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

Custom Exception Classes in Kotlin

📖 Scenario: You are building a simple banking app. You want to handle errors clearly when someone tries to withdraw more money than they have.

🎯 Goal: Create a custom exception class called InsufficientFundsException and use it to show a clear error message when a withdrawal is too large.

📋 What You'll Learn

Create a custom exception class named InsufficientFundsException that inherits from Exception

Create a variable balance with the value 1000

Create a variable withdrawAmount with the value 1500

Write a function withdraw that throws InsufficientFundsException if withdrawAmount is greater than balance

Use a try-catch block to call withdraw and print the exception message

💡 Why This Matters

🌍 Real World

Custom exceptions help apps handle errors clearly, like showing a message when a bank withdrawal fails.

💼 Career

Understanding custom exceptions is important for writing robust Kotlin applications in jobs like Android development or backend services.

1

Create the initial variables

Create a variable called balance and set it to 1000. Then create a variable called withdrawAmount and set it to 1500.

Kotlin

# Create variables balance and withdrawAmount
// Your code here

Need a hint?

Use val to create variables and assign the exact numbers.

2

Create the custom exception class

Create a custom exception class called InsufficientFundsException that inherits from Exception. It should accept a message parameter of type String and pass it to the superclass.

Kotlin

val balance = 1000
val withdrawAmount = 1500

// Create InsufficientFundsException class here
// Your code here

Need a hint?

Define a class with class InsufficientFundsException(message: String) : Exception(message).

3

Write the withdraw function with exception

Write a function called withdraw that takes no parameters. Inside, check if withdrawAmount is greater than balance. If yes, throw InsufficientFundsException with the message "Not enough balance". Otherwise, print "Withdrawal successful".

Kotlin

val balance = 1000
val withdrawAmount = 1500

class InsufficientFundsException(message: String) : Exception(message)

// Write withdraw function here
// Your code here

Need a hint?

Use throw to raise the exception inside the if condition.

4

Call withdraw with try-catch and print error

Use a try-catch block to call the withdraw() function. Catch InsufficientFundsException and print its message.

Kotlin

val balance = 1000
val withdrawAmount = 1500

class InsufficientFundsException(message: String) : Exception(message)

fun withdraw() {
    if (withdrawAmount > balance) {
        throw InsufficientFundsException("Not enough balance")
    } else {
        println("Withdrawal successful")
    }
}

// Use try-catch to call withdraw and print exception message
// Your code here

Need a hint?

Use try { withdraw() } and catch (e: InsufficientFundsException) { println(e.message) }.