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What Are OOP Concepts in Java? How They Work and More

By: Stackify
  |  February 28, 2024
What Are OOP Concepts in Java? How They Work and More

OOP is one of the most fundamental programming concepts. Let’s explore the four main OOP concepts in Java and discuss how each works.

Java is one of many programming languages and technologies supported by Netreo’s leading tools, Retrace and Prefix. At Netreo, we aim to help developers become better. Let’s take a look at some of the foundational Java programming language concepts with a primer on OOP concepts in Java.

Object Oriented Programming (OOP) is a programming paradigm that focuses on the use of objects to represent and manipulate data

What is Object Oriented Programming (OOP)?

Object Oriented Programming (OOP) is a programming paradigm that focuses on the use of objects to represent and manipulate data. In OOP, data is encapsulated within objects, and objects are defined by their properties (attributes) and behaviors (methods). OOP provides several key concepts that enable developers to write modular, reusable, and maintainable code.

Definition of OOP Concepts in Java

The main ideas behind Java’s Object-Oriented Programming, OOP concepts include abstraction, encapsulation, inheritance and polymorphism. Basically, Java OOP concepts let us create working methods and variables, then re-use all or part of them without compromising security. Grasping OOP concepts is key to understanding how Java works.

Java defines OOP concepts as follows:

  • Abstraction. Using simple things to represent complexity. We all know how to turn the TV on, but we don’t need to know how it works in order to enjoy it. In Java, abstraction means simple things like objects, classes and variables represent more complex underlying code and data. This is important because it lets you avoid repeating the same work multiple times.
  • Encapsulation. The practice of keeping fields within a class private, then providing access to those fields via public methods. Encapsulation is a protective barrier that keeps the data and code safe within the class itself. We can then reuse objects like code components or variables without allowing open access to the data system-wide.
  • Inheritance. A special feature of Object-Oriented Programming in Java, Inheritance lets programmers create new classes that share some of the attributes of existing classes. Using Inheritance lets us build on previous work without reinventing the wheel.
  • Polymorphism. Allows programmers to use the same word in Java to mean different things in different contexts. One form of polymorphism is method overloading. That’s when the code itself implies different meanings. The other form is method overriding. That’s when the values of the supplied variables imply different meanings. Let’s delve a little further.

How OOP Concepts in Java Work

OOP concepts in Java work by letting programmers create components that are reusable in different ways while maintaining security.

How Abstraction Works

Abstraction lets programmers create useful and reusable tools. It enables programmers to create complex systems by breaking them down into smaller, more manageable components. For example, a programmer can create several different types of objects, which can be variables, functions or data structures. Programmers can also create different classes of objects as ways to define the objects.

For instance, a class of variable might be an address. The class might specify that each address object shall have a name, street, city and zip code. The objects, in this case, might be employee addresses, customer addresses or supplier addresses. In addition, abstraction provides a mechanism for hiding the implementation details of a class or method from the outside world and providing a simplified interface for clients to interact with. In Java, you can achieve abstraction through two main mechanisms: abstract classes and interfaces.

  1. Abstract Classes: An abstract class is a class that you can’t instantiate and can only extend by subclasses. Abstract classes can have both abstract and non-abstract methods. Abstract methods do not have a body and you must implement them by any subclass that extends the abstract class. Non-abstract methods have a body and you can directly call them by the subclass.
  2. Interfaces: An interface is a collection of methods. You can use it to define a set of behaviors that a class should implement. A class can implement multiple interfaces, and all the methods defined in an interface must be implemented by any class that implements it.
Encapsulation lets us reuse functionality without jeopardizing security. It’s a powerful, time-saving OOP concept in Java.

How Encapsulation Works

Encapsulation lets us reuse functionality without jeopardizing security. It’s a powerful, time-saving OOP concept in Java. For example, we may create a piece of code that calls specific data from a database. It may be useful to reuse that code with other databases or processes. Encapsulation lets us do that while keeping our original data private. It also lets us alter our original code without breaking it for others who have adopted it in the meantime.

Encapsulation provides several benefits, including:

  1. Data hiding: By hiding the implementation details of a class, encapsulation protects the data from unauthorized access and manipulation.
  2. Modularity: Encapsulation helps to break down complex systems into smaller, more manageable components, making the codebase more modular and easier to maintain.
  3. Flexibility: By providing a controlled interface for interacting with a class, encapsulation allows for changes to the internal implementation without affecting the external interface.

Access Modifiers

In Java, encapsulation is implemented using access modifiers, which control the visibility of variables and methods within a class.

The three access modifiers in Java are:

  1. Public: Public variables and methods can be accessed from anywhere, including outside the class.
  2. Private: Private variables and methods can only be accessed within the class they are defined in.
  3. Protected: Protected variables and methods can be accessed within the same class and its subclasses.

Encapsulation enables developers to write cleaner, more organized, and more secure code. By controlling access to variables and methods, encapsulation promotes good software design practices and helps to manage the complexity of large-scale projects.

How Inheritance Works

Inheritance is another labor-saving Java OOP concept that works by letting a new class adopt the properties of another. We call the inheriting class a subclass or a child class. The original class is often called the parent or the superclass. We use the keyword extends to define a new class that inherits properties from an old class.

The subclass inherits all the public and protected variables and methods of the superclass, and it can also define its own variables and methods. This makes it possible to create a hierarchy of classes, where each subclass inherits from its superclass and adds its own unique features.

Benefits of Inheritance

Inheritance provides several benefits, including:

  1. Reusability: By inheriting from a superclass, a subclass can reuse the code and functionality already defined in the superclass, making it easier to write and maintain code.
  2. Polymorphism: Inheritance allows for polymorphism, where objects of different subclasses can be treated as objects of the same superclass, making it easier to write generic code.
  3. Flexibility: Inheritance provides a way to add new features to an existing class hierarchy without modifying the existing code.

Inheritance allows developers to create complex class hierarchies with shared functionality and unique features. By promoting code reuse, polymorphism, and flexibility, inheritance enables developers to write more efficient and maintainable code.

How Polymorphism Works

Polymorphism in Java works by using a reference to a parent class to affect an object in the child class. We might create a class called “horse” by extending the “animal” class. That class might also implement the “professional racing” class. The “horse” class is “polymorphic,” since it inherits attributes of both the “animal” and “professional racing” class.

Two more examples of polymorphism in Java are method overriding and method overloading.

In method overriding, the child class can use the OOP polymorphism concept to override a method of its parent class. That allows a programmer to use one method in different ways depending on whether it’s invoked by an object of the parent class or an object of the child class.

In method overloading, a single method may perform different functions depending on the context in which it’s called. This means a single method name might work in different ways depending on what arguments are passed to it.

Benefits of Polymorphism

Polymorphism provides several benefits, including:

  1. Flexibility: Polymorphism allows for more flexible and adaptable code by enabling objects of different classes to be treated as if they are of the same class.
  2. Code reuse: Polymorphism promotes code reuse by allowing classes to inherit functionality from other classes and to share common methods and properties.
  3. Simplification: Polymorphism simplifies code by enabling the use of generic code that can handle different types of objects.

Polymorphism allows for more flexible and adaptable code. By enabling objects of different classes to be treated as if they are of the same class, polymorphism promotes code reuse, simplification, and flexibility, making it an essential component of Object-Oriented Programming.

Examples of OOP Concepts in Java

Now that we explained the foundational OOP concepts in Java, let’s look at a few common examples.

Short Encapsulation Example in Java

In the example below, encapsulation is demonstrated as an OOP concept in Java. Here, the variable “name” is kept private or “encapsulated.”

//save as
package com.javatpoint;
public class Student {
 private String name;
 public String getName() {
  return name;
 public void setName(String name) { = name
//save as
package com.javatpoint;
class Test {
 public static void main(String[] args) {
  Student s = new Student();
Compile By: javac -d .
Run By: java com.javatpoint.Test

Output: vijay

Example of Inheritance in Java

It’s quite simple to achieve inheritance as an OOP concept in Java. Inheritance can be as easy as using the extends keyword:

class Mammal {

class Aardvark extends Mammal {


For a full tutorial on the different ways to use inheritance in java, see this blog post.

Short Example of Polymorphism in Java

In the example below of polymorphism as an OOP concept in Java, we have two classes: Person and Employee. The Employee class inherits from the Person class by using the keyword extends. Here, the child class overrides the parent class. For the full example, see this blog post.

class Person {
 void walk() {
  System.out.println(“Can Run….”);
class Employee extends Person {
 void walk() {
  System.out.println(“Running Fast…”);
 public static void main(String arg[]) {
  Person p = new Employee(); //upcasting

Differences between OOP and other programming styles

Object-Oriented Programming (OOP) has become widely popular due to its many advantages over other programming styles such as Procedural Programming and Functional Programming.

Procedural Programming

Procedural Programming is a programming style that is based on a set of procedures or functions, where each function is a sequence of instructions that performs a specific task. It focuses on the sequence of must-follow steps that to accomplish a specific task. In contrast, OOP focuses on the objects and their interactions to solve problems.

Functional Programming

Functional Programming is a programming style that focuses on the use of functions that produce output based on their input, without modifying any external state. It is based on mathematical functions and is characterized by immutability and statelessness. In contrast, OOP is based on objects and their states, and it is designed to manage complex, stateful systems.

Benefits of OOP over other programming

Here are some key differences between OOP and other programming styles:

  1. Data and behavior: OOP is based on the idea of encapsulating data and behavior within objects, whereas procedural programming separates data and behavior into different functions or procedures. Functional programming, on the other hand, treats data and behavior as separate entities altogether.
  2. Inheritance and code reuse: OOP uses inheritance to reuse code and build complex systems. Procedural programming and functional programming do not have inheritance concepts built into them.
  3. Flexibility: OOP is more flexible than procedural programming because it allows for changes to be made to the underlying data structures and objects without changing the entire system. In contrast, procedural programming requires a complete restructuring of the program if any changes are made.

OOP enables encapsulation, inheritance, code reusability, and flexibility, making it a powerful tool for building complex, stateful systems.

Best Practices for OOP Concepts in Java

The goal of OOP concepts in Java is to save time without sacrificing security and ease of use. The following best practices are all oriented toward advancing that main goal.

  • DRY (Don’t Repeat Yourself). A core concept in Java, DRY simply means you should never have two blocks of identical code in two different places. Instead, have one method you use for different applications.
  • If you expect your Java code to change in the future, encapsulate it by making all variables and methods private at the outset. As the code changes, increase access to “protected” as needed, but not too public.
  • Single Responsibility. This best practice principle for OOP concepts in Java states that a class should always have only one functionality. That way, the class can be called and/or extended on its own when new uses arise for it, without causing coupling between different functionalities.
  • Open Closed Design. Make all methods and classes Closed for modification but Open for an extension. That way, tried and tested code can remain static but can be modified to perform new tasks as needed.

For a good, full list of best practices for OOP concepts in Java, see this blog post. You can also check out our article about OOP concepts in C#.

These concepts and best practices are only as great as the developers that implement them. To make your work better, you need productivity tools to improve your Java programming.

Try Netreo’s Prefix and Retrace to help you write and maintain the best code ever!

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