In this comprehensive guide to SOLID design principle, we talk about the five fundamental principles to transform your software development skills to ensure that your code is efficient, flexible and scalable.
In the world of software development, creating code that not only works but is also maintainable, flexible, and scalable is a constant pursuit. As software systems grow in complexity, maintaining them becomes increasingly challenging. This is where SOLID design principles come to the rescue.
SOLID is an acronym that represents a set of five fundamental principles of object-oriented programming and design. When applied effectively, these principles can significantly improve the quality of your code and make it easier to manage, extend, and modify. SOLID is not just a catchy term, it’s a guide to writing software that stands the test of time.
This comprehensive guide will delve deep into the world of SOLID design principles. We’ll explore what each principle entails, why they matter, and how they can be applied in real-world software development scenarios. Whether you are a seasoned developer looking to reinforce your design skills or a newcomer eager to grasp the essentials of writing clean and maintainable code, this blog is for you.
Before we dive in, let’s briefly introduce the five SOLID principles:
Each of these principles serves as a building block for creating robust and maintainable software, and understanding them is key to becoming a proficient software developer.
The Single Responsibility Principle (SRP) is the first of the SOLID design principles. It states that a class should have only one reason to change, meaning it should have a single responsibility. In other words, a class should encapsulate a single, well-defined piece of functionality. This principle helps in making your code more maintainable, understandable, and easier to extend.
When a class has multiple responsibilities, it becomes tightly coupled to different parts of your application. This makes it more challenging to maintain and test. If one responsibility changes, it can affect the entire class, potentially introducing bugs or requiring changes in unrelated areas. By adhering to SRP, you create classes that are more modular and less prone to unintended consequences.
Let’s explore an example to demonstrate SRP in action. We’ll create a simple payroll system that calculates employee salaries and generates payroll reports.
In the above example, the PayrollSystem class handles both salary calculation and report generation. This violates SRP because it combines two distinct responsibilities.
To adhere to SRP, we’ll separate the responsibilities into two classes:
SalaryCalculator for salary calculation and PayrollReporter for report generation.
In the refactored code:
By adhering to SRP, this code is more modular and easier to maintain. Changes to salary calculation logic won’t impact report generation, and vice-versa. It also allows for better testing and future extensibility of the payroll system.
The Open-Closed Principle (OCP) is the second principle in the SOLID design principles. It states that software entities (such as classes, modules, and functions) should be open for extension but closed for modification. In other words, you should be able to add new functionality to a system without altering its existing source code.
OCP promotes a design that is both flexible and maintainable. When you adhere to this principle, you can introduce new features or make changes to your software without the risk of introducing bugs or affecting existing, working code. It encourages modular and reusable code, making your software more robust and adaptable.
Let's illustrate OCP with a code example involving shapes and their area calculation
The above code has the following issues:
In essence, the original code violates the OCP because it's not designed to accommodate future changes or additions to the system without modifying existing classes. It lacks the flexibility and extensibility needed in a well-designed software system. Refactoring the code to adhere to OCP, as demonstrated in the refactored example, addresses these issues and provides a more maintainable and extensible solution.
Refactored Code (Adhering to OCP)
To adhere to the Open-Closed Principle, we'll introduce an interface for area calculation (AreaCalculatable) and a separate class (AreaCalculator) responsible for calculating areas. This allows us to add new shapes without modifying existing code.
In the refactored code:
With this design, adding a new shape (e.g. ‘Triangle’) is as simple as creating a new class that implements the ‘AreaCalculatable’ interface, without needing to modify the ‘AreaCalculator’ or existing shape classes. This demonstrates how adhering to the Open-Closed Principle leads to a more extensible and maintainable codebase.
According to LSP, the objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.
In simple terms, if a class B is a subclass of class A, you should be able to use objects of class B wherever you use objects of class A without causing issues.
LSP promotes the idea of substitutability and inheritance. When you adhere to this principle, you ensure that derived classes don’t violate the expected behavior of the base class, leading to a more maintainable and predictable code. Violations of LSP can result in unexpected and erroneous behavior in your code.
Let’s understand this principle with an example
The above code violates LSP and here’s why
To adhere to the Liskov Substitution Principle, you should ensure that derived classes do not modify the behavior of base class methods in a way that violates client expectations. If a method is declared in the base class with a certain behavior, derived classes should maintain or refine that behavior but not change it incompatibly.
Refactored Code (Adhering to LSP)
First we will convert the class Animal to abstract class.
Then we will create 2 more abstract classes. One for herbivorous and another for carnivorous and each of these classes will extend from abstract class A
Now we will create individual animal classes such as Horse and Lion which will extend from Herbivore or Carnivore abstract classes depending on their eating habits.
I'm a Lion and I'm eating Meat!!!
I'm a Horse and I'm eating Grass.
Here you can see that the code is more readable and maintainable. So if we want to add more animals like the Giraffe, Leopard, and Tiger then we can simply extend them from the abstract classes Carnivore or Herbivore depending on their eating habits without any issues.
The Interface Segregation Principle (ISP) is one of the SOLID principles of object-oriented design. It suggests that a class should not be forced to implement interfaces it doesn't use. In other words, clients should not be compelled to depend on interfaces they do not need. This principle aims to prevent classes from being overloaded with unnecessary methods and promotes the creation of lean, focused interfaces.
The ISP helps to keep interfaces small, focused, and cohesive, which leads to better maintainability and flexibility in your software. When you adhere to this principle, changes to one part of your codebase won't affect unrelated parts. It also promotes better code organization and easier testing.
Let's explore the ISP using a code example involving a DocumentProcessor that handles various document types (e.g., Word documents and PDFs). Initially, we'll create a single, bloated interface, and then refactor it to adhere to the ISP.
Original Code (Violating ISP)
In the initial code, we have a single Document interface with methods for editing, saving, and printing documents. However, not all document types support these operations.
In this code:
Refactored Code (Adhering to ISP)
To adhere to the Interface Segregation Principle, we'll split the Document interface into smaller, more focused interfaces tailored to each document type.
In the refactored code,
This design adheres to the Interface Segregation Principle by allowing each document type to implement only the methods that are appropriate for its behavior. Clients can now depend on specific interfaces they need, reducing unnecessary coupling and promoting a more modular and maintainable codebase.
The Dependency Inversion Principle (DIP) is one of the SOLID principles of object-oriented design. It states that high-level modules should not depend on low-level modules; both should depend on abstractions. In other words, software modules (such as classes or components) should rely on abstract interfaces or classes rather than concrete implementations.
Additionally, abstractions should not depend on details; details should depend on abstractions.
DIP promotes flexibility, extensibility, and easier maintenance in software systems. By adhering to this principle, you can change the behavior of a component without modifying its high-level modules. It also encourages the use of interfaces and abstractions, making it easier to replace or extend components.
Let's explore the DIP using a code example involving a messaging system. Initially, we'll create a system that violates DIP by directly depending on concrete messaging services. Then, we'll refactor it to adhere to DIP by introducing abstractions and dependency injection.
In the initial code, we have a MessagingSystem class that sends messages using concrete messaging services (EmailService and SMSService). This violates DIP because the high-level MessagingSystem depends on specific low-level implementations.
In this code
To adhere to the Dependency Inversion Principle, we'll refactor the code by introducing abstractions (MessageService interface) and using dependency injection to provide service implementations.
In the refactored code:
By adhering to DIP, we decouple high-level modules from low-level implementations, making it easier to change or extend the messaging system. New message services can be added without modifying the MessagingSystem class, promoting flexibility and maintainability.
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