CSC/ECE 517 Fall 2011/ch4 4h lp: Difference between revisions
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===Singleton=== | ===Singleton=== | ||
[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. | [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. | ||
====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> ==== | ====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> ==== | ||
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 ? | |||
For an object to be considered as a singleton object, they must satisfy three requirements: | For an object to be considered as a singleton object, they must satisfy three requirements: | ||
#controls concurrent access to a shared resource. | #controls concurrent access to a shared resource. | ||
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#there can be only one object. | #there can be only one object. | ||
==== | ====Applications==== | ||
====Advantages==== | ====Advantages==== | ||
*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. | *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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*Introduces a global state into the application. | *Introduces a global state into the application. | ||
*Can make unit testing classes in isolation difficult. | *Can make unit testing classes in isolation difficult. | ||
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> | |||
===Adapter=== | ===Adapter=== | ||
Revision as of 02:51, 21 October 2011
Introduction
This wiki article discusses about some of the commonly used & easy to understand Desgin patterns[1] in the software industry. Specifically, we will be studying about the Singleton, Adapter, Command & Strategy patterns.
Design Pattern
Definition
Examples
Case Study
Singleton
Singleton is a design pattern which imposes a restriction on the class to instantiate exactly one object of that class.
When should we use Singleton ? <ref>When should we use Singleton ?</ref>
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 ? For an object to be considered as a singleton object, they must satisfy three requirements:
- controls concurrent access to a shared resource.
- access to the resource will be requested from multiple, disparate parts of the system.
- there can be only one object.
Applications
Advantages
- Singletons are often preferred to global variables as they don't pollute the global namespace with unnecessary variables & also can be instantiated only when needed unlike global variables which always consume resources.
Drawbacks
- Introduces a global state into the application.
- Can make unit testing classes in isolation difficult.
Checkout the this video from Misko Hevery which explains in detail when the usage of singletons is not exercised.<ref>Why Singletons are bad</ref>
Adapter
Command
Strategy
Conclusion
References
<references/>