Custom exception classes in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a custom exception message

What is the output of this Ruby code that defines and raises a custom exception?

Ruby

class MyError < StandardError
  def message
    "Custom error occurred"
  end
end

begin
  raise MyError
rescue MyError => e
  puts e.message
end

A Custom error occurred

B MyError

C StandardError

D RuntimeError

💡 Hint

Look at the message method inside the custom exception class.

The custom exception class overrides the message method to return 'Custom error occurred'. When raised and rescued, printing e.message outputs this string.

❓ Predict Output

intermediate

Exception class inheritance check

What will this Ruby code output when checking the inheritance of a custom exception?

Ruby

class MyCustomError < RuntimeError; end

puts MyCustomError < StandardError
puts MyCustomError < RuntimeError
puts MyCustomError < Exception

A

true
true
false

B

false
true
true

C

true
false
true

D

true
true
true

💡 Hint

Remember that RuntimeError inherits from StandardError, which inherits from Exception.

MyCustomError inherits from RuntimeError, which inherits from StandardError, which inherits from Exception. So all comparisons return true.

🔧 Debug

advanced

Identify the error in custom exception raising

What error does this Ruby code raise when executed?

Ruby

class CustomError < StandardError; end

def test
  raise CustomError, 123
rescue CustomError => e
  puts e.message
end

test

A ArgumentError

B NoMethodError

C Outputs '123'

D TypeError

💡 Hint

Check how Ruby converts the second argument of raise to a message string.

Ruby converts the second argument (123) to a string '123' for the exception message. So e.message outputs '123' without error.

❓ Predict Output

advanced

Custom exception with additional data

What is the output of this Ruby code that adds extra data to a custom exception?

Ruby

class DataError < StandardError
  attr_reader :code
  def initialize(msg, code)
    super(msg)
    @code = code
  end
end

begin
  raise DataError.new("Error happened", 404)
rescue DataError => e
  puts "Message: #{e.message}, Code: #{e.code}"
end

A Message: Error happened, Code:

B Message: Error happened, Code: 404

C Message: , Code: 404

D RuntimeError

💡 Hint

Look at how the initialize method sets the message and code attributes.

The initialize method calls super with the message and sets @code. The rescue block prints both correctly.

🧠 Conceptual

expert

Behavior of rescue with multiple custom exceptions

Given these custom exceptions, which rescue clause will catch the raised exception?

Ruby

class ErrorA < StandardError; end
class ErrorB < ErrorA; end

begin
  raise ErrorB
rescue ErrorA
  puts "Caught ErrorA"
rescue StandardError
  puts "Caught StandardError"
end

A Caught ErrorA

B No output, exception not caught

C Caught StandardError

D RuntimeError

💡 Hint

Remember that rescue catches exceptions of the specified class or its subclasses.

ErrorB is a subclass of ErrorA, so the first rescue clause catches it and prints 'Caught ErrorA'.