SOLID principles

SOLID is a OOP design principles proposed by Robert C. Martin in his paper Design Principles and Design Patterns. (2000)

Single responsibility principle (SRP)

A class should handle one and only one job.
i.e. a class should have one and only one reason to change.

Open-Closed principle (OCP)

Objects or entities should be open for extension, but closed for modification.

Liskov substitution principle (LSP)

Every subclass should be substitutable for their superclass.
Formally, if $q(x)$ is a property provable about objects of $x$ of type $T$, then $q(y)$ should be provable for objects $y$ of type $S$ where $S$ is a subtype of $T$.

Interface segregation principle (ISP)

Clients shouldn't be forced to depend on methods they do not use.
i.e. clients should never be forced to implement an interface that it doesn't use.

Dependency inversion principle (DIP)

Entities must depend on abstractions (e.g. interface) not on concretions (e.g. class).

How do I do this???

Solution: split your code! (into interfaces or classes)