Object-Oriented Programming (OOP) in Java: Concepts and Principles.

Object-Oriented Programming (OOP) in Java Concepts and Principles.

As a fundamental concept in software development, object-oriented programming (OOP) offers a strong and well-organized framework for structuring and writing code. Object-oriented programming (OOP) is a programming paradigm centered around the concept of objects – encapsulated entities consisting of data and behaviors represented by fields and methods. OOP aims to increase code reuse and flexibility by creating custom object types matching problem domain concepts.

One of the key ideas in OOP is that it fundamentally changes the way developers design and build software systems, creating a foundation that easily fits the needs and complexity of contemporary programming. Taking a closer look at the core ideas behind object-oriented programming (OOP), this introduction tries to show how important OOP has been in influencing software development practices for a number of different programming languages, including Java.

Key OOP Pillars:

  • Encapsulation
  • Abstraction
  • Inheritance
  • Polymorphism

We will explore how these pillars are implemented in Java, the most popular OOP language. Mastering OOP techniques is critical for effectively designing Java applications, frameworks and APIs.

Encapsulation in Java

Encapsulation involves bundling data states and associated behaviors within a single entity called class. This unifies all attributes and actions related to an object conceptually. In Java, classes encapsulate fields to store internal object state and methods carrying out behaviors related to the object. Access modifiers control exposure of members:

public class Addition {

  // Field encapsulating state

  private double Ans;  

  // Method encapsulating behavior

  public void add(double x) {

    ans += x; 



Benefits of encapsulation:

  • Centralizes object states and behaviors
  • Prevents invalid direct state manipulation
  • Allows changing implementation internals independently

By exposing only selective public methods vs full internal state, objects have more control over how their data gets accessed. This concept is called data hiding. Encapsulation enables changing class internals more safely and flexibly.

Abstraction in Java

Abstraction focuses on exposing only essential details of an entity while hiding internals and complexity – much like real-world objects. This simplified interface helps client interactions without worrying about implementation intricacies.

In Java, abstraction is achieved using

  1. Abstract Classes
  2. Interfaces

Abstract classes contain both concrete and abstract methods – incomplete methods declaring only a signature and return type for subclasses to provide actual implementations. Hire Java Developers to leverage the variables and defined methods encapsulate shared data and utilities derived classes utilize:

public abstract class Shape {

  private String color;

  // Concrete method

  public void setColor(String color) {

    this.color = color;


  // Abstract method signature

  public abstract double area();


public class Circle extends Shape {

  // Inherited fields and methods 

  public double area() {

    return Math.PI * radius * radius;



Interfaces solely contain method signatures that implementing classes must define themselves. This enforces a contract what operations classes can perform without dictating details of the implementation:

public interface Resizable {

    void resize(int width, int height); 


public class Image implements Resizable {

    public void resize(int width, int height) {

      // custom resize implementation



So while abstract classes provide concrete scaffolding shareable via inheritance, interfaces focus exclusively on abstraction through method signatures for diverse implementations.

Inheritance in Java

Inheritance enables new classes to be defined by extending capabilities from existing classes. This promotes reuse allowing customized specializations without reinventing the wheel.

In Java, classes use the `extends` keyword to inherit fields and methods from a base superclass, gaining its capabilities in addition to any new features:

public class Vehicle {

  private int maxSpeed;

  public int getMaxSpeed() {

    return this.maxSpeed;



public class Car extends Vehicle {

  private String model;

  public String getModel() {

    return this.model;



Now Car inherits the maxSpeed field and associated getter from Vehicle in addition to defining its own model field and getter. Inheritance chains build a taxonomy – a generalized parent class with increasing specialization in subclasses.

Java doesn’t allows multiple inheritance, however interfaces help fill that gap. Inheritance should focus on `is-a` relationships modeling specialization matching problem domain concepts.

Polymorphism in Java

Polymorphism lets a variable, field or method handle different data types interchangeably enabling cleaner, more extensible code focused on abstract interfaces over concrete types.

In Java, polymorphism applies via:

  1. Method Overriding
  2. Interfaces

Method overriding occurs when subclasses redefine a method they inherited from a parent class to provide specialized implementations:

public class Shape {

  public void render() {

    // Generic shape render method



public class Circle extends Shape {


  public void render() {

   // Custom circle rendering logic



Now code can simply invoke `render()` on any `Shape` knowing the correct specialized form gets executed without needing awareness of actual runtime type:

Shape shape = new Circle();

shape.render(); // Calls Circle’s render()

This simplifies programming thinking abstractly about behaviors uniformly despite differing underlying forms.

Interfaces also enable polymorphism in Java by letting different classes expose common method signatures the interface defines. This allows the interface type to handle all compatible implementations interchangeably:

interface Renderable {

    void render();


class Image implements Renderable {

  public void render() { /* renders image */ } 


class Circle implements Renderable {

  public void render() { /* renders circle */}


void renderScene(Renderable[] items) {

  for(Renderable item : items) {

     item.render(); //Calls correct render() method



Here `renderScene()` accepts any `Renderable` without concern for actual runtime type, simplifying generalization of behaviors across types.

Java OOP Principles & Best Practices

Beyond core concepts, following principles like DRY, YAGNI, encapsulating variability, favoring composition over inheritance, single responsibility, open/closed, Liskov substitution and depending on abstractions improves OOP code quality and maintainability:

  • DRY Principle – Don’t Repeat Yourself – Factor duplicate logic into shared utilities.
  • YAGNI Principle – You Aren’t Gonna Need It – Build for current needs rather than speculative complexity.
  • Encapsulate What Varies – Isolate parts that change frequently behind stable interfaces.
  • Composition Over Inheritance – Favor object composition rather than deep inheritance hierarchies for reuse.
  • Single Responsibility – Classes should have one purpose only with high cohesion between members.
  • Open/Closed Principle – Classes should be open for any extension but closed for modification through polymorphism.
  • Liskov Substitution – Child classes should not break parent class expectations around interfaces and invariants.
  • Depend on Abstractions – Code against abstract interfaces rather than concrete types for flexibility.
  • Inversion of Control – Decouple objects by injecting dependencies rather than having them create directly.

These patterns increase quality attributes like loose coupling, high cohesion, and separation of concerns. Applying principles judiciously balances tradeoffs solving domains problems elegantly.


Object-oriented programming (OOP) organizes state and behaviors into classes, employing principles like inheritance and polymorphism for efficient system design. Java leverages OOP with features like access modifiers and abstract classes, ideal for constructing adaptable programs and APIs. Best practices recommend encapsulation and composition over inheritance. Hire dedicated developers well-versed in OOP is crucial, ensuring sustained quality attributes and seamless adaptation as systems evolve, aligning technical solutions with domain concepts for improved communication with stakeholders in the ever-growing complexity of software development.

I am a full-time professional blogger from India. I like reading various tech magazines and several other blogs on the internet.

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