CancellationException behavior in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch a CancellationException.

Kotlin

try {
    // some suspend function call
} catch (e: [1]) {
    println("Cancelled")
}

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

ACancellationException

BTimeoutException

CIllegalArgumentException

DIOException

Attempts:

3 left

💡 Hint

Common Mistakes

Catching a generic Exception instead of CancellationException

Using unrelated exception types like IOException

The CancellationException is the specific exception thrown when a coroutine is cancelled.

2fill in blank

medium

Complete the code to rethrow a CancellationException inside a catch block.

Kotlin

try {
    // some suspend function
} catch (e: Exception) {
    if (e is [1]) throw e
    println("Other exception")
}

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

AIOException

BCancellationException

CNullPointerException

DIllegalStateException

Attempts:

3 left

💡 Hint

Common Mistakes

Not rethrowing CancellationException causes coroutine to continue unexpectedly

Rethrowing unrelated exceptions

When catching exceptions, CancellationException should be rethrown to properly cancel the coroutine.

3fill in blank

hard

Fix the error in the code to properly handle coroutine cancellation.

Kotlin

try {
    delay(1000)
} catch (e: [1]) {
    println("Cancelled")
}

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

AException

BRuntimeException

CCancellationException

DError

Attempts:

3 left

💡 Hint

Common Mistakes

Catching generic Exception hides cancellation

Catching Error or RuntimeException is incorrect

The delay function throws CancellationException when the coroutine is cancelled, so it must be caught specifically.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths only if length is greater than 3.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val lengths = words.filter { word -> [2] > 3 }.associate { word -> word to [1] }

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

Aword.length

Blen(word)

Cword.length()

Dword.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using Python's len() function

Using method call syntax like length() which is invalid in Kotlin

In Kotlin, word.length gives the length of the string. The condition checks if length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths if length is greater than 3.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val result = words.filter { word -> [3] > 3 }.associate { word -> [1] to [2] }

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

Aword.uppercase()

Bword.length

Dword.toUpperCase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated toUpperCase() instead of uppercase()

Using Python style functions like len()

word.uppercase() converts the word to uppercase in Kotlin. word.length gives the length. The condition checks if length is greater than 3.

Practice

1. What is the primary purpose of CancellationException in Kotlin coroutines?

easy

A. To stop coroutines cleanly without treating it as an error

B. To indicate a syntax error in coroutine code

C. To force a coroutine to restart automatically

D. To log coroutine execution details

Solution

1. Step 1: Understand coroutine cancellation

   CancellationException is used to stop coroutines without throwing an error that crashes the program.

2. Step 2: Differentiate from errors

   Unlike other exceptions, CancellationException signals a normal cancellation, not a failure.

3. Final Answer:

   To stop coroutines cleanly without treating it as an error -> Option A

4. Quick Check:

   CancellationException stops coroutines cleanly [OK]

Hint: CancellationException means clean stop, not error [OK]

Common Mistakes:

• Confusing it with runtime errors
• Thinking it restarts coroutines
• Assuming it logs info automatically

2. Which of the following is the correct way to catch a CancellationException in Kotlin?

easy

A. catch (Error e) { /* handle */ }

B. catch (Exception e) { /* handle */ }

C. catch (Throwable e) { /* handle */ }

D. catch (CancellationException e) { /* handle */ }

Solution

1. Step 1: Identify exception type

   CancellationException is a specific exception type used for coroutine cancellation.

2. Step 2: Use specific catch block

   To handle cancellation properly, catch CancellationException explicitly.

3. Final Answer:

   catch (CancellationException e) { /* handle */ } -> Option D

4. Quick Check:

   Catch CancellationException explicitly [OK]

Hint: Catch CancellationException by its own type [OK]

Common Mistakes:

• Catching only Exception hides cancellation
• Using catch (Error) which is unrelated
• Catching Throwable catches too broadly

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

import kotlinx.coroutines.*

fun main() = runBlocking {
    val job = launch {
        try {
            repeat(3) { i ->
                println("Working $i...")
                delay(100)
            }
        } catch (e: CancellationException) {
            println("Cancelled!")
        } finally {
            println("Cleanup done.")
        }
    }
    delay(150)
    job.cancelAndJoin()
    println("Done")
}

medium

A. Working 0...
Working 1...
Cancelled!
Cleanup done.
Done

B. Working 0...
Working 1...
Working 2...
Done

C. Cancelled!
Cleanup done.
Done

D. Working 0...
Cancelled!
Done

Solution

1. Step 1: Trace coroutine execution and cancellation

   The coroutine prints "Working 0..." and "Working 1..." with 100ms delay each, then cancellation occurs after 150ms.

2. Step 2: Handle CancellationException and finally block

   CancellationException is caught, printing "Cancelled!" and finally block prints "Cleanup done." before main prints "Done".

3. Final Answer:

   Working 0... Working 1... Cancelled! Cleanup done. Done -> Option A

4. Quick Check:

   CancellationException caught, cleanup runs, then Done [OK]

Hint: CancellationException triggers catch and finally blocks [OK]

Common Mistakes:

• Ignoring finally block output
• Assuming all repeats run before cancel
• Missing cancellation catch output

4. Identify the problem in this Kotlin coroutine snippet that tries to handle cancellation:

val job = launch {
    try {
        delay(1000)
    } catch (e: Exception) {
        println("Cancelled")
    }
}
job.cancel()

medium

A. The coroutine never starts because launch is missing parentheses

B. Calling job.cancel() without join causes a deadlock

C. It catches Exception instead of CancellationException, so cancellation is not handled properly

D. delay(1000) cannot be cancelled

Solution

1. Step 1: Check exception handling

   The catch block catches Exception, which includes CancellationException but does not distinguish it from other exceptions.

2. Step 2: Understand cancellation propagation

   To specifically handle cancellation without catching unrelated exceptions, catch CancellationException explicitly.

3. Final Answer:

   It catches Exception instead of CancellationException, so cancellation is not handled properly -> Option C

4. Quick Check:

   Catch CancellationException explicitly to handle cancellation [OK]

Hint: Catch CancellationException, not just Exception [OK]

Common Mistakes:

• Assuming Exception catch handles cancellation
• Thinking delay is uncancellable
• Confusing cancel() with deadlock

5. You want to cancel a coroutine but still perform some cleanup code without treating cancellation as an error. Which pattern correctly uses CancellationException to achieve this?

hard

A. Avoid catching exceptions and rely on coroutine scope cancellation only

B. Use try-catch with catch (CancellationException) to handle cancellation and run cleanup in finally

C. Use a global exception handler to catch all exceptions including cancellation

D. Use try-catch with catch (Exception) and ignore finally block

Solution

1. Step 1: Understand cancellation handling best practice

   CancellationException should be caught explicitly to avoid treating cancellation as failure.

2. Step 2: Use finally block for cleanup

   Cleanup code belongs in finally to ensure it runs whether coroutine completes or is cancelled.

3. Final Answer:

   Use try-catch with catch (CancellationException) to handle cancellation and run cleanup in finally -> Option B

4. Quick Check:

   Catch CancellationException and cleanup in finally [OK]

Hint: Catch CancellationException and cleanup in finally block [OK]

Common Mistakes:

• Catching only Exception hides cancellation
• Skipping finally block for cleanup
• Relying on global handlers for cancellation