Cold Flow in Kotlin

Introduction

Cold Flow 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.

Cold flow starts emitting only when collector begins. 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

• Cold flow starts emitting only when collector begins.
• Each collector gets its own execution of flow block.
• Different from hot streams like SharedFlow.

Syntax

flow { ... }.collect { }

Cold Flow in Kotlin Example Program in Kotlin

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

fun main(args: Array<String>) = runBlocking {
    val cold = flow {
        println("Flow started")
        emit("A")
        emit("B")
    }
    cold.collect { println(it) }
}

Sample Output

Flow started
A
B

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. val cold = flow { assigns or updates a value used later in the program.

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

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

5. Cold flow starts emitting only when collector begins.

6. 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: cold flow starts emitting only when collector begins.
• 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

• Cold flow starts emitting only when collector begins.
• Each collector gets its own execution of flow block.
• Different from hot streams like SharedFlow.
• 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.