Custom exception classes in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom exception throw and catch

What is the output of this Kotlin code that defines and throws a custom exception?

Kotlin

class MyException(message: String) : Exception(message)

fun test() {
    try {
        throw MyException("Oops!")
    } catch (e: MyException) {
        println("Caught: ${e.message}")
    }
}

fun main() {
    test()
}

A Exception in thread "main" MyException: Oops!

B Caught: null

C Caught: Oops!

D Compilation error

💡 Hint

Look at how the exception message is passed and printed.

The custom exception MyException inherits from Exception and passes the message. When thrown and caught, printing e.message shows "Oops!".

❓ Predict Output

intermediate

Value of variable after catching custom exception

What is the value of variable result after running this Kotlin code?

Kotlin

class CustomError(msg: String) : Exception(msg)

fun checkValue(x: Int): String {
    return try {
        if (x < 0) throw CustomError("Negative")
        "Positive"
    } catch (e: CustomError) {
        e.message ?: "Error"
    }
}

fun main() {
    val result = checkValue(-5)
    println(result)
}

A null

B Error

C Positive

D Negative

💡 Hint

Check what message the exception carries and how it's returned.

The function throws CustomError with message "Negative" for negative input, caught and returns the message.

🔧 Debug

advanced

Identify the error in custom exception class definition

This Kotlin code tries to define a custom exception but causes a compilation error. What is the error?

Kotlin

class BadException : Exception {
    constructor(message: String) {
        super(message)
    }
}

fun main() {
    throw BadException("Error")
}

A Missing primary constructor call to superclass Exception

B super(message) cannot be called inside constructor body

C Exception class cannot be inherited

D No error, code compiles and runs

💡 Hint

In Kotlin, superclass constructor must be called in constructor header.

In Kotlin, calling super(message) inside constructor body is invalid; it must be called in the constructor header.

📝 Syntax

advanced

Which option correctly defines a custom exception with default message?

Choose the Kotlin code that correctly defines a custom exception class with a default message "Default error".

A class MyError(message: String = "Default error") : Exception(message)

B class MyError : Exception { constructor(message: String = "Default error") : super(message) }

C class MyError : Exception(message: String = "Default error")

D class MyError(message: String) : Exception { init { message = "Default error" } }

💡 Hint

Look for correct syntax for default parameter and superclass call.

Option A correctly uses default parameter and passes it to Exception superclass.

🚀 Application

expert

How many items in list after filtering custom exceptions?

Given this Kotlin code, how many exceptions in the list are instances of CustomException after filtering?

Kotlin

open class CustomException(msg: String) : Exception(msg)
class AnotherException(msg: String) : Exception(msg)

fun main() {
    val exceptions = listOf(
        CustomException("A"),
        AnotherException("B"),
        CustomException("C"),
        Exception("D")
    )
    val filtered = exceptions.filter { it is CustomException }
    println(filtered.size)
}

A 2

B 3

C 1

D 4

💡 Hint

Count only instances of CustomException class.

Only two objects are instances of CustomException; others are different exception types.