Classes and Objects

Classes and Objects

In Java, a class is a blueprint that describes the structure and behaviour of objects. An object is an instance of a class — it holds concrete data.

public class BankAccount {
    private String owner;
    private double balance;

    public BankAccount(String owner, double initialBalance) {
        this.owner = owner;
        this.balance = initialBalance;
    }

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

    public double getBalance() {
        return balance;
    }
}

Creating Objects

BankAccount account = new BankAccount("Alice", 1000.0);
account.deposit(500.0);
System.out.println(account.getBalance()); // 1500.0

Key Concepts

• Fields store an object's state
• Methods define what an object can do
• Constructors initialise the object when it's created
• Encapsulation hides internal state with private and exposes behaviour through public methods

this keyword

this refers to the current object. Use it to disambiguate between a field and a parameter with the same name.

public void setOwner(String owner) {
    this.owner = owner; // this.owner = field, owner = parameter
}

Interview Tip

When asked about OOP, explain all four pillars: Encapsulation, Inheritance, Polymorphism, Abstraction. Use BankAccount as your go-to example — it naturally demonstrates encapsulation.