Object-Oriented Design

About

Object-Oriented Design (OOD) is the process of planning a system of interacting objects to solve a software problem while following object-oriented programming (OOP) principles.

It involves:

• Identifying the key objects and their interactions.

• Applying principles like encapsulation, abstraction, inheritance, and polymorphism.

• Designing scalable, maintainable, and efficient systems.

Why is OOD Important in Software Development?

OOD is a fundamental part of software development because it provides a structured and maintainable way to design software systems.

1. Improves Code Maintainability

• Easier Debugging & Updates

• Encapsulation ensures that changes in one class don’t break other parts of the system.

• If a bug is found, it’s localized to a specific object or class, making it easier to fix.

Example: Imagine a banking system where BankAccount is encapsulated:

public class BankAccount {
    private double balance;
    
    public void deposit(double amount) {
        if (amount > 0) balance += amount;
    }
    
    public double getBalance() { return balance; }
}

If we later change how balances are stored (e.g., switch to BigDecimal), other parts of the code won’t break since they interact only through getBalance().

2. Enhances Code Reusability

• Write Once, Use Multiple Times

• With inheritance and polymorphism, you can reuse existing code instead of rewriting it.

• Avoids duplicate logic, reducing the risk of bugs.

Example: If we have different types of users in a system, we can create a base User class and extend it for different roles:

class User { 
    String name; 
    void login() { System.out.println("Logging in..."); } 
}

class Admin extends User { 
    void accessAdminPanel() { System.out.println("Accessing admin panel..."); } 
}

class Customer extends User { 
    void browseProducts() { System.out.println("Browsing products..."); } 
}

Admin and Customer reuse login(), reducing code duplication.

3. Supports Scalability & Extensibility

• Easy to add new features without modifying existing code.

• Ensures long-term scalability as systems grow.

Example: If we need to add more vehicle types to our ParkingLot, we can do so without changing existing code:

abstract class Vehicle {
    String licensePlate;
}

class Car extends Vehicle { }
class Bike extends Vehicle { }
class Truck extends Vehicle { }  // Newly added vehicle type!

The system easily extends without breaking anything.

4. Enhances Team Collaboration

• Modular Design: Teams can work on different classes independently.

• Clear Responsibilities: Each class has a single job (SRP - Single Responsibility Principle).

Example: In a shopping app, different teams can work on separate modules:

• Order Management Team → Order, Payment classes.

• User Management Team → User, Authentication classes.

• Product Management Team → Product, Inventory classes.

Teams can work simultaneously without interfering with each other.

5. Improves Code Readability & Understanding

• OOD makes complex systems more intuitive.

• A properly designed class hierarchy makes it easy for new developers to understand the system.

Example: Which is easier to understand?

Without OOD:

void processOrder(String userType, double amount) { 
    if (userType.equals("Admin")) { 
        // Admin discount logic
    } else if (userType.equals("Customer")) { 
        // Regular discount logic
    }
}

With OOD:

abstract class User { abstract double getDiscount(); }

class Admin extends User { double getDiscount() { return 0.2; } }
class Customer extends User { double getDiscount() { return 0.1; } }

void processOrder(User user, double amount) { 
    double discount = user.getDiscount();
}

The second approach is cleaner, easier to extend, and eliminates if-else clutter.

6. Enables Better Testing

• Modular testing becomes easier when code follows OOD principles.

• Unit tests can be written independently for each class.

Example:

• If a bug occurs in the Payment class, we can test just that class without affecting User or Order.

• Mocking dependencies is easier in OOD-based designs.

Better separation = Easier testing.

7. Encourages the Use of Design Patterns

• OOD naturally aligns with design patterns, which solve common software design problems.

  • Factory Pattern → Used for object creation.

  • Strategy Pattern → Used for dynamic behavior selection.

  • Singleton Pattern → Used for one-instance objects (e.g., database connections).

Example (Factory Pattern for creating shapes):

interface Shape { void draw(); }

class Circle implements Shape {
    public void draw() { System.out.println("Drawing Circle"); }
}

class Rectangle implements Shape {
    public void draw() { System.out.println("Drawing Rectangle"); }
}

class ShapeFactory {
    public static Shape getShape(String type) {
        if (type.equals("Circle")) return new Circle();
        if (type.equals("Rectangle")) return new Rectangle();
        return null;
    }
}

public class Main {
    public static void main(String[] args) {
        Shape shape = ShapeFactory.getShape("Circle");
        shape.draw();
    }
}

Factory Pattern simplifies object creation and makes the system more flexible.

8. Promotes Security & Data Protection

• Encapsulation hides sensitive data.

• Users can only access data via controlled methods.

Example:

public class SecureUser {
    private String password;
    
    public void setPassword(String password) {
        this.password = hashPassword(password);
    }
    
    private String hashPassword(String password) {
        // Hashing logic here...
        return "hashedValue";
    }
}

The raw password is never exposed outside the class.

9. Makes Code More Adaptable to Change

• OOD reduces "ripple effects" when making changes.

• If we update a single class, the rest of the system remains stable.

Example: If we need to change the Payment processing method, we just modify one class instead of changing multiple parts of the system.

Object-Oriented Concepts in OOD

OOP Principles

• Encapsulation

• Abstraction

• Inheritance

• Polymorphism

OOP Principles

Use Design Patterns

Design Pattern

UML Diagrams for OOD

UML (Unified Modeling Language) diagrams help visualize OOD.

Common UML Diagrams

• Class Diagram → Shows class relationships.

• Sequence Diagram → Shows object interactions.

• Use Case Diagram → Defines user interactions.

• Component Diagram → Illustrates system components.

Diagrams

Steps in Object-Oriented Design

Step 1: Understand Requirements

Before jumping into code, ask clarifying questions to refine the problem:

• What are the core functionalities of the system?

• What are the inputs and outputs?

• Should it handle concurrency or scalability?

Step 2: Identify Key Objects & Classes

Extract nouns from the problem statement (these often become classes).

• Example: "A parking lot has multiple floors, each with different parking spots."

  • Classes: ParkingLot, Floor, ParkingSpot, Vehicle.

  • Attributes: floorNumber, spotNumber, isAvailable.

  • Methods: parkCar(), removeCar(), findNearestSpot().

Step 3: Define Relationships Between Objects

Different classes interact in various ways:

• Inheritance (is-a) → A Truck is-a Vehicle.

• Composition (has-a) → A ParkingLot has multiple Floors.

• Association → Driver is associated with Vehicle.

• Aggregation → Library contains multiple Books, but books can exist independently.

Step 4: Define Behaviours & Interactions

• Which objects communicate with each other?

• What methods should each class have?

• What design patterns can help?

Example: Design a Parking Lot

Requirements:

• A parking lot has multiple floors.

• Each floor has different types of spots (compact, large, handicapped).

• A Vehicle can park() and leave().

Class Diagram Approach

class ParkingLot {
    private List<Floor> floors;
}

class Floor {
    private List<ParkingSpot> spots;
}

class ParkingSpot {
    private boolean isAvailable;
    private Vehicle parkedVehicle;
}

abstract class Vehicle {
    String licensePlate;
}

class Car extends Vehicle { }
class Bike extends Vehicle { }

ParkingLot has-a Floor, which has-a ParkingSpot.