How to Create Custom Exception in Kotlin: Simple Guide

In Kotlin, you create a custom exception by defining a class that extends Exception or any of its subclasses. You can add a constructor to pass a message or cause, then throw your custom exception using throw.

Syntax

To create a custom exception, define a class that inherits from Exception. You can add a primary constructor to accept an error message or cause.

• class: declares the class
• CustomException: your exception name
• : Exception(message): inherits from Exception and passes the message

kotlin

class CustomException(message: String) : Exception(message)

Example

This example shows how to define a custom exception and throw it when a condition is met. The try-catch block catches the exception and prints its message.

kotlin

class MyCustomException(message: String) : Exception(message)

fun checkNumber(num: Int) {
    if (num < 0) {
        throw MyCustomException("Negative numbers are not allowed: $num")
    } else {
        println("Number is $num")
    }
}

fun main() {
    try {
        checkNumber(-5)
    } catch (e: MyCustomException) {
        println("Caught exception: ${e.message}")
    }
}

Output

Caught exception: Negative numbers are not allowed: -5

Common Pitfalls

Common mistakes include not extending Exception or forgetting to pass the message to the superclass. Also, throwing exceptions without meaningful messages makes debugging harder.

Wrong way: defining a class without extending Exception will not behave as an exception.

kotlin

class WrongException(message: String) {
    // This is NOT an exception because it does not extend Exception
}

// Correct way:
class RightException(message: String) : Exception(message)

Quick Reference

• Define custom exception by extending Exception or RuntimeException.
• Use throw to raise the exception.
• Catch with try-catch blocks.
• Always provide meaningful messages for clarity.

Key Takeaways

Create custom exceptions by extending the Exception class in Kotlin.

Pass error messages to the superclass constructor for clarity.

Throw your custom exception using the throw keyword.

Catch exceptions with try-catch blocks to handle errors gracefully.

Avoid defining exception classes without extending Exception.