What is Encapsulation?

Encapsulation means keeping an object's data and related behavior together while controlling how outside code can interact with that data.

Short definition:

Encapsulation protects object state behind a safe public interface.

Two Parts of Encapsulation

1. Bundling

Data and functions belong together inside a class.

class BankAccount {
private:
    double balance;

public:
    void deposit(double amount);
    bool withdraw(double amount);
};

2. Controlled Access

Outside code cannot directly change private data.

Instead, it must use public methods.

account.deposit(500);
account.withdraw(200);

Why Direct Access Is Dangerous

If balance were public:

account.balance = -10000;

The object could enter an invalid state.

With encapsulation, the class can reject invalid actions.

Invariant

An invariant is a rule that should always remain true for an object.

For BankAccount:

balance should not become negative

Encapsulation helps protect invariants.

Encapsulation Is More Than Private Data

Private data alone is not enough.

Bad setter:

void setBalance(double value) {
    balance = value;
}

This still allows invalid state.

Better behavior:

bool withdraw(double amount) {
    if (amount <= 0 || amount > balance) {
        return false;
    }

    balance -= amount;
    return true;
}

Viva Answer

Encapsulation is the OOP principle of binding data and methods inside a class while restricting direct access to internal state. Outside code uses public methods, and those methods protect object invariants.

Quick Check

• What is encapsulation?
• What is an invariant?
• Why is public data risky?
• Is private data alone enough for good encapsulation?