Flow builder and collect in Kotlin - Mini Project: Build & Apply

Flow builder and collect

📖 Scenario: You are building a simple Kotlin program that uses Flow to emit a sequence of numbers. This simulates a real-time data stream, like counting seconds or events.

🎯 Goal: Create a Flow that emits numbers from 1 to 5, then collect and print each number as it arrives.

📋 What You'll Learn

Create a Flow using flow builder that emits numbers 1 to 5

Create a val called numberFlow for the flow

Collect the flow using collect inside a runBlocking block

Print each collected number with println

💡 Why This Matters

🌍 Real World

Flows are used in Android apps and backend services to handle streams of data like user inputs, sensor data, or network responses.

💼 Career

Understanding flows and how to collect them is essential for Kotlin developers working with asynchronous data streams and reactive programming.

1

Create a Flow emitting numbers 1 to 5

Create a val called numberFlow using the flow builder. Inside the flow, emit numbers from 1 to 5 using a for loop and emit.

Kotlin

import kotlinx.coroutines.flow.flow

// Create numberFlow emitting 1 to 5
// Your code here

Hint

Use flow { } to create the flow and emit(value) to send values.

2

Add runBlocking to collect the flow

Add a runBlocking block to run the coroutine. Inside it, collect the numberFlow using collect.

Kotlin

import kotlinx.coroutines.runBlocking

import kotlinx.coroutines.flow.collect

runBlocking {
    // Collect numberFlow here
    // Your code here
}

Hint

Use runBlocking { numberFlow.collect { value -> } } to collect values.

3

Print each collected number

Inside the collect lambda, print each collected value using println.

Kotlin

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

val numberFlow = flow {
    for (i in 1..5) {
        emit(i)
    }
}

runBlocking {
    numberFlow.collect { value ->
        // Print value here
        // Your code here
    }
}

Hint

Use println(value) inside the collect block to show each number.

4

Run the program and see the output

Run the program to see the numbers 1 to 5 printed each on a new line.

Kotlin

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

val numberFlow = flow {
    for (i in 1..5) {
        emit(i)
    }
}

runBlocking {
    numberFlow.collect { value ->
        println(value)
    }
}

// Run the program to see output

Hint

Make sure you run the whole program to see the printed numbers.

Practice

1. What is the primary purpose of the flow {} builder in Kotlin?

easy

A. To create a stream of values that can be emitted over time

B. To immediately execute a block of code and return a single value

C. To collect values from a list synchronously

D. To define a suspend function without any emissions

Solution

1. Step 1: Understand the role of flow {}

   The flow {} builder creates a stream that can emit multiple values asynchronously over time.

2. Step 2: Compare with other options

   Options B, C, and D describe synchronous or non-stream behaviors, which do not match flow {} usage.

3. Final Answer:

   To create a stream of values that can be emitted over time -> Option A

4. Quick Check:

   Flow builder = stream of values [OK]

Hint: Remember: flow builds streams, not single immediate values [OK]

Common Mistakes:

• Confusing flow with suspend functions returning single values
• Thinking flow collects values instead of emitting them
• Assuming flow is synchronous

2. Which of the following is the correct syntax to collect values from a flow named myFlow inside a suspend function?

easy

A. myFlow.collect(value -> println(value))

B. myFlow.collect { println(value) }

C. myFlow.collect { value -> println(value) }

D. myFlow.collect { value => println(value) }

Solution

1. Step 1: Identify correct lambda syntax in Kotlin

   Kotlin uses curly braces with parameter and arrow: { param -> action }.

2. Step 2: Match syntax with options

   myFlow.collect { value -> println(value) } correctly uses { value -> println(value) }. Options A lacks braces, B misses parameter declaration (value undefined), D uses wrong arrow symbol (=>).

3. Final Answer:

   myFlow.collect { value -> println(value) } -> Option C

4. Quick Check:

   Correct lambda syntax = myFlow.collect { value -> println(value) } [OK]

Hint: Use { param -> action } syntax for Kotlin lambdas [OK]

Common Mistakes:

• Omitting lambda parameter name
• Using Java-style arrows (->) incorrectly
• Using parentheses instead of curly braces

3. What will be the output of the following Kotlin code?

import kotlinx.coroutines.flow.*
suspend fun main() {
    val numbers = flow {
        emit(1)
        emit(2)
        emit(3)
    }
    numbers.collect { println(it * 2) }
}

medium

A. 1
2
3

B. 2
4
6

C. 3
6
9

D. Compilation error

Solution

1. Step 1: Analyze emitted values

   The flow emits 1, 2, and 3 sequentially.

2. Step 2: Understand the collect block

   Each emitted value is multiplied by 2 and printed, so outputs are 2, 4, and 6.

3. Final Answer:

   2 4 6 -> Option B

4. Quick Check:

   Emitted values * 2 = 2,4,6 [OK]

Hint: Multiply emitted values inside collect to get output [OK]

Common Mistakes:

• Printing original values without multiplication
• Expecting a list output instead of separate prints
• Assuming compilation error due to missing coroutine context

4. Identify the error in this Kotlin flow code snippet:

val flow = flow {
    emit(1)
    emit(2)
}
flow.collect { println(it) }

medium

A. collect should be called with parentheses

B. Incorrect flow builder syntax

C. emit cannot be called inside flow builder

D. Missing suspend modifier on the function calling collect

Solution

1. Step 1: Check flow and collect usage

   The flow builder and emit calls are correct.

2. Step 2: Understand collect context

   Collect is a suspend function and must be called from a suspend function or coroutine scope. Calling it directly without suspend context causes error.

3. Final Answer:

   Missing suspend modifier on the function calling collect -> Option D

4. Quick Check:

   collect requires suspend context [OK]

Hint: Remember: collect is suspend, needs suspend function or coroutine [OK]

Common Mistakes:

• Trying to call collect outside suspend or coroutine
• Confusing emit usage inside flow
• Thinking collect syntax is wrong

5. You want to create a flow that emits only even numbers from 1 to 5 and then collect and print them. Which code snippet correctly achieves this?

hard

A. val flow = flow {
for (i in 1..5) {
if (i % 2 == 0) emit(i)
}
}
suspend fun main() {
flow.collect { println(it) }
}

B. val flow = flowOf(1, 2, 3, 4, 5).filter { it % 2 == 0 }
fun main() {
flow.collect { println(it) }
}

C. val flow = flow {
emit(2)
emit(4)
}
flow.collect { println(it) }

D. val flow = flow {
for (i in 1..5) emit(i)
}
flow.collect { if (it % 2 == 0) println(it) }

Solution

1. Step 1: Check flow emission of even numbers

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } emits only even numbers inside the flow builder using a for loop and condition.

2. Step 2: Verify collection and printing

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } collects the flow inside a suspend function and prints each emitted value correctly.

3. Final Answer:

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } -> Option A

4. Quick Check:

   Emit filtered values inside flow, collect in suspend [OK]

Hint: Filter inside flow builder to emit only needed values [OK]

Common Mistakes:

• Calling collect outside suspend function
• Filtering after emitting all values instead of inside flow
• Not using suspend function for collect