Flow exception handling in Kotlin - Mini Project: Build & Apply

Flow exception handling

📖 Scenario: You are building a simple Kotlin program that uses a Flow to emit numbers. Sometimes, the flow might throw an exception. You want to handle this exception gracefully and provide a fallback value.

🎯 Goal: Create a Kotlin Flow that emits numbers from 1 to 5 but throws an exception at number 3. Then, handle the exception using catch to emit a fallback value of 0. Finally, collect and print all emitted values.

📋 What You'll Learn

Create a Flow that emits numbers 1 to 5

Throw an exception when the number 3 is emitted

Use catch to handle the exception and emit 0 as fallback

Collect the flow and print each emitted value

💡 Why This Matters

🌍 Real World

Flows are used in Kotlin to handle streams of data asynchronously, such as user inputs, network responses, or sensor data. Handling exceptions in flows ensures your app can recover gracefully from errors.

💼 Career

Understanding Flow exception handling is important for Kotlin developers working on Android apps or backend services that require reactive programming and robust error management.

1

Create a Flow that emits numbers 1 to 5

Create a Flow called numberFlow that emits the numbers 1, 2, 3, 4, and 5 using flow { } and emit().

Kotlin

# Create a Flow called numberFlow that emits 1 to 5
// Your code here

Need a hint?

Use flow { } to create the flow and emit() to send each number.

2

Throw an exception when emitting number 3

Modify the numberFlow so that it throws an Exception("Error at 3") when trying to emit the number 3. Emit numbers 1 and 2 normally before the exception.

Kotlin

import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.flow

val numberFlow: Flow = flow {
    emit(1)
    emit(2)
    // Throw exception when number is 3
    // Your code here
    emit(4)
    emit(5)
}

Need a hint?

Use throw Exception("Error at 3") right after emitting 2.

3

Handle the exception with catch and emit fallback

Use the catch operator on numberFlow to catch the exception and emit 0 as a fallback value.

Kotlin

import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.flow.catch

val numberFlow: Flow = flow {
    emit(1)
    emit(2)
    throw Exception("Error at 3")
    emit(4)
    emit(5)
}.catch { e ->
    // Emit 0 as fallback when exception occurs
    // Your code here
}

Need a hint?

Use .catch { e -> emit(0) } after the flow to handle exceptions.

4

Collect and print all emitted values

Collect the numberFlow using collect inside a runBlocking block and print each emitted value with println().

Kotlin

import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.flow.collect

fun main() = runBlocking {
    // Collect numberFlow and print each value
    // Your code here
}

Need a hint?

Use runBlocking to run the coroutine and numberFlow.collect { value -> println(value) } to print each emitted value.