What is Object Oriented Programming?

The simple idea

Imagine you are building an app for a bank. You have customers, accounts, and transactions. In Object Oriented Programming you create a blueprint for each of these things. That blueprint is called a class. When you actually create one customer or one account in memory, that is called an object.

Each object carries two things together. It carries data, which we call fields or attributes, and it carries actions, which we call methods. An Account object might store a balance and offer methods like deposit and withdraw. Everything about an account lives in one place, so the code stays tidy and easy to reason about.

A tiny example

class Account {
    double balance = 0;   // data
public:
    void deposit(double amount) {   // behaviour
        balance = balance + amount;
    }
    double getBalance() {
        return balance;
    }
};

// Using the class
Account myAccount;          // myAccount is an object
myAccount.deposit(500);
cout << myAccount.getBalance();  // 500

class Account {
    private double balance;   // data

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

    double getBalance() {
        return balance;
    }
}

// Using the class
Account myAccount = new Account();  // myAccount is an object
myAccount.deposit(500);
System.out.println(myAccount.getBalance());  // 500.0

class Account:
    def __init__(self):
        self._balance = 0        # data

    def deposit(self, amount):   # behaviour
        self._balance += amount

    def get_balance(self):
        return self._balance

# Using the class
my_account = Account()           # my_account is an object
my_account.deposit(500)
print(my_account.get_balance())  # 500

Here Account is the class, or blueprint. The variable myAccount is a real object built from that blueprint. The balance is hidden inside and can only be changed through the deposit method, which keeps the data safe.

Why teams use it

• Organisation: related data and behaviour stay together, so code is easier to find.
• Reuse: once you write a class you can create many objects from it and reuse it across projects.
• Maintenance: a change to how an account behaves lives in one class, not scattered everywhere.
• Modelling: it maps naturally to real world things, which makes designs easier to talk about.