related functionality, and does not do too much work … it collaborates with others to get work done.
•Low cohesion results in classes that
–are hard to comprehend
–hard to reuse–hard to maintain
–delicate - constantly affected by change
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High Cohesion
•ExampleAssume we need to create a Payment instance … what class should do this?
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First solution
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second solution
Which solution supports higher cohesion for Register?
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Note, when considering which class to assign the responsibility of creating a payment, the text arrives at the same solution when applying both principles