Why SOLID principles guide maintainable design in LLD - Quick Recap

Recall & Review

beginner

What does the 'S' in SOLID stand for?

The 'S' stands for Single Responsibility Principle, which means a class should have only one reason to change, focusing on a single task.

beginner

Explain the Open/Closed Principle in simple terms.

It means software entities like classes should be open for extension but closed for modification, so you can add new features without changing existing code.

intermediate

Why is the Liskov Substitution Principle important for maintainability?

Because it ensures that subclasses can replace their parent classes without breaking the program, making code easier to extend and maintain.

intermediate

What does the Interface Segregation Principle encourage?

It encourages creating small, specific interfaces rather than large, general ones, so classes only implement what they actually need.

intermediate

Describe the Dependency Inversion Principle in simple words.

It means high-level modules should not depend on low-level modules directly; both should depend on abstractions, which helps reduce tight coupling.

Which SOLID principle helps avoid changing existing code when adding new features?

A Open/Closed Principle

B Single Responsibility Principle

C Liskov Substitution Principle

D Dependency Inversion Principle

The Open/Closed Principle states that software should be open for extension but closed for modification.

What does the Single Responsibility Principle ensure?

A A class has only one reason to change

B A class has multiple reasons to change

C Classes depend on concrete implementations

D Interfaces are large and general

Single Responsibility Principle means a class should have only one reason to change.

Which principle promotes using small, specific interfaces?

A Open/Closed Principle

B Dependency Inversion Principle

C Liskov Substitution Principle

D Interface Segregation Principle

Interface Segregation Principle encourages small, client-specific interfaces.

Liskov Substitution Principle means:

A Interfaces should be large and general

B Subclasses can replace parent classes without errors

C High-level modules depend on low-level modules

D Classes should have one responsibility

Liskov Substitution Principle ensures subclasses can replace their base classes safely.

Dependency Inversion Principle helps by:

A Combining multiple responsibilities in one class

B Making classes depend on concrete implementations

C Making high-level modules depend on abstractions

D Creating large interfaces

Dependency Inversion Principle states that both high-level and low-level modules should depend on abstractions.

Explain how SOLID principles help make software easier to maintain.

Think about how each principle reduces problems when changing or extending code.

Describe a real-life example that illustrates the Single Responsibility Principle.

Imagine a team where each member has a clear, single task.

Practice

1. Which of the following best describes the main goal of the SOLID principles in software design?

easy

A. To create code that is easy to understand, maintain, and extend

B. To make code run faster by using complex algorithms

C. To reduce the number of lines of code in a program

D. To enforce strict coding styles and formatting rules

Solution

1. Step 1: Understand the purpose of SOLID principles

   SOLID principles focus on writing code that is clean, flexible, and easy to maintain and extend over time.

2. Step 2: Compare options with the purpose

   To create code that is easy to understand, maintain, and extend matches this goal, while others focus on speed, code length, or style, which are not the main SOLID goals.

3. Final Answer:

   To create code that is easy to understand, maintain, and extend -> Option A

4. Quick Check:

   SOLID = Maintainable code [OK]

Hint: SOLID = clean, flexible, maintainable code [OK]

Common Mistakes:

• Confusing SOLID with performance optimization
• Thinking SOLID is about code formatting
• Believing SOLID reduces code length only

2. Which of the following is the correct description of the Single Responsibility Principle (SRP)?

easy

A. A class should be responsible for error logging only

B. A class should have only one reason to change

C. A class should handle all user interface and database logic

D. A class should inherit from multiple classes

Solution

1. Step 1: Recall the Single Responsibility Principle

   SRP states that a class should have only one reason to change, meaning it should have a single responsibility.

2. Step 2: Evaluate each option

   A class should have only one reason to change correctly states SRP. A class should inherit from multiple classes describes multiple inheritance, not SRP. A class should handle all user interface and database logic mixes responsibilities, violating SRP. A class should be responsible for error logging only is too narrow and not a general principle.

3. Final Answer:

   A class should have only one reason to change -> Option B

4. Quick Check:

   SRP = One reason to change [OK]

Hint: SRP = one responsibility per class [OK]

Common Mistakes:

• Confusing SRP with inheritance rules
• Thinking SRP means one class per feature
• Believing SRP limits class functionality too much

3. Consider this code snippet following the Open/Closed Principle (OCP):

class Shape:
    def area(self):
        pass

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius
    def area(self):
        return 3.14 * self.radius * self.radius

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

shapes = [Circle(2), Square(3)]
areas = [shape.area() for shape in shapes]
print(areas)

What will be the output?

medium

A. [6.28, 9]

B. [12.56, 6]

C. Error: area method not implemented

D. [12.56, 9]

Solution

1. Step 1: Calculate area of Circle with radius 2

   Area = 3.14 * 2 * 2 = 12.56

2. Step 2: Calculate area of Square with side 3

   Area = 3 * 3 = 9

3. Step 3: Collect areas in list and print

   Output list is [12.56, 9]

4. Final Answer:

   [12.56, 9] -> Option D

5. Quick Check:

   Circle area + Square area = [12.56, 9] [OK]

Hint: Calculate each shape's area separately [OK]

Common Mistakes:

• Using wrong formula for circle area
• Mixing side length with radius
• Expecting error due to base class method

4. A developer wrote this code violating the Dependency Inversion Principle (DIP):

class Database:
    def connect(self):
        print('Connecting to MySQL')

class UserService:
    def __init__(self):
        self.db = Database()
    def get_user(self):
        self.db.connect()
        return 'User data'

What is the main problem here?

medium

A. UserService depends directly on a concrete Database class, making it hard to change database types

B. Database class should not have a connect method

C. UserService should not have a get_user method

D. Database connection should be established outside UserService

Solution

1. Step 1: Understand Dependency Inversion Principle

   DIP says high-level modules should depend on abstractions, not concrete classes.

2. Step 2: Analyze the code dependency

   UserService creates a concrete Database instance directly, tightly coupling them and making changes hard.

3. Final Answer:

   UserService depends directly on a concrete Database class, making it hard to change database types -> Option A

4. Quick Check:

   DIP = Depend on abstractions, not concretes [OK]

Hint: Check if high-level class depends on concrete class [OK]

Common Mistakes:

• Thinking connect method is wrong
• Confusing method placement with DIP violation
• Ignoring tight coupling issues

5. You are designing a payment system that must support multiple payment methods (credit card, PayPal, crypto). To follow the Interface Segregation Principle (ISP) and keep the system maintainable, which design approach is best?

hard

A. Use one class with many conditional statements to handle all payment types

B. Create a single large interface with all payment methods combined, and implement it in one class

C. Create separate interfaces for each payment method and implement only the needed ones in each class

D. Avoid interfaces and use global functions for each payment method

Solution

1. Step 1: Recall Interface Segregation Principle

   ISP states clients should not be forced to depend on interfaces they do not use; smaller, specific interfaces are better.

2. Step 2: Evaluate design options

   Create separate interfaces for each payment method and implement only the needed ones in each class creates separate interfaces per payment method, so classes implement only what they need, improving maintainability.

3. Final Answer:

   Create separate interfaces for each payment method and implement only the needed ones in each class -> Option C

4. Quick Check:

   ISP = Small, specific interfaces [OK]

Hint: Split interfaces by feature to avoid unused methods [OK]

Common Mistakes:

• Using one big interface for all methods
• Mixing all logic in one class with conditionals
• Ignoring interface design principles