Advanced Object-oriented Design Patterns

Creational Design Patterns

Summarizing the Design Themes

Think differently about Types Vs. ClassesProgram to an interface (Type), not an implementation

(Class).

Favor object composition over class inheritance, black box over white box reuse.

Design relationships betweenobjects and their types to achieve good run-time structure.

Don’t limit yourself to compile time structures.

Creational Design Patterns

Creational design patterns abstract the instantiation process

Encapsulate knowledge about which concrete classes the system uses ..

Hide how instances of these classes are created and put together

Result: Modules can be designed independent of how objects are created, composed and represented …

Topic 1: Singleton Pattern

Ensure a class only has one instance, and provide a global point of access to it

Singleton: Applicability

Use singleton pattern when:• there must be exactly one instance of a class, and it must be

accessible to clients from a well-known access point. • when the sole instance should be extensible by sub-classing,

and clients should be able to use an extended instance without modifying their code.

Singleton Model

Singleton$ instance : Singleton

getInstance() : Singleton

public static Singleton getInstance() {

if (instance == null) instance = new Singleton (); return instance;

}

protected Singleton() {

// Constructor code }

Singleton: Consequences

Notice that the constructor is protected. A client that tries to instantiate Singleton directly will get an error at compile-time.

Controlled access to sole instance, only through getInstance () method.

The Singleton class may be sub-classed, and it's easy to configure an application with an instance of this extended class. How ?

Permits a variable number of instances. How?

Singleton: Exercise

You are writing a Java application that makes use of a database connection to connect to a data server. Each application instance is allocated a single connection. The database connection is used by multiple data access classes to load and update data.

Variation: Could you used the Singleton pattern to implement a database connection pool manager object ? How ?

Topic 2: Factory Method Pattern

Define an interface for creating an object, but let subclasses decide which class to

instantiate

Factory Method: Applicability

Use the Factory Method pattern when• a class can't anticipate the class of objects it must create. • a class wants its subclasses to specify the objects it creates

Example: You are a class library developer, who is developing a framework for Workflow product with objects such as Workflow, WorkflowTask and WorkflowScheduler. But you obviously do not know all the specific types of workflow tasks that such as system would be used with.

Factory Method: Example

WorkflowManager

createTask() : WorkflowTasknewTask() : WorkflowTask

WorkflowTask

start()stop()init()

InsuranceWorkflowManager

createTask() : WorkflowTask

InsuranceTask

WorkflowTask task = createTask ();task.init ()

return new InsuranceTask ();

Factory Method: Model

ProductCreator

factoryMethod()

ConcreteCreator

factoryMethod()

ConcreteProduct

Factory Method: Collaborations

Product (WorkflowTask) defines the interface of objects the factory method creates.

ConcreteProduct (InsuranceTask) implements the Product interface.

Creator (WorkflowManager)• declares the factory method, which returns an object of type Product. • Typically calls the factory method to create a Product object

ConcreteCreator (InsuranceWorkflowManager) overrides the factory method to return an instance of a ConcreteProduct.

Factory Method: Consequences

Eliminates the need to bind application-specific classes into your code. The code only deals with the Product interface; therefore it can work with any user-defined ConcreteProduct classes.

Factory Method gives subclasses a hook for providing an extended version of an object.

Could be used to connect parallel class hierarchies Factory method can be parameterized to specify a specific

concrete product type.

Factory Method: Exercise

Assume you have written a singleton Database connection pool manager, which manages a pool of Database connection objects. Your project manager comes with a new requirement that your Java application now has to support both DB2 and SQL Server in different installations. Also in the future there is a possibility of having to support Oracle. Your realize the way you connect to these different DB vendors is little different, though your logic within the Database Connection Pool Manager class is reusable. How would you go about addressing the new requirements?

Topic 3: Abstract Factory Pattern

Provide an interface for creating families of related or dependent objects without specifying their

concrete classes.

Abstract Factory: Applicability

Use the Abstract Factory pattern when• a system should be independent of how its products are created,

composed, and represented. • a system should be configured with one of multiple families of

products. • a family of related product objects is designed to be used together, and

you need to enforce this constraint. • you want to provide a class library of products, and you want to reveal

just their interfaces, not their implementations.

Abstract Factory: Example

MotifWidgetFactory

createWindow() : WindowWidgetcreateButton() : ButtonWidget

OpenLookWidgetFactory

createWindow() : WindowWidgetcreateButton() : ButtonWidget

MotifWindow

OpenLookWindow

MotifButton

OpenLookButton

Client

IButtonWidget<<Interface>>

+theButtonWidget

IWindowWidget<<Interface>>

+theWindowWidget

IWidgetFactory

createWindow() : WindowWidgetcreateButton() : ButtonWidget

<<Interface>>

+theWidgetFactory

Abstract Factory: Model

ConcreteFactory1

createProductA()createProductB()

ConcreteFactory2

createProductA()createProductB()

ProductA1

ProductA2

ProductB1

ProductB2

AbstractFactory

createProductA()createProductB() AbstractProductA

Client

AbstractProductB

Abstract Factory: Collaborations AbstractFactory (WidgetFactory) declares an interface for operations

that create abstract product objects ConcreteFactory (MotifWidgetFactory, OpenLookWidgetFactory)

implements the operations to create concrete product objects AbstractProduct (WindowWiget, ButtonWidget) declares an

interface for a type of product object ConcreteProduct (MotifWindow, MotifButton)

• defines a product object to be created by the corresponding concrete factory.

• implements the AbstractProduct interface Client uses only interfaces declared by AbstractFactory and

AbstractProduct classes

Abstract Factory: Consequences

Isolates concrete classes from Client It makes exchanging product families easy: The class of a

concrete factory appears only once in an application, where it's instantiated.

Ensures application use objects from only one family at a time

Supporting new kinds of products is difficult A more flexible but less safe design is to add a parameter

to operations that create objects. This parameter specifies the kind of object to be created.

Abstract Factory: Exercise

You are in charge of designing an Enterprise application that connects to your Magazine Fulfillment Service Providers and uploads/ download subscription lists. Though there are only 3 providers, each of them have different connection mechanism, communication mechanism and file structures. You would like to isolate your application from provider specific differences and and have identified a basic set of objects you want to work with in your application such a ProviderConnection, SubscriptionFile, SubscriptionOrder etc. How would you evolve your design ?

Topic 4: Builder Pattern

Separate the construction of a complex object from its representation so that the same construction process can create different

representations.

Builder: Applicability

Use the Builder pattern when• the algorithm for creating a complex object should be

independent of the parts that make up the object and how they're assembled.

• the construction process must allow different representations for the object that's constructed.

Builder: Example

AcordPolicyRecordBuilder

buildNamedInsured()buildAddress()buildVehicle()buildDriver()buildCoverage()getFinishedProduct()

BillingPolicyRecBuilder

buildNamedInsured()buildAddress()buildVehicle()buildDriver()buildCoverage()getFinishedProduct()

ClaimsPolicyRecBuilder

buildNamedInsured()buildAddress()buildVehicle()buildDriver()buildCoverage()getFinishedProduct()

MasterPolicyRecordReader

AcordRecord BillingRecord ClaimsRecord

MasterPolicyRecord

IPolicyRecordBuilder

buildNamedInsured(name : String)buildAddress(line1 : String, line2 : String, state : String, zip : String)buildVehicle(vin : String)buildDriver(name : String)buildCoverage(name : String)getFinishedProduct(name : String) : Product

<<Interface>>

Builder: Model

ConcreteBuilder

buildPart1()buildPart2()getFinishedProduct()

Director

Builder

buildPart1()buildPart2()getFinishedProduct()

AProduct

for all pieces of the complex object { builder.buildPart()}

prod = builder.getFinishedProduct ();

Builder: Collaborations Builder (PolicyRecordBuilder) - specifies an abstract interface for

creating parts of a Product object. ConcreteBuilder (AcordPolicyRecBuiler)

• constructs and assembles parts of the product by implementing the Builder interface.

• defines and keeps track of the representation it creates. • provides an interface for retrieving the product

Director (MasterPolicyRecordReader) - constructs an object using the Builder interface.

Product (AcordRecord, BillingRecord) - represents the complex object under construction. ConcreteBuilder builds the product's internal representation and defines the process by which it's assembled.

Builder: Consequences

Lets you vary a product's internal representation - Because the product is constructed through an abstract interface, all you have to do to change the product's internal representation is define a new kind of builder

Isolates code for construction and representation - Clients needn't know anything about the classes that define the product's internal structure; such classes don't appear in Builder's interface

Gives you finer control over the construction process - the Builder pattern constructs the product step by step under the director's control. Only when the product is finished does the director retrieve it from the builder.

Builder: Exercise

For your Enterprise application that integrates with your Magazine Fulfillment Service Providers, you need to read in a master subscription record and create multiple provider specific subscription formats. The subscription record structure is complex and has multiple data sections such as Subscribtion Info, Payment History, Credit Info etc. The target formats though containing the same inf. hold it in different order, format and data types. More over specific format have different summary data layouts. Your manager also decides to support an XML based subscription record format for integration with B2B Online subscription services. You know other formats are coming in the future.