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.

Design 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.

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Last updated 4 months ago

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