SOLID Principles Introduction

1Why Does SOLID Matter?

Hi friends, in this article we will learn about SOLID principles in object-oriented programming. SOLID is a collection of five design principles that help us build maintainable, flexible and understandable applications. These principles guide us in organising classes and responsibilities properly. They are useful in enterprise applications, APIs, microservices and large software projects where requirements change regularly. Following SOLID makes applications easier to test, extend and maintain without affecting unrelated functionality.

2A Banking Application Story

Imagine we are developing a banking application. Customers can transfer money, check balances, download statements and receive notifications.

Initially, one large component handles all these responsibilities. It manages customer information, calculates transactions, saves records and sends messages.

As the application grows, the component becomes difficult to maintain. A small change in notifications can affect transaction processing. Adding another payment method also requires changes in several places.

This is where SOLID principles help. We divide responsibilities, depend on abstractions and keep different parts independent. Each component gets a clear purpose, making the banking application easier to extend.

3Meet the Five SOLID Principles

1. Single Responsibility Principle

A class should have only one responsibility and one reason to change. For example, transaction processing and email notifications should not be handled by the same class. Keeping responsibilities separate makes changes easier and reduces unwanted dependencies.

2. Open/Closed Principle

Software entities should be open for extension but closed for modification. This means we should be able to introduce new functionality without repeatedly changing existing, tested functionality. For example, adding a new payment option should not require rewriting the complete payment processing component.

3. Liskov Substitution Principle

Objects of a parent type should be replaceable with objects of its child types without breaking application behaviour. In simple terms, a child type must correctly follow the expectations of its parent type. This helps us create reliable inheritance relationships.

4. Interface Segregation Principle

Clients should not be forced to depend on interfaces they do not use. An interface is a contract that defines available operations. Instead of creating one large interface containing every banking operation, we should create smaller, focused interfaces.

5. Dependency Inversion Principle

High-level business logic should not directly depend on low-level implementation details. Both should depend on abstractions. For example, a transaction service should depend on a payment contract rather than being tightly connected to one particular payment provider.

4Follow the SOLID Design Flow

Let's see how these principles work together when designing a banking transaction application.

1. The customer starts a banking transaction through the application.
2. The transaction service receives the request and handles the business responsibility.
3. The service uses a payment contract to select the required payment implementation.
4. The payment implementation processes the transaction and stores the transaction record.
5. The notification component independently informs the customer about the result.

Each component has a focused responsibility, and the design allows new implementations without tightly coupling the complete application.

5What Does Good Design Achieve?

Clear responsibilities
Flexible payment implementations
Independent notification service
Easier testing and maintenance

6Watch the Components Interact

7Keep These Principles in Mind

SOLID principles help us design software that can grow without becoming difficult to maintain.

They encourage clear responsibilities, flexible implementations and loosely coupled components. They are particularly valuable when multiple developers work on large applications.

Remember that SOLID is a design guideline, not a strict checklist. Apply the principles based on actual application requirements.

• Give every class a clear responsibility.
• Prefer flexible and loosely coupled designs.
• Depend on abstractions rather than concrete implementations.

8Questions Interviewers Love to Ask