Flow Operators in Kotlin

Introduction

Flow Operators 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.

Operators such as map and filter transform flow values. 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

• Operators such as map and filter transform flow values.
• Intermediate operators are applied before terminal collect.
• They follow functional pipeline style.

Syntax

flow.map { }.filter { }

Flow Operators in Kotlin Example Program in Kotlin

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

fun main(args: Array<String>) = runBlocking {
    flowOf(1, 2, 3, 4)
        .filter { it % 2 == 0 }
        .map { it * 10 }
        .collect { println(it) }
}

Sample Output

20
40

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. The println(it) statement writes a line to the console — this produces part of the sample output below.

3. Operators such as map and filter transform flow values.

4. 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

• Operators such as map and filter transform flow values.
• Intermediate operators are applied before terminal collect.
• They follow functional pipeline style.
• 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.