Java OOP — Complete Guide

Java is a class-based, object-oriented language. Every piece of running logic lives inside a class, and every value you work with (except the 8 primitives) is an object. Understanding OOP is not optional in Java — it is the language. The sooner you build a solid mental model of objects, classes, and the four pillars, the easier every other topic becomes.

What Is OOP?

Object-Oriented Programming organises code around objects — self-contained units that bundle data (fields) and behaviour (methods). This mirrors how we think about the real world: a BankAccount knows its own balance and knows how to deposit or withdraw. A Car knows its own speed and knows how to accelerate. This bundling of state and behaviour in one place is what makes large codebases manageable — each object is responsible for itself.

The alternative — procedural code — passes data between separate functions. OOP keeps data and the functions that operate on it together, making large codebases dramatically easier to manage.

The Four Pillars

Each pillar solves a specific design problem. Encapsulation prevents accidental state corruption. Inheritance enables code reuse across related types. Polymorphism lets one piece of code work with many different types. Abstraction lets you work at a higher level without being tangled in implementation details.

Learning Path

Work through these in order. Each page builds on the previous one.

Beginner

1. Introduction to OOP in Java — classes, objects, constructors, this
2. Encapsulation — access modifiers, getters/setters, immutability
3. Inheritance — extends, super, method overriding, final
4. Polymorphism — runtime dispatch, @Override, interface polymorphism
5. Abstraction — abstract classes, interface, default methods

Intermediate

6. Interfaces — Deep Dive — segregation, default methods, functional interfaces
7. Abstract Classes — template method pattern, partial implementations
8. Method Overloading — compile-time polymorphism, varargs, autoboxing
9. Composition vs Inheritance — when to favour composition, Decorator pattern

Advanced

10. SOLID Principles — five design principles every Java engineer must know
11. Design Patterns — Singleton, Factory, Observer, Strategy, Builder
12. Generics and OOP — type-safe collections, bounded wildcards, generic methods

A Minimal First Class

Before diving in, here is the smallest complete example of a Java class to orient you. Every element here — the field, constructor, method, and getter — has a specific role that the pillar tutorials will explain in depth.

public class Dog {
    // Field — state belonging to each Dog object
    private String name;

    // Constructor — runs when you write: new Dog("Rex")
    public Dog(String name) {
        this.name = name;
    }

    // Method — behaviour the Dog can perform
    public void bark() {
        System.out.println(name + " says: Woof!");
    }

    // Getter — controlled read access to private field
    public String getName() { return name; }
}

// Create two independent objects from the same class
Dog rex  = new Dog("Rex");
Dog luna = new Dog("Luna");

rex.bark();  // Rex says: Woof!
luna.bark(); // Luna says: Woof!

Start with the Introduction to OOP in Java to understand how classes and objects work before moving to the individual pillars.