Encapsulation

The bundling of data (state) and the methods that operate on that data into a single unit (class or module), and restricting direct access to the internal state from outside that unit.

Problem / Why It Matters

When any part of a program can directly read and modify another part’s internal state, changes to that state ripple unpredictably throughout the codebase. Encapsulation establishes ownership of state — each piece of data has exactly one owner responsible for its invariants — making the system easier to reason about and safer to change.

Explanation

Grady Booch lists encapsulation as one of the four fundamental software design principles (PHAME: principles of hierarchy, abstraction, modularization, and encapsulation).

Encapsulation has two aspects:

1. Data hiding: internal fields are private; external code cannot read or write them directly. Access is controlled through methods (getters/setters, or more meaningfully, domain methods).
2. Behavioral bundling: the logic that operates on the data lives with the data. A BankAccount class encapsulates its balance field and exposes deposit() and withdraw() methods that enforce business rules (e.g., no negative balance).

This means the class can enforce invariants — rules that must always hold for its state — without relying on external callers to behave correctly.

Encapsulation vs. Information Hiding

These are closely related but not identical:

• Information Hiding is the design principle: hide decisions that are likely to change behind stable interfaces. It applies to modules, services, and APIs, not just classes.
• Encapsulation is the OOP mechanism that implements information hiding at the class level: bundle state and behavior, restrict access to state.

Encapsulation is one implementation of information hiding. A microservice that exposes only an HTTP API and hides its database schema is practicing information hiding without classical OOP encapsulation.

Examples

Good: BankAccount.withdraw(amount) enforces the no-overdraft rule internally. Callers cannot set the balance directly.

Bad: account.balance -= amount called from every site that needs to debit an account. No invariant enforcement; rule changes require finding every call site.

Related

• Information Hiding — the design principle; encapsulation is its OOP implementation
• Abstraction — encapsulation supports abstraction by hiding state details
• Coupling and Cohesion — encapsulation reduces coupling by making state private
• Single Responsibility Principle — tells you what to encapsulate