launch and async builders in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the launch builder do in Kotlin coroutines?

The launch builder starts a new coroutine that runs concurrently without blocking the current thread. It is used for tasks that don't return a result.

beginner

What is the main difference between launch and async builders?

launch starts a coroutine that does not return a result, while async starts a coroutine that returns a Deferred result which can be awaited.

beginner

How do you get the result from an async coroutine?

You call the await() function on the Deferred object returned by async to get the result once the coroutine completes.

beginner

Why should you avoid blocking the main thread in Android apps?

Blocking the main thread freezes the UI, making the app unresponsive. Using coroutines with launch or async helps run tasks asynchronously to keep the UI smooth.

intermediate

What happens if you call launch inside a CoroutineScope?

A new coroutine starts immediately within that scope and runs concurrently. It inherits the scope's context and lifecycle, so it cancels if the scope is cancelled.

Which builder should you use to start a coroutine that returns a result?

A launch

B async

C runBlocking

D delay

async returns a Deferred result that you can await().

What does launch return when called?

A Job

B Deferred<T>

C Result<T>

D Nothing

launch returns a Job representing the coroutine's lifecycle.

How do you wait for the result of an async coroutine?

A call <code>start()</code>

B call <code>join()</code>

C call <code>cancel()</code>

D call <code>await()</code>

await() suspends until the result is ready and returns it.

Which coroutine builder is best for fire-and-forget tasks?

A launch

B async

C runBlocking

D withContext

launch is used for tasks that don't return a result and run concurrently.

What happens if you call await() on a Deferred that is not completed yet?

A It blocks the thread

B It throws an exception

C It suspends the coroutine until completion

D It returns null

await() suspends the coroutine without blocking the thread until the result is ready.

Explain how launch and async builders differ and when to use each in Android development.

Think about whether you need a result or just want to run code concurrently.

Describe how to get a result from an async coroutine and why blocking the main thread is bad.

Consider how Android apps stay smooth while doing background work.