March 6, R. McFadyen1 Pure Fabrication P Problem: You have a responsibility to assign to a class, but assigning it to a class in the conceptual model causes low coupling and/or high cohesion. Solution: Fabricate a class - create a class that is not found in your conceptual model, one that does not necessarily have a business meaning to the business person. Ch 22: GRASP Patterns Pure Fabrication Protected Variations (Law of Demeter)

March 6, R. McFadyen2 Example: Suppose we need to save instances of Sale in a relational database. To which class in the model do you assign this responsibility? Since Sale has the information to be saved, Sale would be suggested by Information Expert. To manage data going to and from a relational database will require a large number of operations … insert, delete, update, select, rollback, commit, buffer management, … (Recall the ACID properties? D - durable) Pure Fabrication

March 6, R. McFadyen3 Pure Fabrication The Expert pattern suggests that Sale is the expert, and so should be the class to do this. For example, to save itself in the database. There’s a lot involved here - save, retrieve, commit, rollback - what about LineItems? Etc. We would end up adding a lot to Sale that has nothing to do with Sale-ness … Sale becomes less cohesive and coupled to more non-domain classes. Sale will become much more complex, and not so focused.

March 6, R. McFadyen4 Pure Fabrication Pure Fabrication suggests to create a new class for these new responsibilities PersistentStorage insert() delete() update()... Sale remains well-designed - high cohesion, low coupling PersistentStorage class is relatively cohesive - sole purpose is to store/retrieve objects to/from a relational database PersistentStorage is a fabrication; it is made up from your imagination; it cannot be found in the Domain Model

March 6, R. McFadyen5 Pure Fabrication Example: see pages of Patterns in Java, Volume 2; Mark Grand; John Wiley & Sons Fig 4.14 shows a conceptual model Fig 4.15 shows an initial assignment of responsibilities, where the ServiceManager does scheduling and invoicing. Fig 4.16 shows the fabrication of a new class, InvoiceGenerator, which results in higher cohesion/less coupling.

March 6, R. McFadyen6 System manages a field service organization: Organization sends technicians who install and repair equipment on service calls to other organizations that use the equipment Some service calls are paid by the organization that uses the equipment; equipment vendors pay from some service calls; others are paid for jointly. Service manager is given field service projects for a user organization Service manager is schedules service technicians to perform the tasks Service manager sends invoices for completed tasks to the paying organizations

March 6, R. McFadyen7 UserOrganization PayingOrganization ServiceTechnician ServiceManager FieldServiceProject Note: ServiceManager is highly coupled to other classes Organizes-work-for Install/repair-equipment schedules invoices compute-invoices-for performs pays-for 1..* 0..* * 0..* ServiceTask getPayor(): Patterns in Java, Volume 2 Fig 4.14

March 6, R. McFadyen8 Consider the tasks assigned to the ServiceManager: scheduling tasks scheduling projects scheduling technicians generating invoices These are central to the function of the service manager no reasonable class in the domain to assign this to, so using Pure Fabrication, fabricate a new class for this purpose

March 6, R. McFadyen9 UserOrganization Paying Organization ServiceTechnician ServiceManager FieldServiceProject Organizes-work-for Install/repair-equipment schedules invoices compute-invoices-for performs pays-for 1..* 0..* * 0..* ServiceTask getPayor(): Invoice Generator generateInvoices(): Note: Pure Fabrication preserves low coupling and high cohesion Patterns in Java, Volume 2 Fig 4.15

March 6, R. McFadyen10 Object Design P Behavioural decomposition Pure Fabrication can lead to this Classes are determined by behavioural grouping: managing persistent storage generating invoices Representational decomposition Classes represent real things found in the problem domain - clerks use registers to create sales comprising line items … Reduces the representational gap Objects do things that, in the real- world, are done to them. Design of objects divided into two groups:

March 6, R. McFadyen11 Protected Variations P Problem: How do we design systems so that changes in its elements does not have an unfavourable impact on other elements? Solution: Identify points of predicted variation/instability and assign responsibilities to create a stable interface around them Example: Law of Demeter (LoD) Special case of this pattern. (p ) If objects traverse long object structure paths and send messages to distant, indirect (stranger) objects, the system is fragile with respect to changes in the object structures - a common point of instability in systems. LoD helps us avoid creating such designs

March 6, R. McFadyen12 Law of Demeter Also called Don’t Talk to Strangers Each class should only use a limited set of other classes: only units “closely” related to the current unit. “Each class should only talk (send messages) to its friends.” “Don’t talk to strangers.”

March 6, R. McFadyen13 Law of Demeter FRIENDS

March 6, R. McFadyen14 Don’t Talk to Strangers PaymentRegisterSale getTenderedAmount() paymentAmount() endSale() enterItem() makePayment()... becomeComplete() makeLineItem() makePayment() getTotal() getPayment... The class diagram shows that Register knows about Sale, and Sale knows about Payments that have been made towards it add a method to get a payment Suppose Register needs to find out the amount of the payment

March 6, R. McFadyen15 Don’t Talk to Strangers Assume: Register has a paymentAmount method which returns the current amount tendered for the payment Sale has a method, getPayment, which returns the Payment instance associated with the Sale Consider: In order to return the payment amount, we could have a paymentAmount method in Register such as: public void paymentAmount() { Payment payment = sale.getPayment() Money amount = payment. getTenderedAmount () } A little different from the text’s example

March 6, R. McFadyen16 Don’t Talk to Strangers :Payment :Register:Sale The previous has messages: Register will have a dependency on Payment This increases the coupling in our system getPayment() getTenderedAmount ()

March 6, R. McFadyen17 Don’t Talk to Strangers :Payment :Register:Sale If getPayment() in Sale would invoke getTenderedAmount() in Payment, and return the payment amount, then we can de- couple Register from Payment make the solution more robust, less sensitive to changes, less coupling Register will get the payment amount it is after, but it won’t know how it was obtained - see Parnas’ concept of information hiding on page 339 Objects are only sending messages to their friends getTenderedAmount () getPayment()

March 6, R. McFadyen18 Law of Demeter presentation: Karl J. Lieberherr; Northeastern University