Law of Demeter
What is the Law of Demeter
- It is a design guideline for developing software
- It is a technique to achieve loose coupling
- It's considered a good design technique, often applied in OOP
- A.K.A principle of least knowledge
Guidelines
- Each object should have only limited knowledge about others
- Each object should only talk to its friends, don't talk to strangers
- Only talk to your immediate friends
Not compliant
Compliant (single dot)
Not compliant (single dot)
Don't lie to yourself!
Not compliant (single dot)
Don't lie to yourself!
Compliant (multiple dots)
- Account only talks to Plan
Example
- Lets suppose we have to refactor the domain model
Users might not have a profile, and the plan belongs to the profile
Example
- And we have code like this...
Example
- We will have to make changes in multiple places
If we would have followed Demeter
- We only need to make a change in one class (The User class)
So the model would look like this
And the services don't need to change
Requirements
- O itself
- m’s parameters
- any objects created/instantiated within m
- O’s direct component objects
- A global variable, accessible by O, in the scope of m
A method m of an object O may only invoke the methods of the following kinds of objects
Summary
We don’t want our functions to know about an entire object graph
Individual functions should have a limited amount of knowledge
We want to tell our neighbor objects what we need to have done and depend on them to propagate that message outwards to the appropriate destination
QA?
PPA - Tech huddle (Demeter law)
By David Molinero
