Definition of SOLID
A quick post on the definition of SOLID, not by me, just a reminder of what it means.
A class should only have a single responsibility, that is, only changes to one part of the software's specification should be able to affect the specification of the class.
Software entities ... should be open for extension, but closed for modification.
Liskov Substitution Principle
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
Interface Segregation Principle
Many client-specific interfaces are better than one general-purpose interface
Dependency Inversion Principle
One should depend upon abstractions, [not] concretions.