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Design Patterns at a Glance

Design Patterns	Singleton	Adapter	Command	Strategy
Purpose	To ensure a class has only one instance, and also to provide a global point of access to it	To convert the interface of a class into another interface the clients expects	To encapsulate method invocations so as to decouple caller from the implementation details	To perform a bunch of different things to do, based on the situation/context
Advantages	Sane usage of global namespace by avoiding unnecessary global variables and providing on-demand (lazy) instantiations.	It lets classes work together that could not otherwise because of incompatible interfaces.	It allows one to decouple the requester of an action from the object that actually performs the action.	It lets the algorithms vary independently from clients that use those.
Disadvantages	Introduces global state into system and complicates unit testing.	Adapter has to implement the entire target interface. Selective functionality implementation is not an option.	Having Command objects specific to each action ends up cluttering the design, especially in the context of MVC architectures <ref>[1]</ref>	Increases the number of objects and all algorithms use the same interface. <ref> [www.cs.toronto.edu/~arnold/407/.../stateStrategy/state_strat_pres.ppt] </ref>