JavaWhat is Object-oriented Programming in Java?

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Object-oriented programming (OOP) is a powerful programming paradigm and feature known for allowing developers to create code that is highly organized, reusable, and maintainable. Java, as a programming language with OOP features, relies on the fundamental principles of object-oriented programming, which has helped it become one of the most widely used coding languages in the world. In this programming tutorial, we highlight the main concepts and principles of OOP in Java. We take a deep dive into its underlying principles and features, and discuss the best practices and design patterns that programmers can use to enhance their code.

What are the Principles of Object-oriented Programming in Java?

Below, we will discuss the key principles of Object-oriented programming in Java, including the following features:

Encapsulation in Java

Encapsulation is one of the main principles of Object-oriented programming. Encapsulation encourages “data hiding” and makes the internal state of an object accessible only through interfaces. In the Java programming language, developers use encapsulation by creating access modifiers including public, private, and protected.

You can learn more about modifiers and access modifiers in our tutorial, Java Modifiers.

Below is a code example demonstrating the use of encapsulation in Java; the encapsulation, in this example, is implemented within a class that represents a bank account:

public class myBankAccount {
	private double accountBalance;	

 	public void deposit(double amount) {
     	accountBalance += amount;

 	public void withdraw(double amount) {
     	if (amount <= accountBalance) {
         	accountBalance -= amount;
     	} else {
             System.out.println("You do not have sufficient funds");

In the code example above, the variable accountBalance has been encapsulated by declaring it as private. The deposit and withdraw methods provide a sort of controlled access that lets us modify the accountBalance while still maintaining the integrity of our object.

You can learn more about encapsulation in our tutorial: Java Encapsulation Overview.

Inheritance in Java

In Java, inheritance uses the extends keyword as a way to enable new classes to utilize existing classes. This encourages the reuse of code and facilitates a structured hierarchy to code. Programmers can leverage the features of an existing class, add to its capabilities, and customize the existing class to meet specific requirements. Using this approach encourages programmers to create more modular and well-organized code. It also reduces the time and work needed to recreate commonly used functions in our codebases.

Below is a common example of inheritance in Java, in which we create a Car subclass that inherits the data and functionality from an existing superclass called Vehicle:

public class Vehicle {
 	protected String brand;

 	public void horn() {
     	System.out.println("Meep, meep!");
public class Car extends Vehicle {
 	private String model;

 	public Car(String brand, String model) {
     	this.brand = brand;
     	this.model = model;
	public void vroom() {
     	System.out.println("You are driving a " + brand + " " + model + “. Vroom, vroom!”);

In our code example above, our Car class inherits the brand field and horn() method from the Vehicle superclass. In addition, it creates its own model field and vroom() method.

You can learn more about inheritance in Java by reading our tutorial: An Overview of Java Inheritance.

Polymorphism in Java

Polymorphism is another one of the key concepts of Object-oriented Java. It ensures developers treat objects of different types as if they were objects of a shared superclass. Polymorphism provides flexibility and extensibility to our codebases. With Java, polymorphism is created using method overriding and method overloading.

Below is a code example demonstrating polymorphism in Java. In the code snippet, we create an abstract class named Shapes and define the subclasses Circle and Rectangle:

public abstract class Shapes {
 	public abstract double calculatingArea();
public class Circle extends Shape {
 	private double radius;

 	public Circle(double radius) {
     	this.radius = radius;
 	public double calculatingArea() {
     	return Math.PI * radius * radius;
public class Rectangle extends Shape {
 	private double width;
 	private double height;

 	public Rectangle(double width, double height) {
     	this.width = width;
     	this.height = height;
 	public double calculatingArea() {
     	return width * height;

Here, the Circle and Rectangle classes inherit the calculatingArea method from the Shape abstract class. Each of our subclasses has its own implementation of the method, which lets each shape calculate their own areas polymorphically.

You can learn more about the OOP concept of polymorphism by reading our tutorial: Polymorphism in Java.

Abstraction in Java

Abstraction is the process of creating abstract classes and interfaces in order to define common behaviors for derived classes and create a contract for them as well. With Java, abstract classes cannot be instantiated. For Java interfaces, developers can define a set of methods implementing classes have to adhere to.

Below is a code example demonstrating the concept of abstraction in Java. The code uses an interface called Instrument and its implementation in a class that we will call Bass:

public interface Instrument {
 	void play();
public class Bass implements Instrument {

 	public void play() {
         System.out.println("Slapping the bass.");

In our code snippet above, the Instrument interface defines a method called play(). The Bass class implements the interface and creates its own implementation of the play() method.

You can learn more about abstraction in our tutorial: What is Abstraction in Java?

You can learn more about interfaces in our tutorial: What are Interfaces in Java?

What are the Key Concepts of OOP in Java?

Java offers the following key concepts designed to support the Object-oriented features of Java:

  • Classes and Objects
  • Constructors and Destructors
  • Access Modifiers
  • Inheritance
  • Polymorphism

We discuss each of these in more detail below:

  • Classes and Objects: A big part of Java’s design is based on the concept of classes and objects. A class can be thought of as a blueprint from which objects are created. Objects, for their part, are the implementations of a class. Defining classes and creating objects from those classes is the primary way a developer can not only organize data, but manipulate it as well.
  • Constructors and Destructors: Constructors act as special methods that initialize objects within memory management. Constructors share the same name as the class and are invoked when an object is created. Destructors, unlike their counterparts constructors, are not required to have explicit definitions. Instead, Java’s garbage collector automatically reclaims memory that is not used for background processes.
  • Access Modifiers: Access modifiers are used to regulate both the visibility and accessibility of classes, fields, and methods. Access modifiers include public, private, and protected. These properties ensure encapsulation in our classes and objects. Packages, meanwhile, group related classes, manage namespaces, and control access.
  • Inheritance: Java supports the concept of single inheritance, in which a class inherits from a single superclass, versus multiple inheritance, which Java does not directly support, although similar functionality can be achieved using interfaces. Using the extends keyword, subclasses can inherit the fields and methods of a superclass. Method overriding is a technique developers can use to allow subclasses to provide their own implementation of methods they inherit from a class.
  • Polymorphism and Interfaces: Polymorphism makes it so that objects can be treated as objects of a superclass. Interfaces define contracts that classes can implement, mimicking the concept of multiple inheritance of behavior, which Java does not natively support. Java interfaces allow programmers to implement polymorphic behavior.

Best Practices of Object-oriented Programming

To properly implement Object-orientation in our Java programs, there are a set of “best practices” developers can follow to make their code “clean”, easier to maintain, and reusable. Here are a few of the best practices for OOP in Java:

  • Encapsulate Data: Encapsulating data using access modifiers and using getters and setters to control access to classes and objects helps promote data integrity and reduces dependencies.
  • Composition instead of Inheritance: When possible, developers should use composition over inheritance. This practice fosters loosely coupled code, code reusability, and enhanced flexibility. Developers should use caution when working with inheritance in order to avoid tight coupling and allow for code to be reusable.
  • Design Patterns: Design patterns are ways to code that provide solutions to common programming issues and challenges. Java has several popular design patterns, including Singleton, Factory, and Observer. Developers can use design patterns for enhanced code structure and better code maintainability.

Other Object-oriented Programming Features in Java

Despite the fact that Java is not a true OOP language as it uses both primitive and non-primitive data types, Java does offer advanced features that enhance its Object-oriented features, including:

  • Exception Handling: Java has its own set of built-in exception handling features that allow developers to handle exceptions and recover from them as well. Using try-catch blocks and creating custom exception classes lets programmers create more robust code that handles errors with more grace.
  • Generics and Collections: Generics are used to create type-safe collections and classes. With generic classes, methods, and wildcards, coders are able to write reusable code that works with a variety of data types. Java collections, which include List, Set, and Map, provide data structures for the storage and manipulation of collections of objects and data.
  • Multithreading: Multithreading allows developers to create code that executes multiple threads concurrently. Using the Thread class, synchronized blocks, and the java.util.concurrent package, programmers can create concurrent programs that take advantage of parallel processing in order to improve application performance.

Final Thoughts on Objected-oriented Programming in Java

Object-Oriented Programming (OOP) principles are a core feature of Java’s design. OOP features let developers write modular, reusable, and maintainable code. Understanding the OOP principles of encapsulation, inheritance, polymorphism, and abstraction, lets coders create powerful and flexible applications. Java’s OOP features of classes, objects, constructors, access modifiers, and interfaces make up the building blocks of Java’s Object-orientation.

Using Object-oriented programming best practices, which include encapsulation, composition before inheritance, and the use of design patterns, makes code more readable, maintainable, and scalable. In addition, using features such as exception handling, generics, and multithreading further expands a programmer’s ability to write more efficient and performant programs that run concurrently, taking advantage of modern day processing power.

Finally, implementing OOP in Java software developers, and adhering to best practices, helps developers create software that is of higher quality and code that is more readable, reusable, and maintainable.

Read: Top Java Frameworks

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