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| ==Introduction==
| | This page has been moved to [http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2011/ch4_4h_kp http://expertiza.csc.ncsu.edu/wiki/index.php/CSC/ECE_517_Fall_2011/ch4_4h_kp] |
| This wiki article discusses about some of the commonly used & easy to understand Desgin patterns[http://en.wikipedia.org/wiki/Design_pattern_(computer_science)] in the software industry. Specifically, we will be studying about the Singleton, Adapter, Command & Strategy patterns. | |
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| ==Design Pattern==
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| ===Definition===
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| ===Examples===
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| ==Case Study==
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| ===Singleton===
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| [http://en.wikipedia.org/wiki/Singleton_pattern Singleton] is a design pattern which imposes a restriction on the class to instantiate exactly one object of that class. | |
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| ====Implementation====
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| *In Java, We can create a [http://en.wikipedia.org/wiki/Thread-safe thread-safe] & [http://en.wikipedia.org/wiki/Lazy_initialization lazy version] of Singleton as follows. The comments in the code help you understand the Lazy & thread-safe aspect of it.
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| <nowiki>
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| public class Singleton {
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| private static Singleton instance;
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|
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| private Singleton() {
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| //Do nothing. Initialize the object only when the first time getInstance() is called.
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| }
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|
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| public static synchronized Singleton getInstance() {
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| //The keyword "synchronized" makes it thread safe so that two threads invoking getInstance()
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| //at the same time cannot create two instances when a context switch happens so as to
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| //facilitate this scenario. In general this can happen even with 'n' threads invoking getInstance().
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| //"synchronized" keyword is used to eliminate such scenarios.
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| if (null == instance) {
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| instance = new Singleton();
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| }
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| return instance;
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| }
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| }
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| </nowiki>
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| *In Ruby, we can achieve singleton classes by mixing-in the 'Singleton' module as follows. Here, "SingletonClass" is the name of the singleton class that is being created.
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| <nowiki>
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| require 'singleton'
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| class SingletonClass
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| include Singleton
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| ...
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| end
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| </nowiki>
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| ====When should we use Singleton ? <ref>[http://stackoverflow.com/questions/228164/on-design-patterns-when-to-use-the-singleton When should we use Singleton ?]</ref> ====
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| There is a lot of criticism to the use of singleton pattern. So, how do we decide whether an object should be singleton or not ?
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| For an object to be considered as a singleton object, they must satisfy three requirements:
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| #controls concurrent access to a shared resource.
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| #access to the resource will be requested from multiple, disparate parts of the system.
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| #there can be only one object.
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| ====Applications====
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| ====Advantages====
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| *Singletons are often preferred to global variables as they don't pollute the global [http://en.wikipedia.org/wiki/Namespace namespace] with unnecessary variables & also can be instantiated only when needed unlike global variables which always consume resources.
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| ====Drawbacks====
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| *Introduces a global state into the application.
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| *Can make unit testing classes in isolation difficult.
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| Checkout the [http://www.youtube.com/watch?v=-FRm3VPhseI this] video from Misko Hevery which explains in detail when the usage of singletons is not exercised.<ref>[http://www.youtube.com/watch?v=-FRm3VPhseI Why Singletons are bad]</ref>
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| ===Adapter===
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| ===Command===
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| ===Strategy===
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| ==Conclusion==
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| ==References==
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| <references/>
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| ==External Links==
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