SOLID principles in Software Engineering - Mini Project: Build & Apply

Understanding SOLID Principles

📖 Scenario: You are learning about the SOLID principles, which help software developers write better, easier-to-maintain code. These principles guide how to organize code in a way that is clear and flexible.

🎯 Goal: Build a simple example that shows each of the five SOLID principles with clear, easy-to-understand code snippets and explanations.

📋 What You'll Learn

Create a simple class example for each SOLID principle

Add a short explanation for each principle

Use clear and simple code structure

Show how each principle improves code design

💡 Why This Matters

🌍 Real World

SOLID principles help developers write code that is easier to maintain, extend, and understand in real software projects.

💼 Career

Understanding SOLID is essential for software engineers to create high-quality, professional code and work effectively in teams.

1

Create a class example for Single Responsibility Principle (SRP)

Write a class called Book that has a method get_title returning the book's title. This class should only handle book data, not printing or saving.

Software Engineering

# Write the Book class with a get_title method
# Your code here

Hint

Keep the class focused on just storing and returning the book title.

2

Add a helper class for Open/Closed Principle (OCP)

Create a class called DiscountCalculator with a method calculate that returns a price after discount. Add a variable discount_rate set to 0.1 (10%).

Software Engineering

class Book:
    def __init__(self, title):
        self.title = title
    
    def get_title(self):
        return self.title

# Create DiscountCalculator class with discount_rate and calculate method
# Your code here

Hint

This class can be extended later without changing existing code.

3

Implement Liskov Substitution Principle (LSP) with shape classes

Create a base class Shape with a method area that returns 0. Then create a subclass Square that overrides area to return side * side. Use side as an attribute.

Software Engineering

class Book:
    def __init__(self, title):
        self.title = title
    
    def get_title(self):
        return self.title

class DiscountCalculator:
    discount_rate = 0.1
    
    def calculate(self, price):
        return price * (1 - self.discount_rate)

# Create Shape base class with area method returning 0
# Create Square subclass with side attribute and area method
# Your code here

Hint

Subclasses should be usable wherever the base class is expected.

4

Demonstrate Interface Segregation Principle (ISP) and Dependency Inversion Principle (DIP)

Create two classes: Printer with method print_document and Scanner with method scan_document. Then create a class AllInOneMachine that uses instances of Printer and Scanner to perform printing and scanning.

Software Engineering

class Book:
    def __init__(self, title):
        self.title = title
    
    def get_title(self):
        return self.title

class DiscountCalculator:
    discount_rate = 0.1
    
    def calculate(self, price):
        return price * (1 - self.discount_rate)

class Shape:
    def area(self):
        return 0

class Square(Shape):
    def __init__(self, side):
        self.side = side
    
    def area(self):
        return self.side * self.side

# Create Printer class with print_document method
# Create Scanner class with scan_document method
# Create AllInOneMachine class that uses Printer and Scanner instances
# Your code here

Hint

Separate interfaces for different functions and depend on abstractions.