CSC/ECE 517 Fall 2011/ch4 4h lp
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/>