Notes from Practical Object Oriented Design in Ruby

POODR.txt

*Objects have their own behavior.
*Objects interact with each other via methods.
*Objects should know as little about each other as possible. 
*Create objects with room for change.
*You can not fully design software before writing it. 

Designing classes with a single responsibilty.
*Classes should have a single responsibilty. 
*Nouns make good classes.
*Try to descibe a class in one sentence, without using and. 
*Make sure code is transparent and readable. 
*Wrap instance variables in accessor methods so you can depend on behavior, not data.
*Methods are free, give them single responsibities so code can be changed easily. 
*If you have behavior that doesn't quite fit the current class, but doesn't quite need it's own class, isolate it for changes
can easily be made later. 

Managing dependencies.
*Keep dependencies light. 
*Don't overly chain messages. 
*Use variable to hold objects within other objects. ex. on page 41
*Isolate instance creation, maybe in an instance variable or with a method using condtional assignment. 
*Isolate external messages. 
*Use hashes for initialize. ex. on page 47
*Use a defaults method to initialize variables.This can also be removed from initialize. ex. on page 49
*Use a module to instantiate a new object with the arguments, this is a factory method.
*Think about which classes are more likely to change and consider their dependents. 
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