Practical Object-Oriented Design: An Agile Primer Using Ruby

by Metz Sandi

Ask the class questions about its purpose to identify if it has a single responsiblity. Single responsibility principal should also apply to the methods within the class.

recognising dependencies in an object it knows the name of another class it knows the name of a message to be passed to another class other than itself it knows the arguments for another objects initializer/constructor it knows the positional arguments for another classes’ constructor

Some classes are more likely than others to have changes in requirements. Concrete classes are more likely to change than abstract classes. Changing a class that has many dependents will result in widespread consequences.

Design experts notice domain objects without concentrating on them; they focus not on these objects but on the messages that pass between them.

Demeter is often paraphrased as “only talk to your immediate neighbors” or “use only one dot.”

Instead of evaluating this phrase by counting the “dots,” evaluate it by checking the types of the intermediate objects.

If you remove the dots from this line of code, your costs may well go up instead of down.