What is Encapsulation?

Definition
Encapsulation is the OOP principle of bundling data (attributes) and methods that operate on that data within a single unit (class), while restricting direct access to internal state through access modifiers.
Core Concepts:
First, Data Hiding: Internal state is hidden from external access using private/protected access modifiers. External code cannot directly manipulate object internals.
Second, Controlled Access: Public methods (getters/setters) provide controlled access to private data, allowing validation and maintaining invariants.
Third, Information Hiding: Implementation details are hidden behind a public interface. Clients depend on behavior, not internal structure.
Why Encapsulation Matters:
Problem Without Encapsulation: If a BankAccount class exposes balance as public, any code can set account.balance = -5000, violating business rules. Changes to internal representation (switching from single balance to multiple currency balances) break all client code.
Solution With Encapsulation: balance is private. Public method withdraw(amount) validates amount, checks sufficient funds, and updates balance. Internal representation can change without affecting clients.
Interview Tip: Encapsulation is about protecting invariants and enabling change. Always explain the business rule or constraint you're protecting.

💡 Key Takeaways

✓Encapsulation bundles data and methods while hiding internal state

✓Data hiding uses access modifiers to prevent direct access to attributes

✓Controlled access through public methods maintains object invariants

✓Enables changing implementation without breaking client code

✓Fundamental to maintainability and reducing coupling

📌 Interview Tips

1BankAccount hiding balance and providing deposit/withdraw methods

2Person class with private age field and validation in setAge()

3Library system where BookCopy availability is computed, not directly set

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