SOLID Principles: Introducing Software Architecture & Design

Understand the single responsibility principle by analyzing a UserService that saves to the database and sends a welcome email, illustrating how one class can violate SRP.

Apply the single responsibility principle by implementing separate email service and user service classes, using constructor-based dependency injection with a repository, saving a user, and sending a welcome email.

Explore the open-closed principle by examining a notification service that must be extended for new channels (email, sms, WhatsApp, Instagram) without modifying existing logic, highlighting design flaws.

Explore abstraction through interfaces and polymorphism to extend email, SMS, and WhatsApp notifications without modifying existing code, following the open-closed principle with a shared notification center interface.

Explore applying the open-closed principle in a real-time notification app by using interfaces, a factory, and dependency injection to extend email, SMS, and WhatsApp delivery at runtime.

Explore the Liskov substitution principle in inheritance, illustrating how a superclass can be replaced by its subclasses without breaking the application, using list, ArrayList, LinkedList, and a payment processor example.

Introduce interface-based design to decouple concrete classes; implement card payment processor and cash payment processor, and explore the Liskov substitution principle.

Implement a flexible payment module with credit card, UPI, and wallets using a base payment service. Enable Liskov substitution and replaceable implementations in the order flow.

explore the interface segregation principle by splitting large interfaces into smaller, usage-specific ones, so clients avoid implementing unused methods, as illustrated by a report interface for excel and pdf.

Apply the interface segregation principle by segregating interfaces for Excel and PDF reports, then implement a real-time Spring Boot order management system with customer, admin, and delivery agent roles.

Apply the interface segregation principle to a real-time order management app by splitting into single-responsibility interfaces for placing, canceling, refunding, and updating delivery status, enabling loosely coupled, testable services.

Explore dependency injection in Spring by learning setter, constructor, and field injection, and how autowired annotation enables runtime, automatic wiring for loosely coupled, immutable components.

Explore how qualifier and primary annotations resolve dependency injection ambiguity by selecting between debit and credit card implementations, enabling a default or explicit bean choice in a Spring app.

Explore inversion of control and IoC container, learn how it creates Spring beans, injects dependencies, manages lifecycle from instantiation to destruction, and reads configurations from xml, Java config, and annotations.

Demonstrate the dependency inversion principle, showing how high-level modules depend on abstractions and dependent interfaces rather than concrete classes to avoid tight coupling in a payment context.

Implement dependency inversion by defining a public IPaymentService with a pay(double amount) method and injecting it into a payment context via constructor or setter injection for loose coupling.

Explore dependency inversion principle by building a Springboard real-time app that injects a notification service into an order service, and exposes a REST endpoint to place orders.

Explore the five solid principles—single responsibility, open-closed, Liskov substitution, interface segregation, and dependency inversion—applied to object oriented programming with concrete examples like email and notification services.