Flat Preloader Icon

Solid Principles Java

  • The SOLID principles are a set of five object-oriented design principles that help developers create more maintainable, flexible, and scalable software.
  • These principles were introduced by Robert C. Martin (also known as Uncle Bob) and have become a fundamental part of modern software development practices.

Each letter in “SOLID” represents one of the five principles, which are as follows:

  • Single Responsibility Principle (SRP)
  • Open-Closed Principle (OCP)
  • Liskov Substitution Principle (LSP)
  • Interface Segregation Principle (ISP)
  • Dependency Inversion Principle (DIP)

Single Responsibility Principle (SRP)

  • The Single Responsibility Principle (SRP) is one of the five principles of the SOLID design principles for object-oriented software development.
  • It states that a class should have only one reason to change, which means that a class should have only one job or responsibility.
  • In other words, a class should have only one single purpose or responsibility.

Applying the Single Responsibility Principle leads to several benefits, including:

  • Increased Cohesion: A class that has a single responsibility tends to be more focused and has higher cohesion, making it easier to understand and maintain.
  • Reduced Coupling: When a class has only one responsibility, it’s less likely to be tightly coupled with other classes, resulting in a more modular and flexible codebase.
  • Improved Testability: Classes adhering to the SRP are often easier to test since they have a clear and focused purpose, making it easier to create isolated unit tests.

Certainly! Below is an example program in Java that demonstrates the Single Responsibility Principle (SRP) by creating separate classes for different responsibilities:

					// UserService class responsible 
for user-related operations
class UserService {
    public void createUser(String username) {
        System.out.println("User " + username +
        " created successfully");
        // Add logic for creating a user in the system

    public void deleteUser(String username) {
        System.out.println("User " + username +
        " deleted successfully");
        // Add logic for deleting a user from the system

// UserLoggingService class responsible for
user logging operations
class UserLoggingService {
    public void logUserCreation(String username) {
        System.out.println("User " + username + 
        " created at " + System.currentTimeMillis());
        // Add logic for logging user creation

    public void logUserDeletion(String username) {
        System.out.println("User " + username +
        " deleted at " + System.currentTimeMillis());
        // Add logic for logging user deletion

// Main class to demonstrate the Single 
Responsibility Principle
public class SRPExample {
    public static void main(String[] args) {
        UserService userService = new UserService();

        UserLoggingService userLoggingService
        = new UserLoggingService();

  • To comply with the Single Responsibility Principle, it’s essential to identify the responsibilities within a system and design classes that have a clear and focused purpose, encapsulating one aspect of the overall functionality. This principle encourages creating smaller, focused, and reusable classes, resulting in a more maintainable and understandable codebase.

Open-Closed Principle

  • The Open-Closed Principle (OCP) is one of the SOLID principles of object-oriented programming. It states that software entities, such as classes, modules, functions, etc., should be open for extension but closed for modification.
  • This principle encourages the design of software components that can be easily extended to add new functionality without modifying the existing codebase.
  • In practical terms, the Open-Closed Principle promotes the use of inheritance, interfaces, and other abstractions to allow the behavior of a module to be extended without modifying its source code.

Applying the Open-Closed Principle offers several advantages, including:

  • Increased Stability: The existing codebase remains unchanged, reducing the risk of introducing bugs or unintended consequences during modifications.
  • Ease of Maintenance: New features can be added without altering the existing code, making it easier to maintain and extend the software.
  • Code Reusability: By creating reusable components, developers can build on existing functionality to add new features, promoting code reusability and reducing redundant code.

Certainly! Below is an example program in Java that demonstrates the Open-Closed Principle by using inheritance and abstraction to allow for easy extension without modification:

					// Abstract class Shape following the 
Open-Closed Principle
abstract class Shape {
    public abstract void draw();

// Rectangle class extending Shape
class Rectangle extends Shape {
    public void draw() {
        System.out.println("Drawing a rectangle");

// Circle class extending Shape
class Circle extends Shape {
    public void draw() {
        System.out.println("Drawing a circle");

// Drawing class utilizing the Shape classes
class Drawing {
    public void drawShape(Shape shape) {

// Main class to demonstrate the Open-Closed Principle
public class OCPExample {
    public static void main(String[] args) {
        Drawing drawing = new Drawing();
        Shape rectangle = new Rectangle();
        Shape circle = new Circle();


  • To follow the Open-Closed Principle, it’s crucial to design modules that can be easily extended through inheritance, polymorphism, or composition, without requiring changes to their existing implementation. This approach facilitates building software that is scalable, adaptable, and more resilient to changes and new requirements.

Liskov Substitution Principle

  • The Liskov Substitution Principle (LSP) is a fundamental principle in object-oriented programming introduced by Barbara Liskov in 1987.
  • It states that objects of a superclass should be replaceable with objects of its subclasses without affecting the functionality of the program.
  • In other words, a derived class must be substitutable for its base class without altering the desirable properties of the program.

In order to adhere to the Liskov Substitution Principle, the following rules should be considered:

  • Behavior Preservation: Subclasses should preserve the behavior of the superclass, and they should be able to respond to any method calls of the superclass without altering the expected outcomes.
  • Inheritance Usage: Inheritance should only be used when the subclass is a true subtype of the superclass, and the subclass is expected to be used wherever the superclass is used.

Certainly! Below is an example program in Java that demonstrates the Liskov Substitution Principle by ensuring that the derived class can be substituted for its base class without affecting the program’s functionality:

					// Superclass representing a Bird
class Bird {
    public void fly() {
        System.out.println("Bird is flying");

// Subclass representing a Ostrich, which is a bird 
that cannot fly
class Ostrich extends Bird {
    public void fly() {
        throw new UnsupportedOperationException
        ("Ostrich cannot fly");

// Subclass representing a Sparrow, which is a bird 
that can fly
class Sparrow extends Bird {
    // Sparrow inherits the fly method from 
    the Bird class

// BirdWatcher class using the Bird class
class BirdWatcher {
    public void watchBird(Bird bird) {

// Main class to demonstrate the Liskov 
Substitution Principle
public class LSPExample {
    public static void main(String[] args) {
        BirdWatcher birdWatcher = new BirdWatcher();

        Bird sparrow = new Sparrow();
        Bird ostrich = new Ostrich();


  • By following the Liskov Substitution Principle, developers can create a more reliable and flexible codebase, ensuring that subclasses can be used interchangeably with their base classes. This principle is closely related to the concept of polymorphism in object-oriented programming.

Interface Segregation Principle

  • The Interface Segregation Principle (ISP) is one of the SOLID principles of object-oriented programming. It emphasizes that clients should not be forced to implement interfaces they do not use.
  • This principle encourages the creation of specific, focused interfaces that are tailored to the needs of the clients, rather than having a single large interface that encompasses all possible behaviors.
  • In practical terms, the Interface Segregation Principle advises the segregation of interfaces into smaller, more specific ones that are relevant to the implementing classes.

Key considerations when applying the Interface Segregation Principle include:

  • Creating fine-grained, client-specific interfaces that contain only the necessary methods for each client.
  • Avoiding the use of general-purpose interfaces that force clients to implement methods they do not need.
  • Promoting the use of multiple, focused interfaces to cater to the specific requirements of different clients.

Certainly! Below is an example program in Java that demonstrates the Interface Segregation Principle by using specific interfaces tailored to the needs of different clients:

					// Interface for performing fly behavior
interface Flyable {
    void fly();

// Interface for performing swim behavior
interface Swimmable {
    void swim();

// Duck class implementing the Flyable and Swimmable
class Duck implements Flyable, Swimmable {
    public void fly() {
        System.out.println("Duck is flying");

    public void swim() {
        System.out.println("Duck is swimming");

// Penguin class implementing the Swimmable interface
class Penguin implements Swimmable {
    public void swim() {
        System.out.println("Penguin is swimming");

// Main class to demonstrate the Interface 
Segregation Principle
public class ISPExample {
    public static void main(String[] args) {
        Duck duck = new Duck();

        Penguin penguin = new Penguin();

  • By adhering to the Interface Segregation Principle, developers can create more robust and maintainable software systems with interfaces that are specific, focused, and tailored to the precise needs of the implementing classes.

Dependency Inversion Principle

  • The Dependency Inversion Principle (DIP) is one of the SOLID principles of object-oriented programming. It states that high-level modules should not depend on low-level modules, but both should depend on abstractions.
  • Abstractions should not depend on details, but details should depend on abstractions. This principle encourages the use of abstractions to decouple higher-level and lower-level modules within the system.

By adhering to the Dependency Inversion Principle, the following goals can be achieved:

  • Reduced Coupling: High-level modules are not directly dependent on low-level modules, leading to reduced coupling between different components of the system.
  • Increased Flexibility: Abstractions allow for the introduction of new implementations without affecting the existing high-level modules, providing more flexibility in the system’s design.
  • Easier Testing and Maintenance: Dependencies on abstractions make it easier to isolate and test different parts of the system, leading to improved maintainability and easier debugging.

Certainly! Below is an example program in Java that demonstrates the Dependency Inversion Principle by using abstractions to decouple high-level and low-level modules:

					// High-level module representing a consumer
class Consumer {
    private Service service;

    // Constructor injection
    public Consumer(Service service) {
        this.service = service;

    public void doSomething() {

// Low-level module representing a service
interface Service {
    void execute();

// Concrete implementation of the Service interface
class MyService implements Service {
    public void execute() {
        System.out.println("MyService is executing");

// Main class to demonstrate the Dependency 
Inversion Principle
public class DIPExample {
    public static void main(String[] args) {
        Service service = new MyService(); 
        // Low-level module
        Consumer consumer = new Consumer(service); 
        // High-level module

  • To follow the Dependency Inversion Principle, it is crucial to design the system with a focus on defining clear and stable abstractions that can serve as a bridge between high-level and low-level modules. This approach enables the system to be more adaptable to changes and allows for the seamless introduction of new features or components

Why should we use SOLID principles?

  • Maintainability: By promoting clean and organized code, the SOLID principles make it easier to understand, update, and modify the software over time, reducing the likelihood of introducing bugs or unexpected behavior.
  • Scalability: Following the SOLID principles results in a more modular and flexible codebase, allowing for the addition of new features or components without impacting the existing functionality. This scalability is crucial for accommodating changing business requirements and future enhancements.
  • Testability: The SOLID principles encourage the creation of code that is easier to test, enabling developers to build comprehensive test suites that validate the behavior of individual components. This facilitates the identification of issues early in the development process.
  • Reduced Coupling: By minimizing dependencies and ensuring that components are loosely coupled, the SOLID principles help prevent changes in one part of the system from causing widespread effects throughout the codebase. This reduces the risk of unintended side effects and simplifies the debugging process.
  • Flexibility: Adhering to the SOLID principles enables the creation of software that can adapt to evolving requirements and technological advancements. This flexibility is crucial for ensuring that the software remains relevant and functional in the face of changing user needs and industry trends.
  • Code Reusability: The SOLID principles emphasize the development of reusable components, allowing developers to leverage existing code across different parts of the application. This reusability minimizes redundancy and promotes the efficient use of resources.