Why SOLID principles guide maintainable design in LLD - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to apply the Single Responsibility Principle (SRP) by defining a class that only handles user data storage.

LLD

class UserDataHandler:
    def __init__(self, user):
        self.user = user
    def [1](self):
        # Save user data to database
        pass

Drag options to blanks, or click blank then click option'

Ahandle_all_tasks

Bsend_email

Cprocess_payment

Dsave_user_data

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a method name that does multiple unrelated tasks.

The Single Responsibility Principle means a class should have one job. Here, the method should only save user data, so save_user_data is correct.

2fill in blank

medium

Complete the code to follow the Open/Closed Principle by allowing new payment methods without changing existing code.

LLD

class PaymentProcessor:
    def process(self, payment_method):
        if payment_method == 'credit_card':
            self.process_credit_card()
        elif payment_method == 'paypal':
            self.process_paypal()
        # To add new methods, use [1] instead of if-else

Drag options to blanks, or click blank then click option'

Ahardcoding

Bpolymorphism

Cglobal variables

Dmonolithic design

Attempts:

3 left

💡 Hint

Common Mistakes

Using if-else statements that require code changes for new methods.

The Open/Closed Principle says software should be open for extension but closed for modification. Using polymorphism allows adding new payment methods without changing existing code.

3fill in blank

hard

Fix the error in the code to respect the Liskov Substitution Principle by ensuring the subclass can replace the superclass without issues.

LLD

class Bird:
    def fly(self):
        print('Flying')

class Penguin(Bird):
    def [1](self):
        print('Penguins swim!')

Drag options to blanks, or click blank then click option'

Afly

Bswim

Cwalk

Dsing

Attempts:

3 left

💡 Hint

Common Mistakes

Raising exceptions in overridden methods that are expected to work.

The Liskov Substitution Principle means subclasses should be replaceable for their base classes without breaking expected behavior. Overriding fly with penguin-appropriate behavior (like swimming) instead of raising an exception respects substitutability.

4fill in blank

hard

Fill both blanks to apply Interface Segregation Principle by splitting a large interface into smaller ones.

LLD

class [1]:
    def read(self): pass
    def write(self): pass

class [2]:
    def print(self): pass

Drag options to blanks, or click blank then click option'

AReadable

BWritable

CPrinter

DDocument

Attempts:

3 left

💡 Hint

Common Mistakes

Combining unrelated methods into one interface.

The Interface Segregation Principle says clients should not depend on methods they do not use. Splitting a large interface into Readable and Printer interfaces helps achieve this.

5fill in blank

hard

Fill all three blanks to apply Dependency Inversion Principle by depending on abstractions, not concrete classes.

LLD

class [1]:
    def connect(self): pass

class DatabaseConnection([2]):
    def connect(self):
        print('Connected to DB')

class Application:
    def __init__(self, db: [3]):
        self.db = db

Drag options to blanks, or click blank then click option'

AIDatabase

BDatabaseConnection

DConcreteDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

High-level classes depending directly on concrete classes.

The Dependency Inversion Principle says high-level modules should depend on abstractions. Here, IDatabase is the abstraction, DatabaseConnection is the concrete class, and Application depends on the abstraction IDatabase.

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