SOLID principle

 S: S stands for single responsibility principle.Each class is responsible for single responsibility.

 O: Class shall be open for extension but closed for modification.

 L: L stands for Liskov substitution.This means that objects of superclass shall be replaced with objects        of  subclasses without breaking the system.

 I: I stands for Interface segregation Principle. This means that Interface shall not have unnecessary             methods which need to be implemented by client forcefully. So if there are methods which are not         always required by clients so better would be to keep them in separate interface and the clients which         want those can override those methods by implementing different interface.

   

D: D stands for dependency inversion principle.It states that class shall depend on abstract                            reference(abstract class or interface) rather than concrete implementation(class) . Here , inversion      refers to the dependent classes on which another class depends. In simple terms we shall use abstract          classes or interface as dependency to one class not concrete classes.

     For eg

      Class Car{

      Engine engine// This Engine either has to be abstract class or interface.

         }