Custom exception classes in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a custom exception class named MyException.

Kotlin

class MyException : [1]("Custom error occurred")

Drag options to blanks, or click blank then click option'

AException

BError

CThrowable

DRuntimeException

Attempts:

3 left

💡 Hint

Common Mistakes

Using Error instead of Exception as the base class.

Forgetting to extend any class.

The custom exception class should inherit from Exception, the standard base class for custom exceptions in Kotlin.

2fill in blank

medium

Complete the code to throw the custom exception MyException.

Kotlin

fun checkValue(value: Int) {
    if (value < 0) {
        [1] MyException()
    }
}

Drag options to blanks, or click blank then click option'

Areturn

Bthrow

Craise

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'raise' which is from other languages.

Using 'return' which just exits the function.

In Kotlin, you use throw to throw exceptions.

3fill in blank

hard

Fix the error in the custom exception class constructor to accept a message parameter.

Kotlin

class MyException(message: String) : Exception([1]) {}

Drag options to blanks, or click blank then click option'

Amessage

B"message"

Cmsg

Dthis.message

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string literal instead of the variable.

Using an undefined variable name.

The constructor parameter message should be passed directly to the base Exception class.

4fill in blank

hard

Fill both blanks to create a custom exception with a default message and a secondary constructor.

Kotlin

class MyException : Exception {
    constructor() : super([1])
    constructor(message: String) : super([2])
}

Drag options to blanks, or click blank then click option'

A"Default error"

Bmessage

C"Error occurred"

Dmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined variable names.

Passing string literals where variables are expected.

The default constructor passes a fixed string, and the secondary constructor passes the message parameter.

5fill in blank

hard

Fill all three blanks to create a custom exception class with a message property and override the toString method.

Kotlin

class MyException(val [1]: String) : Exception([2]) {
    override fun toString(): String {
        return "MyException: " + [3]
    }
}

Drag options to blanks, or click blank then click option'

Amessage

Dmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the property and constructor parameter.

Not returning the message in toString.

The property and constructor parameter are named message, and the toString method returns it.

Practice

1. What is the main purpose of creating a custom exception class in Kotlin?

easy

A. To speed up program execution

B. To define a specific type of error with a clear name

C. To automatically fix errors in code

D. To replace all built-in exceptions

Solution

1. Step 1: Understand what exceptions represent

   Exceptions represent errors or unexpected events in a program.

2. Step 2: Purpose of custom exceptions

   Custom exceptions let you name specific error types clearly, making code easier to read and debug.

3. Final Answer:

   To define a specific type of error with a clear name -> Option B

4. Quick Check:

   Custom exceptions = clear error types [OK]

Hint: Custom exceptions give clear names to specific errors [OK]

Common Mistakes:

• Thinking custom exceptions fix errors automatically
• Believing they speed up code execution
• Confusing them with replacing built-in exceptions

2. Which of the following is the correct way to declare a custom exception class named MyError in Kotlin?

easy

A. exception MyError extends Exception

B. fun MyError() : Exception

C. class MyError inherits Exception

D. class MyError : Exception()

Solution

1. Step 1: Recall Kotlin class declaration syntax

   In Kotlin, classes are declared with class ClassName.

2. Step 2: Inherit from Exception class

   Use colon : to inherit, so class MyError : Exception() is correct.

3. Final Answer:

   class MyError : Exception() -> Option D

4. Quick Check:

   Custom exception syntax = class Name : Exception() [OK]

Hint: Use colon : to inherit Exception in class declaration [OK]

Common Mistakes:

• Using 'fun' instead of 'class' to declare exception
• Using 'extends' or 'inherits' which are not Kotlin keywords
• Missing parentheses after Exception

3. What will be the output of this Kotlin code?

class MyException : Exception("Custom error")

fun test() {
  throw MyException()
}

fun main() {
  try {
    test()
  } catch (e: MyException) {
    println(e.message)
  }
}

medium

A. Custom error

B. MyException

C. null

D. No output, program crashes

Solution

1. Step 1: Understand the custom exception message

   The class passes "Custom error" to Exception constructor, so e.message is "Custom error".

2. Step 2: Catch block prints the exception message

   The catch block catches MyException and prints e.message, which is "Custom error".

3. Final Answer:

   Custom error -> Option A

4. Quick Check:

   Exception message printed = Custom error [OK]

Hint: Exception message prints from e.message in catch block [OK]

Common Mistakes:

• Expecting class name instead of message
• Thinking program crashes without output
• Assuming message is null if not set explicitly

4. Identify the error in this Kotlin custom exception class:

class MyException extends Exception() {}

medium

A. Class body cannot be empty

B. Missing parentheses after class name

C. Using 'extends' instead of ':' for inheritance

D. Exception class cannot be inherited

Solution

1. Step 1: Check Kotlin inheritance syntax

   Kotlin uses colon ':' for inheritance, not 'extends'.

2. Step 2: Identify the incorrect keyword

   The code uses 'extends' which is from Java, causing syntax error.

3. Final Answer:

   Using 'extends' instead of ':' for inheritance -> Option C

4. Quick Check:

   Kotlin inheritance uses ':' not 'extends' [OK]

Hint: Use ':' not 'extends' for inheritance in Kotlin [OK]

Common Mistakes:

• Confusing Java syntax with Kotlin
• Thinking empty class body is invalid
• Believing Exception cannot be subclassed

5. You want to create a custom exception InvalidAgeException that takes an age value as a property and shows a message like "Invalid age: 15". Which Kotlin code correctly implements this?

hard

A. class InvalidAgeException(val age: Int) : Exception("Invalid age: $age")

B. class InvalidAgeException(age: Int) : Exception("Invalid age: ${age}")

C. class InvalidAgeException(age: Int) : Exception("Invalid age: $age")

D. class InvalidAgeException(age: Int) : Exception("Invalid age: " + age)

Solution

1. Step 1: Understand primary constructor properties and string interpolation

   In Kotlin, val param: Type creates a read-only property accessible as e.age.

2. Step 2: Constructor param available in super call

   Params (with or without val) are in scope for superclass constructor, use $age for interpolation (braces optional for simple identifiers).

3. Step 3: Evaluate options

   A correctly uses val age: Int for property + $age interpolation. B/C/D lack val, so no age property (only private param). D uses concatenation (valid but less idiomatic).

4. Final Answer:

   class InvalidAgeException(val age: Int) : Exception("Invalid age: $age") -> Option A

5. Quick Check:

   val age: Int + $age interpolation for property + message [OK]

Hint: Use val age: Int and $age for property + message interpolation [OK]

Common Mistakes:

• Omitting 'val' (no public age property)
• Using unnecessary braces ${age} (optional for simple vars)
• Using string concatenation instead of interpolation