Java SOLID Principles – IQCode Tutorial

Understanding SOLID Principles in Java

In this article, we will explore the SOLID principles in Java – what they are, their importance, and how to implement them. Java is a versatile programming language that finds application in a range of domains, from web development to gaming. By understanding and implementing SOLID principles in Java, you can write code that is modular, testable, and maintainable. We will cover the following topics:

  • What is Java and its features
  • What are the SOLID Principles and their meaning
  • The benefits of using SOLID principles in Java
  • Explanation of each of the five SOLID principles:
    • Single Responsibility Principle
    • Open-Closed Principle
    • Liskov Substitution Principle
    • Interface Segregation Principle
    • Dependency Inversion Principle
  • Frequently asked questions about SOLID principles in Java

We hope this article will help you understand SOLID principles and their implementation in Java.

About Java Programming Language

Java is a widely used programming language and computing platform introduced by Sun Microsystems in 1995. It offers a secure environment to build numerous services and applications, and still employed in innovative digital products for the future. The language was initially designed to run portable devices and set-top boxes but failed under the name “OAK.” In 1995, Sun renamed it “Java,” with updated features for the growing World Wide Web industry. Java’s parent company Sun Microsystems was later acquired by Oracle Corporation in 2009, gaining ownership of Java, MySQL, and Solaris.

Applications of Java

Java is a versatile programming language used in a variety of domains:

  • Android app development
  • Mobile application development
  • Enterprise application development
  • Scientific computing
  • Web application development
  • Server-side technologies such as Apache, JBoss, GlassFish, and more.
  • Big data analytics


Java is a popular programming language known for its user-friendly platform independence. Its features include being an object-oriented language, able to be extended easily, multithreaded, and dynamically adapting to changing environments. These features make it feasible to develop programs capable of running multiple tasks simultaneously and storing a large amount of run-time data for real-time object access verification and resolution.

// Java code example
public class Example {
public static void main(String[] args) {
// program logic here

SOLID Principles in Java

SOLID principles are essential for good software design in Java. These principles were introduced by Robert C. Martin, also known as Uncle Bob. The five SOLID principles are fundamental in object-oriented programming and allow for modular, understandable, debuggable, and refactorable software.

SOLID Principles

SOLID is an acronym for five design principles in object-oriented programming:

  • S: Single Responsibility Principle
  • O: Open-Closed Principle
  • L: Liskov Substitution Principle
  • I: Interface Segregation Principle
  • D: Dependency Inversion Principle

These principles help to write code that is easy to maintain, understand and extend.


Implementing all S.O.L.I.D principles results in easily managed software development.

* CLEAN: S.O.L.I.D principles promote clean and standardized code.
* MAINTAINABLE: Applying S.O.L.I.D principles results in code that is more manageable and easier to maintain.
* SCALABLE: Promotes easy refactoring or changing of code.
* REDUNDANCY: S.O.L.I.D principles prevent redundant code.
* TESTABLE: Code is easily unit testable.
* READABLE: S.O.L.I.D principles improve code readability.
* INDEPENDENT: Code independence is achieved through reduced dependencies.
* REUSABLE: Code becomes more reusable.

SOLID Principles in Java

Here we will look at the five principles of SOLID:

S - Single Responsibility Principle

Each class should only have one responsibility or reason to change.

O - Open/Closed Principle

Classes should be open for extension but closed for modification.

L - Liskov Substitution Principle

Subtypes should be substitutable for their base types.

I - Interface Segregation Principle

Clients should not be forced to depend on interfaces they do not use.

D - Dependency Inversion Principle

Depend upon abstractions, not concretions.

By following these principles, your code can become more maintainable, extensible, and easier to read and understand.

Single Responsibility Principle

The Single Responsibility Principle states that every Java class should have only one responsibility. Combining multiple functionalities in a single class can complicate the code and make it harder to maintain.

For instance, let’s consider a class, FinalExam, that has three methods, i.e., AddQuestion(), ExpectedAnswer(), and Marksdistribution(). All these methods perform different functions. Applying the Single Responsibility Principle suggests dividing them into three separate classes.

Before implementing this principle, the FinalExam class will look like this:


public class FinalExam{
public void AddQuestion() {
//functionality of the method
public void ExpectedAnswer() {
//functionality of the method
public void Marksdistribution() {
//functionality of the method

Implementing a Single Responsibility Principle does not limit the number of methods in a class. Instead, it promotes creating separate classes, with each having one responsibility. By separating functionalities, it becomes easier to maintain and test the code.

After applying this principle to the example class, it will look like this:


public class FinalExam{
public void AddQuestion() {
//functionality of the method
public class Answer{
public void ExpectedAnswer() {
//functionality of the method
public class Marks{
public void Marksdistribution() {
//functionality of the method

Implementing the Single Responsibility Principle simplifies testing, reduces dependencies on other classes, and leads to better code organization.

Open-Closed Principle

The Open-Closed Principle states that a class must be designed to
perform its task without the need for future changes. To achieve
this, the class must remain closed to alteration, but it still
should be extendable in many ways, such as:

– Inheriting from the class.
– Overwriting the required behavior from the class.
– Extending certain behavior of the class.

For example, consider a class named StudentInfo that has a method
called StreamName() to return the name of the stream. If we want
to add a new subclass named ‘Arts’, simply adding one more if
statement violates the open-closed principle. To add this subclass
and follow the principle, we must override the StreamName() method.

The Open-Closed Principle is necessary because classes may come
from third-party libraries, and we should be able to extend them
without affecting their base classes. To avoid issues with changes
in the base class, it’s recommended to use interfaces. This provides
abstraction and loose coupling between classes.


public interface Streamable {
String getStream();

public class Science implements Streamable {
public String getStream() {
return “Science”;

public class Commerce implements Streamable {
public String getStream() {
return “Commerce”;

public class Arts implements Streamable {
public String getStream() {
return “Arts”;

public class StudentInfo {
public String getStreamName(Streamable st) {
return st.getStream();


The Liskov Substitution Principle (LSP) ensures that derived classes can replace base classes without affecting the program’s behavior. In other words, a subtype must be 100% interchangeable with its base type.

To illustrate, let’s consider an example. The Rectangle class and its Square subclass violate the LSP since the Square class has additional constraints that do not apply to Rectangle, and substituting Rectangle with Square can result in unexpected behavior.

This principle is necessary to prevent inheritance abuse and ensure adherence to the “is-a” relationship. It also requires subclasses to adhere to the contract established by their base classes, aligned with Bertrand Meyer’s Design by Contract.

Interface Segregation Principle in Object-Oriented Programming

The Interface Segregation Principle (ISP) states that a client should only implement the interface methods that it needs. ISP favors smaller and client-specific interfaces over large and monolithic ones. The principle ensures that a client is not forced to rely on methods it does not require.

For example, suppose there is an interface for a vehicle and a Bike class. The openDoors() method should not be implemented in the Bike class because a bike does not have doors. To address this, ISP suggests breaking down interfaces into small, coherent ones.

Instead of having one Vehicle interface, we can create four interfaces- Driving, Stopping, Refueling, and Opening- with specific methods. The Bike class can then implement only the required interfaces, and there would be no unnecessary code.

By implementing ISP, our code becomes more readable and maintainable. We only use the required methods, making our class implementation simpler.


The Dependency Inversion Principle (DIP) suggests relying on interfaces and abstract classes instead of real implementations. It means abstractions should be dependent on details instead of vice versa.

For instance, if a WindowMachine class has keyboard and monitor classes inside it, creating instances of both inside WindowMachine constructor would create tightly-coupled classes. Such classes become hard to test. This code can be loosened with the help of the Dependency Inversion principle.

To practice such code, WindowsMachine must be decoupled from keyboard. This can be achieved by using the Keyboard interface. The code should be loosely coupled and extendable, so the application becomes easy to test.

public interface Keyboard {

public class WindowsMachine {
private final Keyboard keyboard; 
private final Monitor monitor;
public WindowsMachine(Keyboard keyboard, Monitor monitor) {
this.keyboard = keyboard;
this.monitor = monitor;

SOLID Principles in Java

In this article, we have explored the basics of SOLID principles in Java and their advantages. SOLID is a design approach that ensures the modularity, clarity, and maintainability of your code. It comprises five principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. Following these principles aids developers and maintainers in saving time and effort in software development and maintenance. It also prevents code inflexibility and fragility, allowing you to create long-lasting software systems.

Frequently Asked Questions

Implementing SOLID Principles in Java

To implement SOLID principles in Java, one should understand the fundamental concepts of object-oriented programming (OOP) like inheritance, polymorphism, abstraction, and encapsulation. One can then utilize these principles in their Java code to improve code quality and maintainability.

Relevance of SOLID Principles

Using SOLID principles is still highly relevant as it improves code maintainability, testability, reusability, and scalability. This approach decreases dependencies, allowing modification to a block of code without affecting other blocks.

SOLID principles are essential to maintain a clean and robust architecture. These principles assist software developers in minimizing the cost of maintenance, bug fixing, and feature additions. As a result, SOLID principles are still relevant for designing and developing high-quality software systems.

Writing SOLID Code

To write SOLID code, follow these principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. By applying these principles, you can ensure that your code is maintainable, scalable, and easy to add to or modify.


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