SOLID Principles

SOLID is acronym used for 5 design principles for object oriented programming. Below we'll look at what they are

Single Responsibility principle

Every class should have only one responsibility. This responsibility should be totally encapsulated by the class.

see wikipedia, this, OODesign, not so SOLID OO Principles.

Open/Closed principle

Software entities should be open for extension, but closed for modification. This is valuable when it comes to production environments where changes to existing code may require code reviews, unit tests etc. which ensures the quality of the product. In design, inheritance is used to achieve open/closed principle. 

see wikipedia, OODesign, Objectmentor,

Liskov Substitution principle

Derived types must be completely substitutable for their base types.

see OODesign,

Interface segregation principle 

Client should not be forced to depend upon interfaces that they don't use

Dependency Inversion principle

High-level modules should not depend on low-level modules.