OO principals

1. Single Responsibility Principle

http://www.oodesign.com/single-responsibility-principle.html

2. Open/Closed Principle

open for extension, but closed for modification

http://joelabrahamsson.com/a-simple-example-of-the-openclosed-principle/

3. Liskov's Substitution Principle(LSP)

http://www.oodesign.com/liskov-s-substitution-principle.html

4. Interface Segregation Principle (ISP)

http://www.oodesign.com/interface-segregation-principle.html

5.Dependency Inversion Principle

http://www.oodesign.com/dependency-inversion-principle.html