Why coroutines matter for async programming in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to launch a coroutine that prints "Hello".

Kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch [1] {
        println("Hello")
    }
}

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

A()

B() -> Unit

C{}

DUnit

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of curly braces {} for the coroutine block.

Passing a function type instead of a lambda block.

The launch function takes a lambda block denoted by curly braces {} to define the coroutine's code.

2fill in blank

medium

Complete the code to suspend the coroutine for 1000 milliseconds.

Kotlin

import kotlinx.coroutines.*

suspend fun waitOneSecond() {
    [1](1000L)
}

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

Adelay

Bsleep

Cpause

Dwait

Attempts:

3 left

💡 Hint

Common Mistakes

Using Thread.sleep which blocks the thread instead of suspending the coroutine.

Using wait or pause which are not coroutine functions.

The delay function suspends the coroutine without blocking the thread.

3fill in blank

hard

Fix the error in the coroutine builder to properly start a coroutine.

Kotlin

import kotlinx.coroutines.*

fun main() {
    GlobalScope.launch [1] {
        println("Running")
    }
    Thread.sleep(500L)
}

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

A{}

B() -> Unit

C()

DUnit

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of curly braces {} for the coroutine block.

Passing a function type instead of a lambda block.

launch requires a lambda block with curly braces {} to define the coroutine's code.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, filtering words longer than 3 characters.

Kotlin

val words = listOf("cat", "house", "dog", "elephant")
val lengths = words.associateWith { [1] }
    .filter { it.value [2] 3 }

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

Ait.length

B>

C<

Dit.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using it.size which is not valid for String.

Using < instead of > for filtering longer words.

Use it.length to get word length and > 3 to filter longer words.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, filtering words longer than 4 characters.

Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val result = words.associateBy(
    keySelector = { [1] },
    valueTransform = { [2] }
).filter { it.value [3] 4 }

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

Ait.uppercase()

Bit.length

C>

Dit.toUpperCase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated it.toUpperCase() instead of it.uppercase().

Using < instead of > for filtering.

Mixing keySelector and valueTransform parameters.

Use it.uppercase() for uppercase keys, it.length for values, and > 4 to filter.

Practice

1. Why are coroutines important for asynchronous programming in Kotlin?

easy

A. They allow tasks to run without blocking the main thread.

B. They make the code run slower but safer.

C. They replace all functions with loops.

D. They force the program to wait for each task to finish.

Solution

1. Step 1: Understand asynchronous programming

   Asynchronous programming means running tasks without stopping the main program flow.

2. Step 2: Role of coroutines in Kotlin

   Coroutines let tasks run in the background without blocking the main thread, improving responsiveness.

3. Final Answer:

   They allow tasks to run without blocking the main thread. -> Option A

4. Quick Check:

   Coroutines = non-blocking tasks [OK]

Hint: Coroutines keep the app responsive by not blocking main thread [OK]

Common Mistakes:

• Thinking coroutines slow down the program
• Confusing coroutines with loops
• Believing coroutines force waiting

2. Which of the following is the correct way to start a coroutine in Kotlin?

easy

A. startCoroutine { println("Hello") }

B. run { println("Hello") }

C. launch { println("Hello") }

D. beginCoroutine { println("Hello") }

Solution

1. Step 1: Recall coroutine builders in Kotlin

   The common builder to start a coroutine is launch.

2. Step 2: Check syntax correctness

   launch { ... } is valid, while others like startCoroutine or beginCoroutine are not standard coroutine starters.

3. Final Answer:

   launch { println("Hello") } -> Option C

4. Quick Check:

   Use launch to start coroutine [OK]

Hint: Remember: launch is the coroutine starter function [OK]

Common Mistakes:

• Using non-existent coroutine functions
• Confusing run with launch
• Trying to start coroutine without a builder

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

import kotlinx.coroutines.*
fun main() = runBlocking {
  launch {
    delay(100)
    println("World")
  }
  println("Hello")
}

medium

A. World\nHello

B. Hello\nWorld

C. Hello

D. World

Solution

1. Step 1: Understand runBlocking and launch

   runBlocking blocks main thread until all coroutines inside finish. launch starts a coroutine that delays 100ms before printing.

2. Step 2: Trace execution order

   "Hello" prints immediately. After 100ms delay, "World" prints. So output is "Hello" then "World".

3. Final Answer:

   Hello World -> Option B

4. Quick Check:

   Immediate print then delayed print = Hello then World [OK]

Hint: Immediate prints appear before delayed ones [OK]

Common Mistakes:

• Assuming launch blocks main thread
• Expecting World before Hello
• Ignoring delay effect

4. Identify the error in this Kotlin coroutine code:

fun main() {
  launch {
    println("Running")
  }
}

medium

A. launch requires a delay before printing.

B. println cannot be used inside launch.

C. main function must be suspend to use launch.

D. launch must be called inside a coroutine scope or suspend function.

Solution

1. Step 1: Check coroutine context requirements

   launch needs a coroutine scope or suspend function to run properly.

2. Step 2: Analyze main function usage

   Here, launch is called directly in main without a scope, causing an error.

3. Final Answer:

   launch must be called inside a coroutine scope or suspend function. -> Option D

4. Quick Check:

   launch needs coroutine scope [OK]

Hint: launch always needs a coroutine scope or suspend context [OK]

Common Mistakes:

• Thinking println is invalid inside launch
• Believing main must be suspend always
• Adding unnecessary delay

5. You want to fetch user data and user posts asynchronously in Kotlin using coroutines, then combine results. Which approach is best?

hard

A. Use async for both tasks and await their results before combining.

B. Use two separate launch blocks and combine results after both finish.

C. Run both tasks sequentially in the main thread.

D. Use delay to wait between fetching user data and posts.

Solution

1. Step 1: Understand async vs launch

   async returns a deferred result you can await; launch is for fire-and-forget tasks.

2. Step 2: Combining asynchronous results

   Using async for both tasks lets you start them concurrently and then await both results to combine efficiently.

3. Final Answer:

   Use async for both tasks and await their results before combining. -> Option A

4. Quick Check:

   async + await = concurrent results combined [OK]

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

Common Mistakes:

• Using launch when results are needed
• Running tasks sequentially wasting time
• Using delay instead of proper async control