launch and async builders in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to launch a coroutine in the Main scope.

Android Kotlin

GlobalScope.[1] {
    println("Hello from coroutine")
}

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

Alaunch

Basync

CrunBlocking

Ddelay

Attempts:

3 left

💡 Hint

Common Mistakes

Using async instead of launch when no result is needed.

Using delay as a builder, which is a suspending function.

The launch builder starts a new coroutine without blocking the current thread.

2fill in blank

medium

Complete the code to start an async coroutine that returns a value.

Android Kotlin

val deferred = GlobalScope.[1] {
    delay(1000)
    return@[1] 42
}

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

Aasync

Blaunch

CrunBlocking

DwithContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using launch when a result is needed.

Using runBlocking inside GlobalScope.

The async builder starts a coroutine and returns a Deferred which holds a future result.

3fill in blank

hard

Fix the error in the code by choosing the correct coroutine builder.

Android Kotlin

val result = GlobalScope.[1] {
    delay(500)
    10 + 20
}
println(result.await())

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

Aproduce

Basync

CrunBlocking

Dlaunch

Attempts:

3 left

💡 Hint

Common Mistakes

Using launch and trying to call await() on Job.

Using runBlocking inside GlobalScope.

Only async returns a Deferred that supports await() to get the result.

4fill in blank

hard

Fill both blanks to create a coroutine that launches and then waits for an async result.

Android Kotlin

GlobalScope.[1] {
    val deferred = async {
        delay(300)
        5 * 5
    }
    val result = deferred.[2]()
    println(result)
}

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

Alaunch

Basync

Cawait

Djoin

Attempts:

3 left

💡 Hint

Common Mistakes

Using async for the outer coroutine when launch is better for fire-and-forget.

Using join() instead of await() to get the result.

Use launch to start the coroutine and await to get the async result.

5fill in blank

hard

Fill all three blanks to create an async coroutine that returns a computed value after delay.

Android Kotlin

val deferred = GlobalScope.[1] {
    delay(200)
    val x = 10
    val y = 20
    x [2] y
}
println(deferred.[3]())

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

Aasync

B+

Cawait

Dlaunch

Attempts:

3 left

💡 Hint

Common Mistakes

Using launch instead of async when a result is needed.

Using join() instead of await() to get the result.

Use async to start coroutine returning a value, + to add, and await to get the result.

Practice

1. What is the main difference between launch and async builders in Kotlin coroutines?

easy

A. launch is used only for UI updates, async for background tasks.

B. launch blocks the main thread, async does not.

C. launch returns a value immediately, async never returns.

D. launch does not return a result, async returns a Deferred result.

Solution

1. Step 1: Understand launch behavior

   launch starts a coroutine that runs code asynchronously but does not return a value.

2. Step 2: Understand async behavior

   async starts a coroutine and returns a Deferred object, which you can await to get a result.

3. Final Answer:

   launch does not return a result, async returns a Deferred result. -> Option D

4. Quick Check:

   launch = no result, async = returns result [OK]

Hint: Remember: async returns a result, launch does not [OK]

Common Mistakes:

• Thinking launch returns a value
• Confusing async with blocking calls
• Assuming launch blocks the main thread

2. Which of the following is the correct syntax to start a coroutine with async in Kotlin?

easy

A. async.launch { fetchData() }

B. val result = launch { fetchData() }

C. val result = async { fetchData() }

D. launch.async { fetchData() }

Solution

1. Step 1: Identify correct coroutine builder usage

   async is called as a function returning a Deferred, so val result = async { fetchData() } is correct.

2. Step 2: Check other options for syntax errors

   launch does not return a value, so val result = launch { fetchData() } is wrong for async usage. Options C and D have invalid syntax.

3. Final Answer:

   val result = async { fetchData() } -> Option C

4. Quick Check:

   async returns Deferred, so assign it [OK]

Hint: async returns Deferred, assign it to a variable [OK]

Common Mistakes:

• Using launch instead of async when expecting a result
• Trying to chain async.launch or launch.async
• Not assigning async result to a variable

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

fun main() = runBlocking {
  val deferred = async {
    delay(100)
    10 + 20
  }
  println("Result: ${deferred.await()}")
}

medium

A. Result: 0

B. Result: 30

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand async and await behavior

   The async block runs asynchronously and returns a Deferred. Calling await() waits for the result.

2. Step 2: Calculate the expression inside async

   The code delays 100ms then returns 10 + 20 = 30, which is printed.

3. Final Answer:

   Result: 30 -> Option B

4. Quick Check:

   async + await returns computed value [OK]

Hint: await() gets async result after delay [OK]

Common Mistakes:

• Forgetting to call await() to get result
• Expecting immediate result without delay
• Confusing launch with async return

4. Identify the error in this Kotlin coroutine code:

fun main() = runBlocking {
  val result = launch {
    val data = fetchData()
  }
  println(data)
}

suspend fun fetchData(): String {
  delay(100)
  return "Hello"
}

medium

A. Variable 'data' is not accessible outside launch block

B. fetchData() cannot be called inside launch

C. runBlocking cannot be used with launch

D. Missing await() on launch

Solution

1. Step 1: Check variable scope of 'data'

   The variable 'data' is declared inside the launch coroutine block, so it is not visible outside.

2. Step 2: Analyze println(data) usage

   Trying to print 'data' outside launch causes a compilation error because 'data' is undefined there.

3. Final Answer:

   Variable 'data' is not accessible outside launch block -> Option A

4. Quick Check:

   Variables inside launch are local to its block [OK]

Hint: Variables inside launch are local; can't access outside [OK]

Common Mistakes:

• Assuming launch returns a value
• Trying to await launch (which returns Job)
• Ignoring variable scope rules

5. You want to fetch two independent data sources concurrently and combine their results. Which Kotlin coroutine approach is best?

val data1 = async { fetchData1() }
val data2 = async { fetchData2() }
val combined = data1.await() + data2.await()

Why is this approach preferred over launching two launch coroutines and combining results later?

hard

A. async returns Deferred results that can be awaited and combined immediately

B. launch blocks the main thread, async does not

C. launch cannot run multiple coroutines concurrently

D. async automatically caches results, launch does not

Solution

1. Step 1: Understand concurrency with async

   Using async starts coroutines that return Deferred results, allowing you to wait for and combine their outputs.

2. Step 2: Compare with launch behavior

   launch returns Job and does not provide a direct result, so combining outputs requires extra state management.

3. Final Answer:

   async returns Deferred results that can be awaited and combined immediately -> Option A

4. Quick Check:

   async = awaitable result, launch = no result [OK]

Hint: Use async to get results you can combine easily [OK]

Common Mistakes:

• Thinking launch returns a value
• Assuming launch blocks main thread
• Believing async caches results automatically