Extend for class methods in Ruby - Mini Project: Build & Apply

Extend a Module for Class Methods in Ruby

📖 Scenario: You are building a simple Ruby program to track books in a library. You want to add a feature that counts how many books have been created.

🎯 Goal: Create a module with a class method to count books. Then extend this module for class methods in the Book class.

📋 What You'll Learn

Create a module called BookCounter with a class method count that returns the number of books.

Create a class called Book.

Extend the BookCounter module for class methods in the Book class.

Create a class variable @@book_count in Book to track the number of books.

Increment @@book_count each time a new Book object is created.

Use the count class method to print the total number of books.

💡 Why This Matters

🌍 Real World

Counting objects like books, users, or orders is common in software to track totals or inventory.

💼 Career

Understanding how to add class methods via modules and track shared data is useful for Ruby developers building maintainable code.

1

Create the Book class with a class variable

Create a class called Book with a class variable @@book_count set to 0. Also, define an initialize method that increments @@book_count by 1 each time a new book is created.

Ruby

# Create the Book class with @@book_count and initialize method
# Your code here

Hint

Use @@book_count to keep track of the number of books. Increment it inside initialize.

2

Create the BookCounter module with a class method

Create a module called BookCounter with a class method count that returns the value of @@book_count from the Book class.

Ruby

class Book
  @@book_count = 0

  def initialize
    @@book_count += 1
  end
end

# Create the BookCounter module with a class method count
# Your code here

Hint

Use Book.class_variable_get(:@@book_count) inside the count method to access the class variable.

3

Extend BookCounter for class methods in Book

Extend the BookCounter module for class methods in the Book class using extend BookCounter.

Ruby

class Book
  @@book_count = 0

  def initialize
    @@book_count += 1
  end
end

module BookCounter
  def count
    Book.class_variable_get(:@@book_count)
  end
end

# Extend BookCounter module here
# Your code here

Hint

Use Book.extend BookCounter after the class and module definitions to add class methods to Book.

4

Create books and print the total count

Create three Book objects named book1, book2, and book3. Then print the total number of books using Book.count.

Ruby

class Book
  @@book_count = 0

  def initialize
    @@book_count += 1
  end
end

module BookCounter
  def count
    Book.class_variable_get(:@@book_count)
  end
end

Book.extend BookCounter

# Create three Book objects and print the count
# Your code here

Hint

Create three books with Book.new and print the count with puts Book.count.

Practice

1. What does the extend keyword do in Ruby when used inside a class?

easy

A. Includes the module's constants only

B. Adds the module's methods as instance methods to the class

C. Adds the module's methods as class methods to the class

D. Removes methods from the class

Solution

1. Step 1: Understand extend usage

   The extend keyword adds methods from a module as class methods to the class that calls it.

2. Step 2: Differentiate from include

   Unlike include, which adds methods as instance methods, extend adds them as class methods.

3. Final Answer:

   Adds the module's methods as class methods to the class -> Option C

4. Quick Check:

   extend = class methods [OK]

Hint: Remember: extend = class methods, include = instance methods [OK]

Common Mistakes:

• Confusing extend with include
• Thinking extend adds instance methods
• Assuming extend removes methods

2. Which of the following is the correct syntax to add module methods as class methods using extend?

easy

A. class MyClass; def self.extend MyModule; end

B. class MyClass; include MyModule; end

C. class MyClass; MyModule.extend(self); end

D. class MyClass; extend MyModule; end

Solution

1. Step 1: Identify correct extend usage

   The correct syntax is to call extend inside the class body with the module name.

2. Step 2: Check each option

   class MyClass; extend MyModule; end uses extend MyModule inside the class, which is correct syntax.

3. Final Answer:

   class MyClass; extend MyModule; end -> Option D

4. Quick Check:

   Correct extend syntax = class MyClass; extend MyModule; end [OK]

Hint: Use 'extend ModuleName' inside class body for class methods [OK]

Common Mistakes:

• Using include instead of extend
• Calling extend on module instead of class
• Incorrect method definition syntax

3. What will be the output of the following Ruby code?

module Greetings
  def hello
    "Hello from module"
  end
end

class Person
  extend Greetings
end

puts Person.hello

medium

A. NoMethodError: undefined method 'hello' for Person

B. Hello from module

C. Hello from module called on instance

D. SyntaxError

Solution

1. Step 1: Understand extend Greetings effect

   The module's method hello becomes a class method of Person.

2. Step 2: Check method call

   Person.hello calls the class method, which exists and returns "Hello from module".

3. Final Answer:

   Hello from module -> Option B

4. Quick Check:

   Class method from extend = "Hello from module" [OK]

Hint: extend adds module methods as class methods, so Person.hello works [OK]

Common Mistakes:

• Thinking hello is an instance method
• Calling hello on an instance instead of class
• Expecting NoMethodError

4. Identify the error in this Ruby code snippet:

module Tools
  def tool_name
    "Hammer"
  end
end

class Worker
  extend Tools

  def self.tool_name
    "Wrench"
  end
end

puts Worker.tool_name

medium

A. No error; output is 'Wrench'

B. NoMethodError because tool_name is undefined

C. SyntaxError due to extend usage

D. Output is 'Hammer' ignoring class method

Solution

1. Step 1: Analyze method definitions

   The module Tools defines tool_name, which is added as a class method to Worker by extend Tools.

2. Step 2: Check method override

   Worker defines its own class method tool_name, which overrides the module method.

3. Final Answer:

   No error; output is 'Wrench' -> Option A

4. Quick Check:

   Class method override works; output = 'Wrench' [OK]

Hint: Class methods override extended module methods if defined later [OK]

Common Mistakes:

• Assuming module method overrides class method
• Expecting NoMethodError
• Confusing instance and class methods

5. You want to share a method info as a class method across multiple classes without repeating code. Which approach using extend is best?

module InfoModule
  def info
    "Shared class info"
  end
end

class Alpha
  # What to do here?
end

class Beta
  # What to do here?
end

hard

A. Add extend InfoModule inside both Alpha and Beta classes

B. Add include InfoModule inside both Alpha and Beta classes

C. Define def self.info separately in Alpha and Beta

D. Use InfoModule.extend(self) outside classes

Solution

1. Step 1: Understand sharing class methods

   Using extend InfoModule inside each class adds info as a class method.

2. Step 2: Evaluate options

   Add extend InfoModule inside both Alpha and Beta classes correctly uses extend in both classes to share the method without repetition.

3. Final Answer:

   Add extend InfoModule inside both Alpha and Beta classes -> Option A

4. Quick Check:

   Use extend in each class to share class methods [OK]

Hint: Use extend inside each class to share class methods from a module [OK]

Common Mistakes:

• Using include instead of extend for class methods
• Defining methods separately instead of sharing
• Calling extend on module instead of class