Classes and Objects

Object-oriented programming (OOP) helps organize related data and behaviour. In Java, a class defines a type, and an object is an instance of that class.

Define a class#

public class Product {
    String name;
    double price;

    void displayDetails() {
        System.out.println(name + ": " + price);
    }
}

The class contains two fields (name and price) and one method (displayDetails). This example keeps the fields package-private for simplicity. In production code, fields are commonly made private and exposed through intentional methods.

Create an object#

Product keyboard = new Product();
keyboard.name = "Keyboard";
keyboard.price = 799.0;
keyboard.displayDetails();

The new expression creates an object and returns a reference to it. The variable keyboard stores that reference, allowing the program to access the object's fields and methods.

Encapsulation#

Encapsulation keeps an object's internal state controlled by its own API. For example:

public class BankAccount {
    private double balance;

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }
}

Callers cannot directly assign to balance; they use the public method, which checks that the deposit is positive. Real financial software needs more safeguards, but this illustrates the principle.

Class versus object#

A class is the definition; an object is one instance built from that definition. Two Product objects can have different names and prices even though they share the same class.

Practice#

Create a Student class with fields for a name and a roll number. Add a method that prints the student's details, then create two different student objects.