Flow operators (map, filter, transform) in Kotlin

Introduction

Flow operators help you change or pick data from a stream step by step. They make working with lists of data easy and clear.

When you want to change each item in a list to a new form.

When you want to keep only some items from a list based on a rule.

When you want to do complex changes or multiple steps on data flowing through.

When you want to process data one piece at a time without waiting for the whole list.

When you want to write clean and readable code for data streams.

Syntax

Kotlin

flowOf(1, 2, 3, 4, 5)
    .map { it * 2 }
    .filter { it > 5 }
    .transform {
        emit(it)
        emit(it + 1)
    }

map changes each item to something new.

filter keeps only items that match a condition.

transform lets you emit zero, one, or many items for each input.

Examples

This multiplies each number by 10.

Kotlin

flowOf(1, 2, 3)
    .map { it * 10 }

This keeps only even numbers.

Kotlin

flowOf(1, 2, 3, 4)
    .filter { it % 2 == 0 }

This emits the original number and then the number times 100.

Kotlin

flowOf(1, 2)
    .transform {
        emit(it)
        emit(it * 100)
    }

Sample Program

This program creates a flow of numbers 1 to 5. It multiplies each by 3, keeps only those greater than 7, then for each number it emits the number and the number plus one. Finally, it prints all emitted numbers.

Kotlin

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

fun main() = runBlocking {
    flowOf(1, 2, 3, 4, 5)
        .map { it * 3 } // multiply each by 3
        .filter { it > 7 } // keep numbers greater than 7
        .transform {
            emit(it) // emit the number
            emit(it + 1) // emit number plus one
        }
        .collect { println(it) } // print each emitted number
}

Important Notes

Use map when you want exactly one output per input.

Use filter to remove unwanted items.

transform is more flexible and can emit many or no items per input.

Summary

map changes each item in the flow.

filter keeps only items that match a condition.

transform lets you emit any number of items for each input.