Custom exception classes in Ruby - Mini Project: Build & Apply

Custom exception classes

📖 Scenario: Imagine you are building a simple banking system. You want to handle errors like withdrawing too much money or depositing invalid amounts in a clear way.

🎯 Goal: You will create custom exception classes to represent specific banking errors, then use them to raise and catch errors in your code.

📋 What You'll Learn

Create custom exception classes inheriting from StandardError

Raise custom exceptions with meaningful messages

Rescue and handle the custom exceptions

Print the error messages when exceptions occur

💡 Why This Matters

🌍 Real World

Custom exceptions help make error handling clearer and more specific in real applications like banking systems, improving user experience and debugging.

💼 Career

Understanding how to create and use custom exceptions is important for writing robust, maintainable code in many programming jobs.

1

Create custom exception classes

Create two custom exception classes called InsufficientFundsError and InvalidDepositError that both inherit from StandardError.

Ruby

# Your code here

Need a hint?

Use class ClassName < StandardError to create a custom exception class.

2

Set up account balance variable

Create a variable called balance and set it to 100 to represent the starting money in the account.

Ruby

class InsufficientFundsError < StandardError
end

class InvalidDepositError < StandardError
end

# Create a variable called balance and set it to 100
# Your code here

Need a hint?

Just write balance = 100 to set the starting amount.

3

Raise exceptions based on conditions

Write a method called withdraw that takes an amount parameter. Inside the method, raise InsufficientFundsError with the message "Not enough money" if amount is greater than balance. Also write a method called deposit that raises InvalidDepositError with the message "Deposit must be positive" if amount is less than or equal to zero.

Ruby

class InsufficientFundsError < StandardError
end

class InvalidDepositError < StandardError
end

balance = 100

# Define withdraw(amount) method with exception raising
# Define deposit(amount) method with exception raising
# Your code here

Need a hint?

Use raise ExceptionClass, "message" if condition inside the methods.

4

Rescue exceptions and print messages

Call withdraw(150) and deposit(-20) inside begin blocks. Rescue InsufficientFundsError and InvalidDepositError separately, printing their error messages with puts.

Ruby

class InsufficientFundsError  balance
end

def deposit(amount)
  raise InvalidDepositError, "Deposit must be positive" if amount <= 0
end

# Call withdraw(150) and rescue InsufficientFundsError
# Call deposit(-20) and rescue InvalidDepositError
# Your code here

Need a hint?

Use begin ... rescue ExceptionClass => e ... puts e.message ... end to catch and print errors.