Java Classes and Objects Explained

Classes and objects are the heart of Java. A class is a blueprint, and an object is a real thing built from it. Once this clicks, the rest of Java makes far more sense. In this guide, we'll build a small bank account class step by step and see exactly how Java classes and objects work.

What is a class in Java?

A class describes what something has and what it can do. Think of an architect's drawing of a house. The drawing shows rooms, doors, and windows, but you can't live in it. It's a plan.

In Java, a class has two main parts:

• Fields — the data each object holds (also called instance variables or state).
• Methods — the actions an object can perform (its behaviour).

public class BankAccount {
    // Fields: what every account has
    String owner;
    double balance;

    // Method: what every account can do
    void deposit(double amount) {
        balance = balance + amount;
    }
}

This code creates no accounts yet. It only tells Java what a bank account looks like.

What is an object in Java?

An object is an actual instance of a class that exists while your program runs. If the class is the house drawing, objects are the real houses built from it. They share a layout, but each house has its own furniture.

Every object has three things:

How do you create an object in Java?

You create objects with the new keyword:

BankAccount account = new BankAccount();
account.owner = "Asha";
account.deposit(250.0);

System.out.println(account.owner + " has $" + account.balance);

Asha has $250.0

• new BankAccount() — builds a fresh object in memory.
• BankAccount account — declares a variable that can refer to a BankAccount object.
• account.owner and account.deposit(...) — the dot (.) means "reach into this object" to read a field or call a method.

Let's see it in action: one class, many objects

This is the idea beginners most often miss: each object keeps its own data. Create a few dogs below and make each one bark.

class Dog {
    String name;
    int age;

    void bark() {
        System.out.println(
            name + " says Woof!");
    }
}

What is a constructor?

Setting fields one by one after creating an object is clumsy, and it's easy to forget one. A constructor lets you set everything up the moment the object is created.

public class BankAccount {
    String owner;
    double balance;

    // Constructor: same name as the class, no return type
    BankAccount(String owner, double openingBalance) {
        this.owner = owner;
        this.balance = openingBalance;
    }

    void deposit(double amount) {
        balance += amount;
    }
}

// Usage
BankAccount asha = new BankAccount("Asha", 100.0);
BankAccount ben  = new BankAccount("Ben", 40.0);

• BankAccount(String owner, double openingBalance) — runs automatically whenever new BankAccount(...) is called.
• this.owner = owner — this.owner is the field, and plain owner is the parameter. The this keyword tells them apart.

What does a variable really hold? References explained

Object variables don't hold the object itself. They hold a reference, which is like a house address written on a sticky note. This matters as soon as you copy a variable:

BankAccount a = new BankAccount("Asha", 100.0);
BankAccount b = a;        // copies the address, NOT the account

b.deposit(50.0);
System.out.println(a.balance);   // a and b point to the same object

150.0

How do classes fit into object-oriented programming?

Classes and objects are the foundation for four ideas you'll meet throughout Java:

Quick recap

What's next?

Right now anyone can write account.balance = -1000. Learn how to protect your fields in Java access modifiers, or go back to the Java overview.