Encapsulation

Encapsulation means bundling data (fields) and the methods that operate on that data into a single unit (a class), while restricting direct access to the internal state from outside that class.

The Problem Encapsulation Solves

// WITHOUT encapsulation — anyone can set invalid data
public class Account {
    public double balance;
}

Account acc = new Account();
acc.balance = -5000; // invalid, but nothing stops it!

Achieving Encapsulation: Private Fields + Public Getters/Setters

public class Account {
    private double balance; // hidden from outside access

    public double getBalance() {
        return balance;
    }

    public void setBalance(double balance) {
        if (balance >= 0) {
            this.balance = balance;
        } else {
            System.out.println("Balance cannot be negative");
        }
    }
}

Account acc = new Account();
acc.setBalance(-5000);  // rejected — prints the warning
acc.setBalance(1000);   // accepted
System.out.println(acc.getBalance()); // 1000.0

Access Modifiers Recap

Why Encapsulation Matters

• Validation: setters can reject invalid values before they're ever stored
• Flexibility: internal implementation can change later without breaking code that uses the getters/setters
• Security: sensitive fields (like passwords or balances) are never exposed directly

Interview tip: A common question is "What's the difference between encapsulation and abstraction?" — Encapsulation is about HIDING data via access control; abstraction is about HIDING implementation complexity by exposing only essential behavior. They work together but solve different problems.