Class.new for dynamic class creation in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using Class.new?

Consider this Ruby code that creates a class dynamically and uses it:

Ruby

MyClass = Class.new do
  def greet
    "Hello from dynamic class!"
  end
end

obj = MyClass.new
puts obj.greet

A Hello from dynamic class!

B NoMethodError

C SyntaxError

D nil

💡 Hint

Look at how the method greet is defined inside the Class.new block.

The Class.new block defines a method greet. When we create an instance and call greet, it returns the string.

❓ Predict Output

intermediate

What does this code print when using Class.new with inheritance?

This Ruby code creates a subclass dynamically. What is printed?

Ruby

class Parent
  def message
    "Parent says hi"
  end
end

Child = Class.new(Parent) do
  def message
    super + " and Child says hello"
  end
end

puts Child.new.message

A NoMethodError

B Child says hello

C Parent says hi

D Parent says hi and Child says hello

💡 Hint

Remember that super calls the method from the parent class.

The Child class overrides message but calls super to get parent's message, then adds its own text.

🔧 Debug

advanced

Why does this dynamic class creation code raise an error?

This Ruby code tries to create a class dynamically but raises an error. Identify the cause.

Ruby

MyClass = Class.new do
  def initialize(name)
    @name = name
  end
end

obj = MyClass.new
puts obj.instance_variable_get(:@name)

A ArgumentError: wrong number of arguments (given 0, expected 1)

B SyntaxError: unexpected end-of-input

C NoMethodError: undefined method 'initialize'

D nil

💡 Hint

Check how many arguments are passed to new versus initialize method parameters.

The initialize method expects one argument, but new is called without arguments, causing ArgumentError.

📝 Syntax

advanced

Which option correctly defines a dynamic class with a class method using Class.new?

Select the Ruby code that correctly creates a class with a class method 'info' returning 'Dynamic class'.

A

MyClass = Class.new do
  def self.info
    'Dynamic class'
  end
end

B

MyClass = Class.new do
  def info
    'Dynamic class'
  end
end

MyClass.info

C

MyClass = Class.new do
  class << self
    def info
      'Dynamic class'
    end
  end
end

D

MyClass = Class.new do
  def info()
    'Dynamic class'
  end
end

MyClass.info

💡 Hint

Class methods can be defined inside 'class << self' block or with 'def self.method'.

Options A and C correctly define a class method (A using `def self.info`, C using `class << self`). Options B and D define an instance method but call it on the class, causing NoMethodError.

🚀 Application

expert

How many methods does this dynamically created class have?

Given this Ruby code, how many methods does the class MyClass respond to (excluding inherited methods)?

Ruby

MyClass = Class.new do
  def a
    1
  end

  def b
    2
  end

  def c
    3
  end
end

obj = MyClass.new
methods = obj.methods - Object.new.methods
puts methods.size

A 1

B 3

C 6

D 0

💡 Hint

Subtracting Object's methods from obj's methods leaves only methods defined in MyClass.

The class defines three instance methods: a, b, and c. Subtracting inherited methods leaves these three.