1. Object Oriented Software Development
7. Implementing a model

2. UML models
UML may be used to visualise, specify, construct and document the artefacts of a software system
Part of a software development method
To produce a working application the artefacts in the model must be implemented using development tools, languages and frameworks
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3. Implementing a model
We will use Visual Studio, C# and the .NET framework to begin the implementation of a UML model
Based on examples from Software Modelling Analysis and Design 1
That module and this one represent two closely related aspects of the same process
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4. UML diagrams and C# code
Class Diagram – a C# application is constructed as classes, which generally correspond to classes in the class diagram
Sequence/Collaboration Diagrams – these describe the messages which pass between objects, and relate to method calls in C# code
Object Diagram – represents the objects which exist as the program runs
Use Case Flow of Events/Activity Diagrams – describe sequences of events and workflows which must be implemented in C# code
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5. Example – order system
In the system, an order consists of a number of order lines
An order line specifies a single product and the quantity of that product ordered
Each order is associated with a customer, who has a discount level which is applied to all his or her orders
Sample code in OrderSystem project
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6. Use case – calculate order price
We will look at the use case, the classes involved in it and the sequence diagram for it
Use Case Description
Calculates the total price of a single order including the customer’s discount
Use Case Flow of events
Calculate the price of each order line as (product price) x (quantity) and add to total
Apply the customer’s discount to the total and calculate the final price
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7. Class diagram customer can be associated with many orders
orderline only exists as part of an order - composition
order line contains one product only
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8. Sequence diagram collaborates with other objects to do so
responsibility for calculating its price belongs to order
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9. Create classes using class diagram
Create a new C# class for each class in the diagram
Declare instance variables based on the attributes in the diagram
Define (empty) methods based on the operations in the diagram
Methods which simply access instance variables (e.g. GetDiscount) can be implemented in C# as properties
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10. Create classes using class diagram
Consider whether properties should be read-write or read-only
Define constructor to set values of instance variables
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11. Implement relationships
Add code to implement relationships between classes shown in the diagram
May need to refine model to clarify the meaning of the relationship in some cases
Purpose of relationship is to allow objects to collaborate, so sequence diagram may help clarification by showing what collaborations are needed
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12. OrderLine – Product association
OrderLine is associated with a single product
OrderLine sends a message to Product
Product does not need to send message to OrderLine, so doesn’t need an OrderLine reference
Association is navigable in one direction only, OrderLine to Product
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13. OrderLine-Product implementation
Implement this with an instance variable in OrderLine of type Product
Product has no reference to OrderLine
C# implementation option – create LinePrice property instead of a GetLinePrice method
Cleaner syntax but no difference conceptually – accessing a property is a message, just like a method call
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14. OrderLine-Product
Note – when constructor would only set properties, can omit it and create objects with object initialiser syntax in C#
sends message to product object to ask for its price
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15. Coding patterns This is an example of a one-to-one “has-a” association
OrderLine has-a Product
Common type of association, usually implemented using the same coding pattern
One class has an instance variable whose type is the name of the other class
Can also provide a property or accessor method for this instance variable if required
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16. Coding patterns
A pattern is a general reusable solution to a common problem
Patterns can be applied to many aspects of software design and programming, from loops to complex groups of collaborating classes
Here we are using patterns for relationships between two classes (binary relationship coding patterns)
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17. Coding patterns The coding pattern for a relationship includes:
Implementation
The code artefact without which the relationship does not exist
Helpers
Properties and methods which may be required to make use of the relationship in particular situations
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18. Coding pattern: one-to-one “has-a” association
Implementation
Instance variable in one class whose type is the name of the other class
Helpers
Property in the first class which can be used to get or set the associated object
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19. Order – OrderLine association
Order is associated with many OrderLines
Order “has-a” set of OrderLines
OrderLine only exists as part of an order – composition
This is a whole-part relationship
Order sends a message to OrderLine (GetLinePrice)
Association is navigable in one direction only, Order to OrderLine
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20. Order – OrderLine implementation
Implement this with an instance variable in Order which is a collection of type OrderLine
OrderLine has no reference to Order
Order will call GetLinePrice method of each of its OrderLines
C# implementation option – what kind of collection should we use?
No need to search, will only add to end of collection, best option is List<OrderLine>
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21. Order - OrderLine
sends message to OrderLine object to get its line price
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22. Order – OrderLine helper method
Need a method which can add a new OrderLine to an Order
Composition – the new OrderLine object is created within this method
OrderLine is added to List using its Add method
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23. Coding pattern: one-to-many “has-a” association
Implementation
Instance variable in one class whose type is a collection which holds instances of the other class
Helpers
Method to add objects to the collection (will create objects if the association is composition)
Method to remove an object
Method to return a specific object
Method to return the whole collection
Only need to implement helper(s) as needed
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24. Coding pattern example code
CodingPatterns solution
OneToManyCompositionPattern project
Demonstrates pattern with a full set of helpers
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25. Order-Customer association
Customer can have many Orders
Customer “has-a” set of Orders
But Order is not part of a Customer
Customer does not create Order
Association or aggregation, not composition
Order is associated with one Customer
Order “has-a” Customer
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26. Order-Customer association
In this use case, Order needs to send message to Customer
Order class needs reference to Customer object
Order is associated with one Customer
Order “has-a” Customer
Association or aggregation
So the relationship between Order and Customer is navigable in both directions
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27. Order-Customer association
Actually, this use case does not require any messages from Customer to Order
Don’t strictly need to implement association in that direction
Implementing it here because it is likely that other use cases will require it
e.g. Show orders for a customer
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28. Aggregation and association
Implies ownership
Whole-part, or “has-a” relationship
Part may be shared with other Wholes, unlike composition
Association
No implication of ownership
“uses-a” relationship
Difference in meaning - makes little difference to code
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29. Aggregation and association
“Few things in the UML cause more consternation than aggregation and composition, in particular how they vary from regular association...”
Martin Fowler, 2003
See link below for full quote:
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30. Class diagram with navigabilities
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31. Order-Customer implementation
Need an instance variable in Customer which is a collection of type Order
Need an instance variable in Order which is a reference to a Customer
Order will call GetDiscount method of its Customer
Need some code to make relationship consistent
Order object should be part of the orders collection belonging to its associated Customer
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32. Order-Customer implementation
implemented as a Dictionary for fast searching
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33. Order-Customer consistency
constructor of Order
when you create an order, you specify the customer it’s for, and add it to that customer’s orders
method in Customer
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34. Creating objects create Customer object create Order object
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35. Coding pattern: one-to-many bidirectional association
Implementation
Implement as a one-to-many and a one-to-one in opposite directions
Code in constructor of one class ensures consistency
Helpers
Same as one-to-many
Remove method may need additional code to deal with consistency
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36. Coding pattern example code
OneToManyAssociationPattern project
OneToManyBidirectionalPattern project
Demonstrate patterns with a full set of helpers
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37. Order – Product association
We didn’t show an association on the class diagram between Order and Product
No message passed between Order and Product in sequence diagram
But there is a relationship
Order “uses-a” Product to create an OrderLine
Simple association
Sometimes modelled as a dependency
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38. Order – Product association
AddOrderLine method in order has a parameter of type Product
Temporary association – only exists while this method runs
Sets up permanent association between Product and OrderLine
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39. Class diagram with Order-Product
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40. Coding pattern: simple association
Implementation
One class has a reference to instance(s) of the other in one of the following places:
Method parameter
Method return value
Local variable
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41. Coding pattern example code
SimpleAssociationPattern project
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42. Other relationship examples
These examples and patterns don’t capture all possible types of relationship
Provide examples on which to base solutions to real problems
Object oriented systems model the real world, and reality can be complicated!
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43. What’s next?
We will go on to look at some practical issues which are important in developing robust programs, including handling error situations with exceptions, debugging and testing
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