Exception hierarchy in Ruby - Mini Project: Build & Apply

Exception hierarchy

📖 Scenario: Imagine you are building a simple program that handles different types of errors. You want to organize these errors in a clear way so your program can respond properly when something goes wrong.

🎯 Goal: You will create a small exception hierarchy with a base error class and two specific error classes. Then, you will raise and rescue these errors to see how the hierarchy works.

📋 What You'll Learn

Create a base exception class called MyAppError that inherits from StandardError.

Create two subclasses called DatabaseError and NetworkError that inherit from MyAppError.

Write code to raise a DatabaseError exception.

Write code to rescue the exception using the base class MyAppError and print a message.

💡 Why This Matters

🌍 Real World

Organizing errors in a clear hierarchy helps large programs handle problems gracefully and makes debugging easier.

💼 Career

Understanding exception hierarchies is important for writing robust software and is a common skill required in software development jobs.

1

Create the base exception class

Create a class called MyAppError that inherits from StandardError.

Ruby

# Your code here

Hint

Use class MyAppError < StandardError to create the base exception class.

2

Create subclasses for specific errors

Create two classes called DatabaseError and NetworkError that inherit from MyAppError.

Ruby

class MyAppError < StandardError
end

# Your code here

Hint

Define each subclass with class ClassName < MyAppError.

3

Raise a DatabaseError exception

Write code to raise a DatabaseError exception with the message "Database connection failed".

Ruby

class MyAppError < StandardError
end

class DatabaseError < MyAppError
end

class NetworkError < MyAppError
end

# Your code here

Hint

Use raise DatabaseError, "Database connection failed" to raise the exception.

4

Rescue the exception and print a message

Wrap the code that raises the DatabaseError inside a begin block. Rescue the exception using MyAppError and print "Caught an error: " followed by the exception message.

Ruby

class MyAppError  e
  # Your code here
end

Hint

Use begin and rescue MyAppError => e to catch the error and puts to print the message.

Practice

1.

Which class is the root of the exception hierarchy in Ruby?

easy

A. RuntimeError

B. StandardError

C. Exception

D. IOError

Solution

1. Step 1: Understand Ruby's exception hierarchy

   In Ruby, all exceptions inherit from the Exception class, which is the root of the hierarchy.

2. Step 2: Identify the root class

   Other exception classes like StandardError and RuntimeError are subclasses of Exception.

3. Final Answer:

   Exception -> Option C

4. Quick Check:

   Root exception class = Exception [OK]

Hint: Remember: all errors come from Exception root [OK]

Common Mistakes:

• Confusing StandardError as root
• Thinking RuntimeError is root
• Choosing specific error classes instead of root

2.

Which of the following is the correct way to rescue only StandardError exceptions in Ruby?

begin
  # some code
rescue ???
  puts 'Error caught'
end

easy

A. StandardError

B. Exception

C. RuntimeError

D. IOError

Solution

1. Step 1: Understand rescue syntax

   The rescue keyword can specify which exception class to catch by naming it after rescue.

2. Step 2: Choose the correct exception class

   To catch only StandardError exceptions, write rescue StandardError.

3. Final Answer:

   StandardError -> Option A

4. Quick Check:

   Rescue specific error = StandardError [OK]

Hint: Use rescue with exact error class name [OK]

Common Mistakes:

• Using Exception catches too many errors
• Using RuntimeError misses other StandardErrors
• Using IOError catches unrelated errors

3.

What will be the output of this Ruby code?

begin
  raise IOError, 'File error'
rescue StandardError => e
  puts e.class
end

medium

A. StandardError

B. Exception

C. RuntimeError

D. IOError

Solution

1. Step 1: Identify the raised exception

   The code raises an IOError with message 'File error'.

2. Step 2: Check rescue clause

   The rescue catches StandardError and its subclasses. IOError is a subclass of StandardError, so it is caught.

3. Step 3: Output the exception class

   The code prints e.class, which is IOError.

4. Final Answer:

   IOError -> Option D

5. Quick Check:

   Raised error class = IOError [OK]

Hint: Raised error class prints even if rescued [OK]

Common Mistakes:

• Assuming rescue prints StandardError
• Confusing Exception with StandardError
• Thinking RuntimeError is printed

4.

Find the error in this Ruby code snippet:

begin
  raise 'Oops!'
rescue IOError
  puts 'IO error caught'
end

medium

A. The raise statement must specify an exception class

B. The rescue clause will not catch the raised error

C. The rescue clause syntax is incorrect

D. No error; code runs fine

Solution

1. Step 1: Analyze the raised error

   Raising with a string 'Oops!' creates a RuntimeError by default.

2. Step 2: Check rescue clause type

   The rescue only catches IOError, but the raised error is RuntimeError, so it is not caught.

3. Final Answer:

   The rescue clause will not catch the raised error -> Option B

4. Quick Check:

   Rescue type mismatch causes uncaught error [OK]

Hint: Raised error type must match rescue type [OK]

Common Mistakes:

• Thinking rescue catches all errors by default
• Believing string raise is invalid syntax
• Assuming rescue syntax is wrong

5.

You want to catch all exceptions except system-exiting ones like SystemExit and Interrupt. Which class should you rescue to catch most errors but allow system exits?

hard

A. StandardError

B. Exception

C. RuntimeError

D. NoMethodError

Solution

1. Step 1: Understand exception classes for system exit

   Classes like SystemExit and Interrupt inherit directly from Exception, not StandardError.

2. Step 2: Choose rescue class to exclude system exits

   Rescuing StandardError catches most runtime errors but excludes system-exiting exceptions.

3. Step 3: Confirm other options

   Rescuing Exception catches everything including system exits, which is usually not desired.

4. Final Answer:

   StandardError -> Option A

5. Quick Check:

   Rescue StandardError to exclude system exits [OK]

Hint: Rescue StandardError to avoid catching system exits [OK]

Common Mistakes:

• Rescuing Exception catches system exits too
• Choosing RuntimeError misses some errors
• Confusing NoMethodError as base class