SOLID Software Design Principles

SOLID Software Design Principles helps to develop understandable, maintainable, and scalable software systems. Each letter of SOLID stands for one principle. The principles are a subset of many principles promoted by American software engineer and instructor Robert C. Martin

Design principles provide a proven solution for most common design problems and we only need to consider how we can transform our requirement to a solution with the components of these proven design principles.

“It is not enough for code to work”

Robert C. Martin

Application should be maintainable, understandable, and should be able to change with minimum effort without breaking existing functionalities.

What happens when we don’t follow design principles?

  • Code fragility – Tendency to break after changes.

  • Code rigidity – Difficult to change. Take a lot of time and effort to do a change.

Technical debt

The cost of prioritizing fast delivery over quality delivery for a long period of time

Fast delivery and Quality code are two different things that we can’t accommodate both at the same time. 

If you prefer fast delivery in the early stage of the project, it will look effective. Fast delivery reduces the quality of the code. Hence in the long run, when you receive change requests, the time and effort to do even a simple change will cost you lots of time and effort. It won’t happen if you maintain a quality code from the beginning of the project. 

Technical Debt - Cost of change over time - SOLID Software Design Principles -
Technical Debt – Cost of change over time
Technical debt - Customer responsiveness over time - SOLID Software Design Principles -
Technical debt – Customer responsiveness over time

Fast delivery leads to increase technical debt

Quality delivery won’t increase the technical debt

Benefits of SOLID Software Design Principles

  • Clean code – easy to understand

  • Can change faster with minimal risk

  • Maintainable over long period of time

  • Cost effective

Alternative Methods to Improve Code Quality

  • Continues refactoring

  • Design patterns

  • Unit testing

SOLID Software Design Principles

  • Single Responsibility Principle

  • Open Closed Principle

  • Liskov Substitute Principle

  • Interface Segregation Principle

  • Dependency Inversion Principle

Single Responsibility Principle

The single responsibility principle states that one component(class, module, method or microservice) should have only one responsibility. It also says that there should be a single reason to change the component. 

This doesn’t mean that the class should only have one method. This means that all the functions in a component should align with a single purpose. 

For example, imagine a class where there are methods to place an order for a product and do the payment. If we need to change the ordering mechanism we need to change this class. If we need to change the payment mechanism we need to change this class. Placing an order and doing a payment are two very different causes to change this class. Having multiple reasons to change a single class can increase the danger of breaking the existing code. 

By keeping these two tasks in two different components decrease the danger of breaking the existing functionalities. It made your components clean and easy to understand.

Open Closed Principle

Software entities should be open for extension but close for modification.

Modules, classes, or methods should be extendable without changing their behavior and should be able to add new features to the extended components.

By following this principle, new features can be added easily and with minimal cost. It also minimize the regression bug since it isolates the changes to a specific component.

Liskov Substitute Principle

This states that any object of a type must be substitutable by objects of a derived type without altering the correctness of that program. In other words, any implementation of an abstraction should be substitutable by any of the implementation of that particular abstraction. 

We should clearly identify what the actual hierarchy and should eliminates falls hierarchies. 

Interface Segregation Principle

An object should only depend on interfaces it requires and should not be forced to depend on methods or properties that they don’t require.

We need to identity FAT interfaces to break them into more focused lean interfaces. If an interface has lot of methods, if clients of that interface need to throw not implementing method exception or need to provide empty implementation and client forces to implement methods that are not relevant then you have a fat method that needs to be broken into pieces.

Dependency Inversion Principle

This states that high level modules should not depend on low level modules, both should depend on abstractions. Abstractions should not depend on details.

public class Gear{

public class Engine{
  Gear gear;

public class Car{
  Engine engine;

Class Car is tightly coupled with Engine class and Engine class is tightly coupled with Gear class. And this is a tightly coupled system. We can overcome this by introducing interfaces.

This principle improves loos coupling between components.


Writing great software is difficult. Even with the mature object-oriented principles, your program can be a mess. Doing so many things in a single code block, huge classes and methods, duplicate configurations are some common things that we face frequently. By using these proven solutions we can develop a great design.

Related articles :

Factory Method Design Pattern with Real-world example

OOP Design Patterns – All you need to know.

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