SOLID Programming Principles

Acronym describing clean coding principles for OOP languages, coined by Uncle Bob Martin.

Definition

• S - Single Responsibility
  • A class and/or method should have a single responsibility, and shouldn’t do more than 1 thing
• O - Open/Closed
  • The architecture should be “Open” for enhancement through adding additional code, but “Closed” to modifying the existing code
  • Realistically, it’s encouragement to do your best to design the solution in such a way that through inheritance or composition you can leave your code open to further evolution without having to modify existing code
• L - Liskov Substitution
  • When a class inherits from another class, the overall program shouldn’t break; and it shouldn’t require hacky conditionals to work
  • Rather than passing additional arguments to methods or classes, can additional subclasses, interfaces, or updating constructors help?
• I - Interface Segregation
  • Make interfaces or parent classes more specific, rather than generic
  • Code smell for this one is if your abstract class or interface starts to have a bunch of methods that account for odd edge cases, the question should be: “Should I break this out into a couple of interfaces / classes, instead of one monster one?”
• D - Dependency Inversion
  • Classes should depend on abstractions, not on concrete classes
    • E.g., A class depends on (meaning it has it passed in as a constructor parameter or something similar) a specific concrete class, but instead should depend on an interface or abstract class

Additional References

• Uncle Bob’s SOLID Principles Made Easy using Python

Other Related Topics

• Development
• Software Design Principles