Flow Basics in Kotlin

Introduction

Flow Basics is a fundamental concept every Kotlin developer should understand. Flow streams values over time, similar to RxJava observables but with structured concurrency and coroutine integration.

Flow emits multiple values asynchronously like a stream. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Flow emits multiple values asynchronously like a stream.
• Created using flow { } builder.
• Represents cold stream that starts on collection.

Syntax

fun numbers(): Flow<Int> = flow { emit(1) }

Flow Basics in Kotlin Example Program in Kotlin

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun main(args: Array<String>) = runBlocking {
    flow {
        emit(1)
        emit(2)
        emit(3)
    }.collect { value ->
        print("$value ")
    }
}

Sample Output

1 2 3

When to use

Use Flow for streams of values over time — sensor readings, paginated API pages, or search-as-you-type results.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. Flow emits multiple values asynchronously like a stream.

3. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Flow emits multiple values asynchronously like a stream.
• Created using flow { } builder.
• Represents cold stream that starts on collection.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Add the kotlinx-coroutines-core dependency when running coroutine examples outside Android or IntelliJ.
• Semicolons at the end of statements are optional in Kotlin.