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'.

Practice

1. What is the main purpose of creating a custom exception class in Ruby?

easy

A. To define specific error types for clearer error handling

B. To speed up the program execution

C. To replace all built-in exceptions

D. To automatically fix errors in the code

Solution

1. Step 1: Understand the role of exceptions

   Exceptions represent errors or unexpected events in a program.

2. Step 2: Purpose of custom exceptions

   Custom exceptions let you create specific error types to handle different problems clearly.

3. Final Answer:

   To define specific error types for clearer error handling -> Option A

4. Quick Check:

   Custom exceptions = clearer error handling [OK]

Hint: Custom exceptions clarify error types for better handling [OK]

Common Mistakes:

• Thinking custom exceptions speed up code
• Believing they replace built-in exceptions
• Assuming they fix errors automatically

2. Which of the following is the correct way to define a custom exception class in Ruby?

easy

A. class MyError < StandardError; end

B. class MyError < Exception; end

C. def MyError; end

D. exception MyError inherits StandardError

Solution

1. Step 1: Check Ruby syntax for class inheritance

   In Ruby, custom exceptions inherit from StandardError for best practice.

2. Step 2: Identify correct class definition

   class MyError < StandardError; end correctly defines a class inheriting from StandardError using Ruby syntax.

3. Final Answer:

   class MyError < StandardError; end -> Option A

4. Quick Check:

   Custom exception syntax = class MyError < StandardError [OK]

Hint: Custom exceptions inherit StandardError with class syntax [OK]

Common Mistakes:

• Inheriting directly from Exception (not recommended)
• Using def instead of class
• Wrong inheritance syntax

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

class MyError < StandardError; end

begin
  raise MyError, "Something went wrong"
rescue MyError => e
  puts e.message
end

medium

A. MyError

B. StandardError

C. No output

D. Something went wrong

Solution

1. Step 1: Understand raise and rescue

   The code raises a MyError with message "Something went wrong" and rescues it as variable e.

2. Step 2: Output from exception message

   Inside rescue, puts e.message prints the error message string.

3. Final Answer:

   Something went wrong -> Option D

4. Quick Check:

   e.message prints error message string [OK]

Hint: e.message prints the error message string [OK]

Common Mistakes:

• Printing class name instead of message
• Expecting no output
• Confusing exception class with message

4. Identify the error in this Ruby code defining a custom exception:

class MyError
  def initialize(msg)
    super(msg)
  end
end

medium

A. Incorrect method name 'initialize'

B. Missing inheritance from StandardError

C. super should not be called in initialize

D. Class name should be lowercase

Solution

1. Step 1: Check class inheritance

   The class does not inherit from StandardError or any exception class, so it won't behave as an exception.

2. Step 2: Confirm other parts are correct

   Method name and super call are correct for passing message to parent class.

3. Final Answer:

   Missing inheritance from StandardError -> Option B

4. Quick Check:

   Custom exceptions must inherit StandardError [OK]

Hint: Always inherit StandardError for custom exceptions [OK]

Common Mistakes:

• Forgetting to inherit StandardError
• Thinking initialize method name is wrong
• Believing super call is unnecessary

5. You want to create a custom exception that stores an error code along with the message. Which code correctly implements this?

hard

A. class CodeError < StandardError
def initialize(msg)
@code = msg
end
end

B. class CodeError
def initialize(msg, code)
@code = code
super(msg)
end
end

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

D. class CodeError < Exception
def initialize(code)
@code = code
end
end

Solution

1. Step 1: Check inheritance and attribute

   class CodeError < StandardError attr_reader :code def initialize(msg, code) super(msg) @code = code end end inherits from StandardError and defines attr_reader for code to access it.

2. Step 2: Verify initialize method

   It calls super(msg) to set the message and stores code in @code correctly.

3. Final Answer:

   class CodeError < StandardError with attr_reader and initialize(msg, code) -> Option C

4. Quick Check:

   Custom exception with extra data = class CodeError < StandardError attr_reader :code def initialize(msg, code) super(msg) @code = code end end [OK]

Hint: Use attr_reader and call super(msg) in initialize [OK]

Common Mistakes:

• Not inheriting from StandardError
• Calling super after instance variables
• Missing attr_reader for code