SOLID is a mnemonic acronym introduced by Michael Feathers for five of the principles named by Robert C. Martin, and when applied together, these intend to make it more likely that a programmer will create a system that is easy to maintain and extend over time.

1. (++) Single responsibility principle (SRP)

   • ​Every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility. 

2. Open Close Principle (OCP)

   • Open for extension, closed for modification. Our code should be open to change (or extension) and closed for modification to accomodate that change.

3. (++) Liskov substitution principle

   • ​​Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

4. Interface segregation principle

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

5. Dependency inversion principle (DIP)

   • Depend upon abstractions. Do not depend upon concretions. Spring's frameworks popular dependency injection feature is based on this principle.

Note: Latest version of this note can be found @ http://codingarchitect.com/introduction-to-the-solid-principles-of-design.