SKP's GoF Design Patterns a Day - 11 - Facade

[GitHub Repository for Code Samples]
https://github.com/sumithpuri/skp-code-marathon-pattaya

Was going through the book ‘Head First Design Patterns’, came up with my own examples to understand them further. Try downloading the code and see if it helps you in comprehending these in a better way.

 

 

Facade Pattern [Sample Code]
Consider a scenario where we require multiple method invocations on various classes, to achieve the desired functionality. Also, consider that this set of functionality is repeatedly being used in your code. If you are thinking of an option where you will perform direct invocations, you are bound to end up with code maintenance issues and tightly coupled code. If these invocations are remote, it is going to be worse with respect to the performance.

 

Fig. 2 : Facade (Structural) Design Pattern - Class Diagram 

[Source : Wikipedia]

 

 

Under the above mentioned conditions is where the facade comes into play.  Herein multiple method invocations are encapsulated into a single method of the facade class, to achieve the desired functionality. It provides us with a single point of change and looser coupling, with respect to the individual implementations. Remote method invocation patterns like SessionFacade (EJB) adapt from here to improve the overall performance and lower complexity.  

 

Fig. 3 : Facade (Structural) Design Pattern - Sequence Diagram 

[Source : Wikipedia]

The example attached is a very simple scenario of a InvoiceManagerFacade which has addInvoice() and deleteInvoice() methods. To achieve the desired result, each of these methods encapsulates the method invocations from OrderManager, LedgerManager and BillingManager classes.

AccountsCentral is the main class. Try adding your own method to the facade class, or try plugging in a new type of facade.

 

SKP's GoF Design Patterns a Day - 10 - Decorator

[GitHub Repository for Code Samples]
https://github.com/sumithpuri/skp-code-marathon-phuket

Was going through the book ‘Head First Design Patterns’, came up with my own examples to understand them further. Try downloading the code and see if it helps you in comprehending these in a better way.

Decorator Pattern [Sample Code]
Decorator Pattern provides an elegant way to use composition for enhancing functionality, where the result expected has direct dependency on the composed and composing class. A chain relation (via composition) or decoration can be finally used to achieve the desired output at runtime. In real-time, when the functionality of one particular product is expected to be built form a base product and various other related sub-products or fixtures, we can rely on the Decorator.

 

 

Fig. 2 : Decorator (Structural) Design Pattern - Class and Sequence Diagram 

[Source : Wikipedia]

The attached example is that of a Pizza application. Here, the pizzas in the shop are made with various combinations of bases and topping combinations. This is a classical example for usage of the decorator pattern. Pizza is the abstract base class for each of the pizza bases to implement and ToppingDecorator is another abstract class that inherits from Pizza for each of the toppings to implement. Hawaiian, Italian and Mexican are the concrete implementation of Pizza whereas Mushroom, Onion and Chicken are the concrete implementations of ToppingDecorator. Each of these toppings encapsulates a Pizza instance. This instance, at runtime, will hold another topping or the pizza base instance. Finally, it is when the cost has to be calculated on the entire pizza that the real value of decorator pattern is seen and just one call suffices to calculate the entire bill value.

PizzaWorld is the main class. Try adding more decorators and pizza base classes to see if you can get a real taste of the Decorator!

SKP's GoF Design Patterns a Day - 05 - Adapter

[GitHub Repository for Code Samples]
https://github.com/sumithpuri/skp-code-marathon-pattaya

Was going through the book ‘Head First Design Patterns’, came up with my own examples to understand them further. Try downloading the code and see if it helps you in comprehending these in a better way.

Adapter Pattern [Sample Code]

What can one do if he needs to use an Asian Hairdryer in a European Country, each with different socket types? I would seek an Adapter! As in real life, when we want to plug and play with similar but incompatible interfaces we use the Adapter. The Adapter adapts the Adaptee to the desired interface, by composing the Adaptee object and inheriting the desired interface or by multiple inheritance.

 

 

Fig. 1 : Adapter (Structural) Design Pattern - Class and Sequence Diagram 

[Source : Wikipedia]

The attached example is a real world Computer scenario, where I want to plug in an external hard drive (pre-usb era!), SeagateDrive of interface type SeagateGeneric to an incompatible computer, SamsungComputer of type Computer. SeagateGeneric provides read() and write() methods for the specified purposes, which needs to be adapted to the actual bufferData(), flushData() and purgeData() methods of the Computer. Note that there is no equivalent of purgeData(). The ideal way to handle this scenario is to throw an exception, whenever this method is invoked on the hard drive as it would do in the real world. The adapter to perform the translation in this scenario is the SeagateAdapter, which implements the Computer interface. It encapsulates a SeagateGeneric instance reference, and adapts it to the Computer interface. Whenever a bufferData() method is invoked on the Computer interface, it actual requires three invocations of read() on the SeagateGeneric implementation to match up to the Computer’s standards. These kinds of translations are done by the adapte.

PCAssembler is the main class here. Try adding your own device and its adapter to the computer.