1. February 200491.3913 Ron McFadyen1 From the Merriam-Webster’s online dictionary (www.m-w.com): Main Entry: an·thro·po·mor·phism Pronunciation: -"fi-z&m Function: noun Date: 1753 : an interpretation of what is not human or personal in terms of human or personal characteristics : HUMANIZATIONHUMANIZATION Anthropomorphism

2. February 200491.3913 Ron McFadyen2 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.

3. February 200491.3913 Ron McFadyen3 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?

4. February 200491.3913 Ron McFadyen4 Figure 3.1 on page 31 - Architectural Layers

5. February 200491.3913 Ron McFadyen5 Sample Problem 1 What object should receive this message? How should objects interact to fulfill the request? How do we justify out decision?

6. February 200491.3913 Ron McFadyen6 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?

7. February 200491.3913 Ron McFadyen7 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

8. February 200491.3913 Ron McFadyen8 Guiding principles to help us assign responsibilities GRASP: –General Responsibility Assignment Software Patterns –Very very fundamental, simple, basic principles of object design. GRASP Patterns Fundamental Principles of Object Design

9. February 200491.3913 Ron McFadyen9 GRASP Patterns Expert Creator Controller Low Coupling High Cohesion

10. February 200491.3913 Ron McFadyen10 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 or objects has the majority of this information.

11. February 200491.3913 Ron McFadyen11 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 Pages 221-226

12. February 200491.3913 Ron McFadyen12 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.

13. February 200491.3913 Ron McFadyen13 Expert leads us to place the method getTotal() in Sale

14. February 200491.3913 Ron McFadyen14 A line item knows its quantity and the associated Product, so it is the expert … SalesLineItem should determine the line item subtotal

15. February 200491.3913 Ron McFadyen15 Only the Product Specification knows the price; so Product Specification needs a method...

16. February 200491.3913 Ron McFadyen16 Creator What object should have the responsibility to create an X? –Ignores special-case patterns such as Factory. Choose an object C, such that: –C contains or aggregates X –C closely uses X –C records instances of X objects –C closely uses X objects –C has the initializing data for X

17. February 200491.3913 Ron McFadyen17 Example: Who should be responsible for creating a SalesLineItem? Since Sale “contains” SalesLineItems, Creator suggests Sale as the candidate for this responsiblity

18. February 200491.3913 Ron McFadyen18 :Register :SalesLineItem makeLineItem(qty) create(qty) :Sale We assign the responsibility of creating a SalesLineItem to Sale – Sale will have a method makeLineItem

19. February 200491.3913 Ron McFadyen19 What object in the domain (or application layer) receives requests for work from the UI layer? Controller

20. February 200491.3913 Ron McFadyen20 Basic Principle: Interface objects should not have responsibility for handling system events Assign responsibility to a Controller, an object in the application/domain layer. Choose or invent an object in the application layer for this. 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

21. February 200491.3913 Ron McFadyen21 makeNewSale enterItem endSale makePayment Part of Figure 9.1 System Operations

22. February 200491.3913 Ron McFadyen22 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.

23. February 200491.3913 Ron McFadyen23 F Register, Store, Cashier F Register is chosen in the text

24. February 200491.3913 Ron McFadyen24 Figure 16.16 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

25. February 200491.3913 Ron McFadyen25 public void endSale ( ) { sale.becomeComplete(); } public void enterItem ( String id, int quantity ) { ProductSpecification spec = catalog.getSpecification( id ); sale.makeLineItem( spec, quantity ); } public void makeNewSale ( ) { sale = new Sale(); } public void makePayment (Money cashTendered ) { sale.makePayment( cashTendered ); } System event handling in Register The Controller doesn’t do much … delegates work to other objects … it receives the request & coordinates fulfillment We examine the code just to get an idea of its organization

26. February 200491.3913 Ron McFadyen26 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

27. February 200491.3913 Ron McFadyen27 F Every business system should have a controller F A controller is class whose job it is to coordinate the system events F The controller sees to it the messages are sent to the correct objects in the model – it delegates F The reason to have a controller is to separate the business model from the visual logic called a view F This is often called a MVC (Model View Controller) separation

28. February 200491.3913 Ron McFadyen28 F Advantage - is that the changes to the model do not affect the GUI (view) logic F Advantage - is that the changes to the GUI (view) do not affect the model logic – could have multiple GUIs – GUI is replaceable F It provides a buffer between the visual (view) and the business logic (model)

29. February 200491.3913 Ron McFadyen29 Figure 16.7 - desirable coupling of interface layer to domain layer

30. February 200491.3913 Ron McFadyen30 Low Coupling High Cohesion We use the same example, “creating a payment”, for both of these patterns. Both patterns happen to suggest the same collaboration

31. February 200491.3913 Ron McFadyen31 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

32. February 200491.3913 Ron McFadyen32 Low Coupling Example Assume we need to create a Payment instance … what class should do this?

33. February 200491.3913 Ron McFadyen33 Creator pattern suggests Register Figure 16.9 Register creates Payment (Collaboration diagram)

34. February 200491.3913 Ron McFadyen34 An alternative design is given in Figure 16.10 Which of the two designs, figures 16.9 and 16.10, supports lower coupling?

35. February 200491.3913 Ron McFadyen35 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 behavior in classes be related. A class with highly related responsibilities and which does not do excessive amounts of work has high cohesion

36. February 200491.3913 Ron McFadyen36 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

37. February 200491.3913 Ron McFadyen37 High Cohesion Example Assume we need to create a Payment instance … what class should do this?

38. February 200491.3913 Ron McFadyen38 First solution given in Figure 16.11 (note this Sequence Diagram represents the same logic as the Collaboration diagram in Figure 16.9)

39. February 200491.3913 Ron McFadyen39 Figure 16.12 gives a second solution (note this Sequence Diagram represents the same logic as the Collaboration diagram in Figure 16.10) Which solution supports higher cohesion for Register?

40. February 200491.3913 Ron McFadyen40 Note, when considering which class to assign the responsibility of creating a payment, the text arrives at the same solution when applying both principles