Flow collect in Kotlin

Introduction

Flow collect 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.

Collect is terminal operator that receives emitted 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

• collect is terminal operator that receives emitted values.
• It is suspend function and runs until flow completes.
• Processing happens sequentially by default.

Syntax

flow.collect { value -> }

Flow collect in Kotlin Example Program in Kotlin

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

fun main(args: Array<String>) = runBlocking {
    flowOf("Red", "Green", "Blue").collect {
        println("Color: $it")
    }
}

Sample Output

Color: Red
Color: Green
Color: Blue

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("Color: $it") statement writes a line to the console — this produces part of the sample output below.

3. Collect is terminal operator that receives emitted 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

• Understand the core idea: collect is terminal operator that receives emitted values.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• collect is terminal operator that receives emitted values.
• It is suspend function and runs until flow completes.
• Processing happens sequentially by default.
• 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.