What is Object-Oriented Programming?

Question

Accepted Answer

**Object-Oriented Programming (OOP)** is a paradigm that organizes code around **objects** — bundles of *state* (data) and *behavior* (methods) — instead of around standalone functions and global data. A **class** is the blueprint; an **object** is a concrete instance created from it.

## The core idea

You model a problem as a set of cooperating objects, each responsible for its own data. Code that needs that data asks the object to do something rather than reaching into it.

```python
class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner          # state held by the object
        self._balance = balance     # convention: leading _ = internal

def deposit(self, amount):      # behavior that owns the data
        self._balance += amount

def balance(self):
        return self._balance

acct = BankAccount("Mai")           # acct is an OBJECT (instance)
acct.deposit(100)                   # ask the object to act on its own state
print(acct.balance())               # 100
```

The data (`_balance`) and the rules that change it (`deposit`) live together, so the object stays internally consistent.

## Trade-offs

- Great fit for domains with clear "things" and relationships (users, orders, accounts).
- Can add ceremony for simple data transformations, where a functional or procedural style is leaner.
- Poorly designed hierarchies become rigid — a real risk, not an automatic win.

## Why it matters

OOP is the dominant paradigm in mainstream languages like Java, C#, and Python, so reading and writing it is a baseline skill.

Bundling state with behavior is what makes large systems navigable: you reason about one object at a time instead of tracking global data flow everywhere.