Encapsulation & Abstraction

About

Encapsulation and Abstraction are two closely related but distinct object‑oriented design principles that work together to reduce complexity, improve maintainability, and enforce clean boundaries in a system’s design.

Encapsulation

Encapsulation is about bundling data (state) and methods (behavior) that operate on that data into a single unit—typically a class—while restricting direct access to the internal details. It uses access modifiers (private, protected, public) to control visibility, ensuring that an object’s internal state can only be modified in controlled ways.

Example: A BankAccount class hides its balance field and only allows deposits and withdrawals through defined methods.

Key goals of Encapsulation:

  • Information hiding – Keep internal representation private to avoid accidental misuse.

  • Controlled access – Expose only the operations that make sense for the object.

  • Reduced coupling – Changes to internal implementation do not require changes in external code.

Common Violations

  1. Public Fields (Data Exposure)

    • Exposing class variables directly instead of controlling access through getters/setters or methods.

    • Example:

      public class User {
    public String name; // Anyone can modify directly
}

  2. Setters That Break Invariants

    • Providing setters that allow the object’s state to be changed to invalid values.

  3. Excessive Getters/Setters

    • Making every field accessible with public getters and setters defeats the purpose of encapsulation.

  4. Leaking Internal References

    • Returning references to internal mutable objects, allowing external modification.

    • Example: Returning a modifiable List from a getter without defensive copying.

  5. Mixing Multiple Responsibilities

    • A class that contains unrelated data and logic, making it hard to protect and reason about internal state.

  6. Bypassing Access Control

    • Using reflection or package-private hacks to manipulate internal state inappropriately.

How to Apply Encapsulation ?

1. Identify the Boundaries of Responsibility

  • Define clear ownership of data and logic within each module, class, or service.

  • Example: In a microservices architecture, each service encapsulates its own database and exposes data only through APIs.

2. Use Access Modifiers Properly

  • Mark internal state as private or protected.

  • Expose only the operations required by external code.

3. Control State Through Public Methods

  • Use methods that enforce business rules when changing the state instead of giving direct access to variables.

4. Avoid Exposing Internal Representations

  • Return copies of mutable collections or wrap them in read‑only views instead of exposing them directly.

5. Encapsulate External System Interactions

  • Wrap external APIs, databases, or message queues in dedicated classes to hide integration details from the rest of the system.

6. Protect Domain Logic from Infrastructure

  • Keep persistence logic (ORM, SQL) separate from business rules.

  • Example: Use a repository layer to abstract away database operations.

7. Maintain Stable Interfaces

  • Even if the internal implementation changes, the public interface should remain consistent to avoid breaking dependencies.

Example

Bad Example (Encapsulation Violation)

public class BankAccount {
    public double balance; // Public field - anyone can change it directly
}

public class Main {
    public static void main(String[] args) {
        BankAccount account = new BankAccount();
        account.balance = -1000; // Invalid state possible
    }
}

Problems

  • Internal state (balance) is exposed.

  • No control over valid or invalid operations.

Good Example (Encapsulation Applied)

public class BankAccount {
    private double balance;

    public void deposit(double amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException("Deposit must be positive");
        }
        balance += amount;
    }

    public boolean withdraw(double amount) {
        if (amount > balance) {
            return false; // insufficient funds
        }
        balance -= amount;
        return true;
    }

    public double getBalance() {
        return balance; // read-only access
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount account = new BankAccount();
        account.deposit(500);
        account.withdraw(200);
        System.out.println(account.getBalance()); // 300
    }
}

Benefits

  • Internal state is protected from invalid modifications.

  • All changes to state go through controlled methods.

  • Business rules are enforced at the point of modification.

Abstraction

Abstraction is about hiding unnecessary details while exposing only the essential features of an object or system. It focuses on what an object does rather than how it does it.

Example: A List interface allows us to add, remove, and retrieve elements without revealing the underlying implementation (ArrayList, LinkedList, etc.).

Key goals of Abstraction:

  • Simplification – Reduce complexity by providing a clear interface.

  • Flexibility – Allow different implementations without affecting the client code.

  • Focus on behavior, not implementation – Encourages thinking in terms of responsibilities, not mechanics.

Common Violations

  1. Leaking Implementation Details

    • Interfaces or APIs reveal how something works internally instead of focusing on what it does.

    • Example: An API returning a specific database record format instead of a domain model.

  2. Too Much Detail in Public Contracts

    • Public methods require knowledge of internal workings to be used correctly.

  3. Over‑Abstraction

    • Creating unnecessary layers of abstract classes or interfaces when there is no foreseeable variation.

  4. Inconsistent Abstractions

    • Mixing high-level and low-level operations in the same interface.

  5. Tight Coupling to a Specific Implementation

    • Claiming to be abstract but forcing consumers to know about a concrete class.

  6. Exposing Internal Models Directly

    • Returning raw entity objects from persistence layers instead of mapping to DTOs or view models.

How to Apply Abstraction ?

1. Define Clear Interfaces

  • Create interfaces or abstract classes that describe what a component does, not how it does it.

  • Example: A PaymentProcessor interface with processPayment() method, implemented differently for PayPal, Stripe, etc.

2. Hide Implementation Details

  • Keep the internal workings of a module private and expose only the necessary methods.

  • Example: A search API that hides whether it uses SQL, Elasticsearch, or an in‑memory search.

3. Use Dependency Inversion

  • Depend on abstractions (interfaces) rather than concrete implementations.

  • This makes swapping implementations easier without changing dependent code.

4. Apply Layered Design

  • Separate the what (API, service contracts) from the how (internal algorithms, data structures).

5. Keep Public APIs Minimal

  • Expose only the essential operations—avoid adding “just in case” methods that reveal too much about the internals.

6. Support Multiple Implementations

  • Ensure that the abstraction can be implemented in different ways without breaking existing code.

7. Combine with Encapsulation

  • Abstraction defines the contract; encapsulation protects the implementation details behind it.

Example

Bad Example (Abstraction Violation)

public class FileStorage {
    public void saveFile(String path, byte[] content) throws IOException {
        FileOutputStream fos = new FileOutputStream("/var/data/" + path);
        fos.write(content);
        fos.close();
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        FileStorage storage = new FileStorage();
        storage.saveFile("report.txt", "Hello".getBytes());
    }
}

Problems:

  • Calling code is tied to a specific file system storage implementation.

  • If storage moves to the cloud (e.g., AWS S3), all client code must be rewritten.

Good Example (Abstraction Applied)

public interface StorageService {
    void save(String filename, byte[] content) throws IOException;
}

public class LocalFileStorage implements StorageService {
    public void save(String filename, byte[] content) throws IOException {
        FileOutputStream fos = new FileOutputStream("/var/data/" + filename);
        fos.write(content);
        fos.close();
    }
}

public class S3Storage implements StorageService {
    public void save(String filename, byte[] content) {
        // AWS S3 upload logic
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        StorageService storage = new LocalFileStorage(); // Could be swapped to S3Storage
        storage.save("report.txt", "Hello".getBytes());
    }
}

Benefits

  • Code depends on the StorageService abstraction, not a specific implementation.

  • Swapping from local storage to cloud storage requires no changes in calling code.

  • Implementation details are hidden from clients.

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