Choose design patterns

 As you learn more and more design patterns it becomes increasingly difficult to understand which one to use in a given scenario. 

Below are some of the observations:

CoR Vs Decorator:

CoR (Chain of Responsibility): To be used when there is a sequence of activities to be performed in a given sequence. These activities and their sequence can change based on a given business scenario. Each activity is a done by a separate class in the chain and we can add new classes as and when they are discovered. 

Decorator: Initially I was quite confused between CoR and Decorator as both of them provided classes that can be added to a base object and these can be changed at runtime. I later realized that the difference between the two is that while Decorator is designed to change the behavior of the base object, CoR is designed to perform different activities with each class. Which means that if the given functionality has to be enhanced based on conditions then decorators should be used, and if different functionalities have to be triggered in a given sequence CoR should be used. 

Summary: Decorators are additive in nature and every decorator handles every message. This is the basic behavior of the pattern. Hence when a client wishes to enhance one or more additional behaviors decorator is used. Decorator would exhibit at least a minimum of two behaviors.

Chain of Responsibility will exhibit one behavior or less. The potential for multiple links to handle one message is possible but not the intention. Hence ideally the cardinality is zero to one so when a CoR pattern is being used the message should be processed by one chain.

Strategy Vs Visitor

Strategy: Involves a class objects being induced into a method as a parameter and this contains an algorithm to do a certain process in a certain way. By parameterizing the algorithm the functionality can be enhanced later to accommodate new algorithms. 

Visitor: Visitor involves the current object being passed as an argument to the visitor class method and later invoking a specific method in the visitor class which can then access the current objects and perform any desired operation. 

So while strategy is used to change the behaviour, visitor is used to add new behavior