march 200391.3913 ron mcfadyen1 ch 17: use case realizations with grasp patterns assigning...

March 2003 91.3913 Ron McFadyen 1

Ch 17: Use Case Realizations with GRASP Patterns

Assigning responsibilities to objects to achieve user goals

Section 17.4 makeNewSale

Section 17.5 enterItem

Section 17.6 endSale

Section 17.7 makePayment

Section 17.8 Start Up

You should review these sections



Who/What is going to be responsible for :

•Handling the system operation makeNewSale?

•Creating a Sale?

•Who/What should create the container of SalesLineItems?

•Handling the System operation enterItem?

•Creating a Saleslineitem?

•Finding a match for a product specification? For doing a lookup?

•Handling the system operation endSale?

•Setting the isComplete attribute of Sale to true?

•Calculating the sale total?

•Calculating the SalesLineItem subtotal?

•Providing the ProductSpecification price?

•Handling the system operation makePayment?

•Creating a Payment instance?

•……….Which GRASP pattern justified the decision?



Section 17.4 makeNewSale

1st concern: who/what is going to be responsible for handling the system operation makeNewSale?

Decision: Using the Controller Pattern

… since there aren’t very many system operations and since Register (in our domain) represents the overall system, we choose Register as a “façade” controller.



By “Controller”, Register is our “façade” controller.

makeNewSale()

:Register



Page 314

public class Register

{ …

public makeNewSale()

{

…

}

We have just decided that Register must have a method makeNewSale



2nd concern: who/what is going to be responsible for creating a Sale?

Should Register delegate the responsibility or …?

Since Register (in the domain) actually records a Sale then, by “Creator”, we decide that Register will do this. Register has the data and it needs to keep track of a Sale, so …



By “Creator”, Register creates a Sale.

makeNewSale()

:Register

create():Sale



Page 314

public class Register

{ …

Private Sale sale;

…

public makeNewSale()

{

sale = new Sale();

}

The method makeNewSale creates the sale object



3rd concern: Sale needs to know of its SalesLineItems. A container for these is required.

Who/What should create this?

Since Sale will contain the lines, by “Creator”, we decide that Sale will do this…



By “Creator”, Register creates a Sale.

makeNewSale()

:Register

create():Sale

create() ::SalesLineItem



Page 315

public class Sale

{ private List lineItems = new ArrayList();

private Date date = new Date();

private boolean isComplete = false;

private Payment payment;

…

The constructor for Sale creates the container for the line items




1st concern: who/what is going to be responsible for handling the system operation enterItem?

We continue using the Controller Pattern … Register is responsible for handling enterItem.



Contract for enteritem specifies

Preconditions:

•A Sale is underway

Postconditions:

•salesLineItem is created

•It is associated with the current Sale

•Its quantity is set

•It is associated with a ProductSpecification

By Creator, Sale can do this

Sale stores the new sales line item in its collection

The product specification will need to be found



enterItem():Register :Sale

::SalesLineItem

: SalesLineItem

2:makeLineItem()

2.1:create()

2.2:add()

Message 2 … see later slide



Page 315

public class Sale

{ …

public void makeLineItem (…, …)

{

lineItems.add ( new SalesLineItem (…, …) ) ;

}

The “add” message sent to the multiobject

The “create” message for a new sales line item



Page 316

public class SalesLineItem

{ private int quantity;

private ProductSpecification productSpec;

public void SalesLineItem (…spec, …quantity)

{

this.productSpec = spec ;

this.quantity = quantity

}

The constructor for a sales line item arranges that the sales line item knows its product specification and its quantity




2nd concern: who/what is going to be responsible for finding a match for a product specification? For doing a lookup?

Who has knowledge of product specifications? Who is capable of doing a lookup?

Expert suggests the product catalogue is the best candidate. The product catalogue contains the product specifications … the product catalogue has the information, it is the expert.



enterItem():Register

::ProductSpecification

:ProductCatalog

1:getSpecification()

1.1:find()



Page 316

public class ProductCatalog

{ private Map productSpecifications = new HashMap();

…

public ProductSpecification getSpecification (ItemID id)

{

Return( …. productSpecifications.get ( id );

}

Collection of product specifications

Method to find a specific item

Handles the “find” message



enterItem():Register :Sale

::ProductSpecification

:ProductCatalog

::SalesLineItem

:SalesLineItem

1:getSpecification()

1.1:find()

2:makeLineItem()

2.1:create()

2.2:add()



Given the previous collaboration, what methods/responsibilities have been assigned to the various classes?

Register Sale

ProductCatalogSalesLineItem

march 200391.3913 ron mcfadyen1 ch 17: use case realizations with grasp patterns assigning...

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