SupervisorJob for independent failure in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a SupervisorJob instance.

Kotlin

val supervisor = [1]()

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

AJob

Blaunch

CSupervisorJob

DCoroutineScope

Attempts:

3 left

💡 Hint

Common Mistakes

Using Job() instead of SupervisorJob()

Using CoroutineScope() which is not a Job

Using launch() which is a coroutine builder

The SupervisorJob() creates a job that allows child coroutines to fail independently.

2fill in blank

medium

Complete the code to launch a child coroutine with the supervisor job.

Kotlin

val scope = CoroutineScope(Dispatchers.Default + [1]())
scope.launch {
    println("Child coroutine running")
}

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ADispatchers.IO

BJob

CCoroutineScope

DSupervisorJob

Attempts:

3 left

💡 Hint

Common Mistakes

Using Job() which cancels all children on failure

Using CoroutineScope() inside CoroutineScope constructor

Using Dispatchers.IO which is a dispatcher, not a job

Adding SupervisorJob() to the CoroutineScope allows child coroutines to fail independently.

3fill in blank

hard

Fix the error in the code to ensure child coroutines fail independently.

Kotlin

val supervisor = Job()
val scope = CoroutineScope(Dispatchers.Default + [1])
scope.launch {
    throw Exception("Failure")
}

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

ASupervisorJob()

BJob()

Csupervisor

DDispatchers.IO

Attempts:

3 left

💡 Hint

Common Mistakes

Using Job() which cancels all children on failure

Using Dispatchers.IO which is a dispatcher, not a job

Using variable name instead of a job instance

Replacing Job() with SupervisorJob() allows child coroutines to fail independently without cancelling siblings.

4fill in blank

hard

Fill all three blanks to create a supervisor scope and launch two child coroutines where one fails independently.

Kotlin

val supervisor = [1]()
val scope = CoroutineScope(Dispatchers.Default + supervisor)
scope.[2] {
    println("Child 1 running")
}
scope.[3] {
    throw Exception("Child 2 failure")
}

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

ASupervisorJob

BJob

CCoroutineScope

Dlaunch

Attempts:

3 left

💡 Hint

Common Mistakes

Using Job() instead of SupervisorJob()

Using CoroutineScope instead of launch for starting coroutines

SupervisorJob() creates the supervisor job, and launch starts child coroutines.

5fill in blank

hard

Fill all three blanks to create a supervisor scope, launch a child coroutine that fails, and another that continues.

Kotlin

val supervisor = [1]()
val scope = CoroutineScope(Dispatchers.Default + supervisor)
scope.[2] {
    throw Exception("Failure in child")
}
scope.[3] {
    println("Independent child running")
}

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

AJob

Blaunch

CSupervisorJob

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Using Job() instead of SupervisorJob()

Using async instead of launch which changes coroutine behavior

SupervisorJob() creates the supervisor job. Both children are started with launch, so one failing does not cancel the other.

Practice

1. What is the main purpose of SupervisorJob in Kotlin coroutines?

easy

A. To cancel all child coroutines when one fails

B. To allow child coroutines to fail independently without cancelling siblings

C. To delay coroutine execution until all children complete

D. To automatically restart failed coroutines

Solution

1. Step 1: Understand SupervisorJob behavior

   SupervisorJob lets child coroutines run independently so one failure doesn't affect others.

2. Step 2: Compare with regular Job

   Regular Job cancels all children if one fails, but SupervisorJob prevents this.

3. Final Answer:

   To allow child coroutines to fail independently without cancelling siblings -> Option B

4. Quick Check:

   SupervisorJob = independent failure [OK]

Hint: Remember: SupervisorJob isolates failures among children [OK]

Common Mistakes:

• Thinking SupervisorJob cancels all children on failure
• Confusing SupervisorJob with CoroutineScope
• Assuming SupervisorJob restarts failed coroutines

2. Which of the following is the correct way to create a CoroutineScope with a SupervisorJob?

easy

A. val scope = CoroutineScope(SupervisorJob() + Dispatchers.Default)

B. val scope = CoroutineScope(SupervisorJob)

C. val scope = CoroutineScope(Job() + Dispatchers.Default)

D. val scope = CoroutineScope(Dispatchers.Default)

Solution

1. Step 1: Identify correct SupervisorJob usage

   Creating a scope with SupervisorJob() instance plus a dispatcher is correct syntax.

2. Step 2: Check options for syntax correctness

   val scope = CoroutineScope(SupervisorJob() + Dispatchers.Default) uses SupervisorJob() constructor and adds dispatcher, which is valid.

3. Final Answer:

   val scope = CoroutineScope(SupervisorJob() + Dispatchers.Default) -> Option A

4. Quick Check:

   SupervisorJob() + Dispatcher = correct scope [OK]

Hint: Use SupervisorJob() with + operator to add dispatcher [OK]

Common Mistakes:

• Using SupervisorJob without parentheses
• Not adding a dispatcher to CoroutineScope
• Using Job() instead of SupervisorJob()

3. Consider this Kotlin code snippet using SupervisorJob:

val scope = CoroutineScope(SupervisorJob() + Dispatchers.Default)
scope.launch {
  launch { throw Exception("Child 1 failed") }
  launch { delay(100); println("Child 2 completed") }
}

What will be printed when this code runs?

medium

A. Child 2 completed

B. Child 1 failed

C. Exception thrown and program crashes

D. No output, both coroutines are cancelled

Solution

1. Step 1: Analyze SupervisorJob effect on children

   With SupervisorJob, failure in one child does not cancel siblings.

2. Step 2: Trace coroutine execution

   Child 1 throws exception immediately, but Child 2 delays and then prints "Child 2 completed".

3. Final Answer:

   Child 2 completed -> Option A

4. Quick Check:

   SupervisorJob allows sibling to finish [OK]

Hint: Child failure won't cancel siblings with SupervisorJob [OK]

Common Mistakes:

• Assuming all children cancel on one failure
• Expecting no output due to exception
• Confusing exception print with println output

4. Identify the error in this Kotlin coroutine code using SupervisorJob:

val scope = CoroutineScope(SupervisorJob)
scope.launch {
  launch { throw Exception("Error") }
  launch { println("Running") }
}

medium

A. launch cannot be called inside another launch

B. Child coroutines cannot throw exceptions

C. CoroutineScope cannot be created without a dispatcher

D. SupervisorJob is used without parentheses, causing a syntax error

Solution

1. Step 1: Check SupervisorJob usage

   SupervisorJob is a function and must be called with parentheses SupervisorJob().

2. Step 2: Validate CoroutineScope creation

   Using SupervisorJob without parentheses is a syntax error; it must be an instance.

3. Final Answer:

   SupervisorJob is used without parentheses, causing a syntax error -> Option D

4. Quick Check:

   SupervisorJob() requires parentheses [OK]

Hint: Always call SupervisorJob() with parentheses [OK]

Common Mistakes:

• Omitting parentheses on SupervisorJob
• Thinking nested launch is invalid
• Assuming dispatcher is mandatory (optional)

5. You want to launch three child coroutines where failure of one should NOT cancel the others. Which setup correctly achieves this using SupervisorJob and handles exceptions independently?

hard

A. Use CoroutineScope(SupervisorJob()) but do not handle exceptions in children

B. Use CoroutineScope(Job()) and rely on default cancellation

C. Use CoroutineScope(SupervisorJob()) and wrap each child launch in try-catch blocks

D. Use GlobalScope without SupervisorJob

Solution

1. Step 1: Choose SupervisorJob for independent failure

   Using SupervisorJob() allows children to fail independently without cancelling siblings.

2. Step 2: Handle exceptions in each child

   Wrapping each child launch in try-catch blocks ensures exceptions are caught and handled independently within each coroutine.

3. Final Answer:

   Use CoroutineScope(SupervisorJob()) and wrap each child launch in try-catch blocks -> Option C

4. Quick Check:

   SupervisorJob + try-catch = safe independent children [OK]

Hint: Combine SupervisorJob with try-catch in children [OK]

Common Mistakes:

• Ignoring exception handling in children
• Using Job() which cancels siblings on failure
• Using GlobalScope without SupervisorJob