• Five agile principles that should guide you every time you need to write code.

1. Single Responsibility (SRP)
2. Open/Closed (OCP)
3. Liskov's Substitution
4. Interface Segregation
5. Dependency Inversion

• A class should have only single responsibility.
• A Class should have only one reason to change.
• Yes, it is quite important for a class to have no more than one task.

• Software entities should be open for extension, but closed for modification.

Software entities: classes, modules, functions, etc.

• Write your code so that you will be able to add new functionality without changing the existing code.

• It uses interfaces instead of superclasses.

• Store any subclass reference into a base class variable and the behavior won’t change which is the goal of LSP.

• Child class objects should be able to replace parent class objects without compromising application integrity.

• What this means essentially, is that we should put an effort to create such derived class objects which can replace objects of the base class without modifying its behavior.

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

• If S is a subtype of T, then objects of type T may be replaced with objects of type S (in other words, objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, and so on).

• The interface-segregation principle states that no client should be forced to depend on methods it does not use.
• Many client-specific interfaces are better than one general-purpose interface.

• One should depend upon abstractions, not concretions.

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

• Both should depend on abstractions.

• Abstractions should not depend on details. * Details should depend on abstractions.

• So High-level and low-level modules And Details depends on Abstractions

High-level + Low-level modules + Details: Abstractions

• High-level modules: describe those operations in our application that has more abstract nature and contain more complex logic. These modules orchestrate low-level modules in our application.

• Low-level modules: contain more specific individual components focusing on details and smaller parts of the application. These modules are used inside the high-level modules in our app.