Chapter 26

Applying Gang of Four Design Patterns

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The “Gang of Four”

• The book was Design Patterns by Gamma, Helm, Johnson, and Vlissades

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Pattern Overload

• As of 2000, there were over 500 published design patterns. Therefore, we’ll also be looking at how the following patterns are related to simpler ones and at underlying principles.

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Adapter

• 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.

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Adapter related to:

• Façade

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Factory

• Problem: Who should be responsible for creating objects when there are special considerations, such as complex creation logic, a desire to separate the creation responsibilities for better cohesion, and so forth?

• Solution: Create a Pure Fabrication object called a Factory that handles the creation.

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Factory

• Maintain a separation of concerns in your programs. That is, modularize or separate concepts so that each has high cohesion

• Using a domain object to create adapters does not support high cohesion

• Hides complex logic and may allow caching or other memory-use efficiencies

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Factory

• For example, a Factory can create a TaxAdapter based upon knowledge of which tax module is being used. The program just calls the Factory’s method and gets an Adapter.

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Singleton

• 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.

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Singleton

• The Factory pattern raises the issue of who creates it and how it is accessed

• We need only one instance of the Factory, whose methods are called from various places in the code

• One solution is to pass the Factory object as a parameter, but this can be messy

• In the UML class diagram, use a “1” in the upper right to indicate creation of 1 instance

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Singleton

• Singleton has a getInstance method that is static and returns an instance of itself if one does not exist.

• Wrap this in concurrency control• Reasons not to make all methods static:

cannot subclass; remote methods interface doesn’t support static; class may not always be a singleton in all contexts

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Strategy

• 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.

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Strategy

• In the POS system, pricing can be complex and rule-based. For example, there may be a tax-free day, a senior citizen discount, $10 off if the sale is over $200, etc.

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Strategy

• Create multiple SalePricingStrategy classes with a polymorphic getTotal method.

• This takes a Sale object and applies the discounting rule for its class.

• A Strategy object is attached to a context object, to which it applies its algorithm

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Strategy

• Since pricing strategies change over time, use a PricingStrategyFactory to create them.

• It can read a parameter from an external source and create the appropriate class.

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Composite

• Problem: How to treat a group or composition structure of objects the same way (polymorphically) as a non-composite (atomic) object?

• Solution: Define classes for composite and atomic objects so that they implement the same interface.

• The outer composite object contains a list of inner objects, and both inner and outer implement the same interface

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Composite

• Pricing rules can be complex and conflicting. For example, on Monday this is in effect:

• 20% senior discount policy• preferred customer discount of 15% off sales

over $400• on Monday, there is $50 off purchases over

$500• buy 1 case of Darjeeling tea, get 15% discount

off of everything

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Composite

• Suppose a senior buys $600 worth of veggieburgers and a case of Darjeeling tea

• Store must have a conflict resolution strategy, usually “Best for the customer.”

• Composite class: CompositeBestForCustomerPricingStrategy implements iSalesPricingStrategy

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Composite

• Attach any object that implements ISalePricingStrategy that has a getTotal method to the Sale object and it won’t know the difference

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Composite

• How to create the strategies?• Design has a composite with the basic pricing

strategy. Then:– Store-defined discount, perhaps initially 0– Customer type discount– Product type discount– Two for one– Any others?

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GRASP and Composite

• IDs to objects• Pass aggregate object as parameter

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Façade• 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 subsystem may change. What to do?

• Solution: Define a single point of contact to the subsystem: a Facade object that wraps the subsystem. This facade object presents a single unified interface and is responsible for collaborating with the subsystem components.

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Façade

• This is similar to Adapter. It provides a single interface to what may be disparate subsystems. This provides a single point of entry

• Pluggable business rules, for example• Gives you a way to do low coupling

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Observer (Publish-Subscribe)• 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. Publisher wants to maintain low coupling to the subscribers.

• Solution: Define a "subscriber" or "listener" interface which Subscribers implement. The publisher can dynamically register subscribers who are interested in an event and notify them when an event occurs.

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Observer

• For example, when the total amount of a sale changes, the total displayed on the screen should change. You could just send a message from the domain object to the UI object

• This would violate Model-View separation

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Observer

• An interface is defined; in this case, PropertyListener with the operation onPropertyEvent.

• Define the window to implement the interface. SaleFrame1 will implement the method onPropertyEvent.

• When the SaleFrame1 window is initialized, pass it the Sale instance from which it is displaying the total.

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Observer

• The SaleFrame1 window registers or subscribes to the Sale instance for notification of "property events,“ via the addPropertyListener message

• Note that the Sale does not know about SaleFrame1 objects; rather, it only knows about objects that implement the PropertyListener interface. This lowers the coupling of the Sale to the window. The coupling is only to an interface, not to a GUI class.

• The Sale instance is thus a publisher of "property events." When the total changes, it iterates across all subscribing PropertyListeners, notifying each.

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