Presentation on GoF Design Patterns Submitted by WWW. ASSIGNMENTPOINT Presentation on GoF Design Patterns Submitted by WWW.ASSIGNMENTPOINT.COM www.assignmentpoint.com

GoF Patterns GoF stands for Gang-of-Four According to the four authors of “Design Patterns” book Twenty three GoF patterns such as Adapter Factory Singleton Strategy Composite Facade Observer/Publish-Subscribe/Delegation Event Model www.assignmentpoint.com

Adapter Context/Problem: How to resolve incompatible interfaces, or provide a stable interface to similar components with different interfaces? Solution: Convert the original interface of a component into another interface, through an intermediate adapter object. www.assignmentpoint.com

Adapter www.assignmentpoint.com

Adapter Adapter pattern is a specialization of Polymorphism, Indirection and Protected Variations (GRASP Patterns) www.assignmentpoint.com

Factory Context/Problem: Who creates the adapters? How to determine which class of adapter to create, such as TaxMasterAdapter or GoodAsGoldTaxProAdapter? Solution: Create a Pure Fabrication object called a Factory that handles the creation. Design Principle: Design to maintain a separation of concerns. www.assignmentpoint.com

UML Style www.assignmentpoint.com

Advantages of Factory Separate the responsibility of complex creation into cohesive helper objects. Hide potentially complex creation logic Allow introduction of performance-enhancing memory management strategies, such as object caching or recycling. www.assignmentpoint.com

Singleton Context/Problem: Exactly one instance of a class is allowed – it is a “singleton”. Objects need a global and single point of access. Solution: Define a static method of the class that returns the singleton. www.assignmentpoint.com

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Implementation and Design Issues Lazy initialization public class ServiceFactory { static ServiceFactory instance = null ; // other variable definition public static synchronized ServiceFactory getIntance () { if (instance == null) { instance = new ServiceFactory() ; } return instance ; // other methods Eager initialization public class ServiceFactory { private static ServiceFactory instance = new ServiceFactory (); public static ServiceFactory getIntance () { return instance ; } // other methods www.assignmentpoint.com

Lazy vs. Eager Approach Lazy one is preferable Creation work (and perhaps holding on to “expensive” resources is avoided in the instance is never actually accessed. The getInstance () lazy initialization sometimes contains complex and conditional creation logic. www.assignmentpoint.com

Static Instance vs. Static Method public class ServiceFactory { static ServiceFactory instance = null ; // other variable definition public static synchronized ServiceFactory getIntance () { if (instance == null) { instance = new ServiceFactory() ; } return instance ; // other methods public class ServiceFactory { private static ServiceFactory instance = new ServiceFactory (); public static ServiceFactory getIntance () { return instance ; } // other methods www.assignmentpoint.com

Static Instance vs. Static Method Static instance is preferred for the following reasons: Static methods are not polymorphic (virtual) and don’t permit overriding in sub-classes in most languages. Most OO remote communication mechanisms, e.g. Java RMI, only support remote-enabling of instance methods, not static methods. You create an object X as singleton, but latter on discovered that it is should not be a Singleton i.e. multiple instances needed. Thus, instance-side solution offers flexibility www.assignmentpoint.com

Use of Adapter, Factory and Singleton public class Register { public void initialize () { // do some work …………….. accAdapter = ServiceFactory.getInstance().getAccountingAdapter () ; } // other methods www.assignmentpoint.com

GoF Patterns Adapter Factory Singleton Strategy Composite Facade Observer/Publish-Subscribe/Delegation Event Model www.assignmentpoint.com

Strategy Context/Problem: How to design for varying, but related, algorithms or policies? How to design for the ability to change these algorithms or policies? Solution: Define each algorithm/policy/strategy in a separate class, with a common interface. www.assignmentpoint.com

Strategy Public Class Sale { LinkedList list = new LinkedList () ; void addSalesLineItem (SalesLineItem sli) { list.add (sli) ; } int getTotal () { total = 0 for each item in list total = total + item.getSubTotal () ; www.assignmentpoint.com

Strategy Public Class Sale { LinkedList list = new LinkedList () ; void addSalesLineItem (SalesLineItem sli) { list.add (sli) ; } int getTotal () { total = 0 for each item in list total = total + item.getSubTotal () ; Strategy Public Class Sale { int total = 0 ; LinkedList list = new LinkedList () ; ISalePricingStrategy isps = (ISalePricingStrategy) new AbsoluteDiscountOverTrresholdPricingStrategy () ; void addSalesLineItem (SalesLineItem sli) { list.add (sli) ; } int getPreDiscountTotal () { return total ; int getTotal () { total = 0 for each item in list total = total + item.getSubTotal () ; return isps.getTotal (this) ; www.assignmentpoint.com

Strategy Public Class Sale { LinkedList list = new LinkedList () ; void addSalesLineItem (SalesLineItem sli) { list.add (sli) ; } int getTotal () { total = 0 for each item in list total = total + item.getSubTotal () ; Strategy Public Class Sale { int total = 0 ; LinkedList list = new LinkedList () ; ISalePricingStrategy isps = (ISalePricingStrategy) new AbsoluteDiscountOverTrresholdPricingStrategy () ; void addSalesLineItem (SalesLineItem sli) { list.add (sli) ; } int getPreDiscountTotal () { return total ; int getTotal () { total = 0 for each item in list total = total + item.getSubTotal () ; return isps.getTotal (this) ; www.assignmentpoint.com

Creating a Strategy with a Factory www.assignmentpoint.com

Composite Example: Suppose a store has the following policies in efeect today: 20% senior disocunt policy Preferred customer discount of 15% off sales over $400 Today there is $50 off purchases over $500 Buy 1 case of Darjeeling tea, get 15% discount off of everything Suppose a senior who is also a preferred customer buys 1 case of Darjeeling ea and $600 of veggieburgers. What pricing policy should be applied? Policy may be different Best for the customer (lowest price) Best for the store (highest price), probably during recession www.assignmentpoint.com

Composite Context/Problem: How to treat a group or composition structure of objects the same way (poly-morphically) as a non- composite (atomic) object? Solution: Define classes for composite and atomic objects so that they implement the same interface. www.assignmentpoint.com

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Collaboration with a Composite www.assignmentpoint.com

GoF Patterns Adapter Factory Singleton Strategy Composite Facade Observer/Publish-Subscribe/Delegation Event Model www.assignmentpoint.com

Strategy Context/Problem: How to design for varying, but related, algorithms or policies? How to design for the ability to change these algorithms or policies? Solution: Define each algorithm/policy/strategy in a separate class, with a common interface. www.assignmentpoint.com

Façade Context/Problem: A common, unified interface to a disparate set of implementations or interfaces – such as within a subsystem – is required. There may be undesirable coupling to many things in the subsystem, or the implementation of the sub-system may change. What to do? Solution: Define a single point of contact to the subsystem – a façade object that wraps the subsystem. Example: “Rule engine” subsystem, whose specific implementation is not yet known. While creating a line item may be several rules may need to be checked. www.assignmentpoint.com

Façade Public Class Sale { public void makeLineItem (ProductSpecification spec, int quantity) { SalesLineItem sli = new SalesLineItem (spec, quantity) ; if (POSRuleEngineFacade.getInstance ().isInvalid ( sli, this ) ) return ; lineItem.add ( sli) ; } // ….. } // end of class www.assignmentpoint.com

Façade Rule engine subsystem may contain one to hundreds of classes. www.assignmentpoint.com

Observer/ Publish – Subscribe/Delegation Even Model A probable solution When Sale changes its total, it sends a message to a window, asking to refresh its display But then Sale object, a domain model object, knows an UI object Low coupling is not maintained www.assignmentpoint.com

Another Example For Alarm Clock the following things needs to do Show a display dialog box Beep or ring a bell Run a reliability watch dog For upcoming train / plane schedule Display the schedule at different display boards Automatically announce the time thorough mikes www.assignmentpoint.com

Observer/ Publish – Subscribe / Delegation Even Model Context/Problem: Different kinds of subscriber objects are interested in the state changes or events of a publisher object, and want to react in their own unique way when the publisher generates an event. Moreover, the publisher wants to maintain low coupling to the subscribers. What to do? Solution: Define a “subscriber” or “listener” interface. Subscribers implement this interface. The publisher can dynamically register subscribers who are interested in an event, and notify them when an event occurs. www.assignmentpoint.com

Registration Desk Publish Subscribe and consume Registration www.assignmentpoint.com

Who is the observer, listener, subscriber and publisher www.assignmentpoint.com

Registration Desk Publish Subscribe and consume www.assignmentpoint.com

Assignment Design and implement one of more use cases so that you use the following patterns Singleton Factory Strategy Publish - Subscribe www.assignmentpoint.com