SOLID principles Object Oriented Programming

Single Responsibility Principle

- Every class should have a single responsibility.

- There should never be more than one reason for a class to change.

- Your classes should be small. No more than a screen full of code.

- Avoid 'god' classes.

- Split big classes into smaller classes.

Open Closed Principle

- Your classes should be open for extension.

- But closed for modification.

- You should be able to extend a classes behavior, without modifying it.

- Use private variables with getters and setters only when you need them.

- Use abstract base classes.

Liskov Substitution Principle

- Objects in a program would be replaceable with instances of their subtypes without altering the correctness of the program.

- Violations will often fail the "is a" test.

- A square "is a" rectangle.

- However, a rectangle "is not" a square.

Interface Segregation Principle

- Make fine grained interfaces that are client specifc.

- Many client specific interfaces are better than one "general purpose" interface.

- Keep your components focused and minimize dependencies between them.

- Notice relationship to the Single Responsibility Principle?

- Avoid 'god' interfaces.

Dependency Inversion Principle

- Abstractions should not depend upon details.

- Details should depend upon abstractions.

- Important than higher level and lower level objects depend on the same abstract interaction.

- This is not the same as Dependency Injection - which is how objects obtain dependent objects.