# Solid Design * `s`: Single Responsibility Principle * Each class should have only one sole purpose, and not be filled with excessive functionality * ex : loan class, there can be multiple loans, car, personal, home. * ex : send otp class, there can be multiple ways to send otp, email, sms, whatsapp. * `o`: Open & Close Principle * Classes should be open for extension, closed for modification. * In other words, you should not have to rewrite an existing class for implementing new features. * ex: using interfaces, can can implement create new class to implement new features. * `l`: Leskov's Subsititution Principle * This means that every subclass or derived class should be substitutable for their base or parent class. * ex: we have 3 social media: facebook, instagram & whatsapp * whatsapp doesnt support features like publish posts * there is no calling on instagram * we create interface that contain common functionality, have our classes extend from them * for specific features, we create new interfaces for them * `i`: Interfae Segragation Principle * Interfaces should not force classes to implement what they can’t do. * Large interfaces should be divided into small ones. * `d`: Dependency Inversion Principle * The principle states that we must use abstraction (abstract classes and interfaces) instead of concrete implementation. * High level modules should not depend on the low level module but both should depend on the abstraction. * ex: when perform payment, it doesnt matter whether you visa or mastercard, debit or credit card, payment provider will take it. * instead of taking input a specific class, take a interface which can be implemented by classes For examples with code :