Read Lines from File in Kotlin

Introduction

Read Lines from File is a fundamental concept every Kotlin developer should understand. File I/O reads and writes persistent data on disk. Kotlin wraps Java file APIs with extension functions for cleaner syntax.

ReadLines returns list of lines from a text file. 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

• readLines returns list of lines from a text file.
• forEachLine reads file line by line without loading all lines at once.
• Useful for logs and large text files.

Syntax

file.readLines()

Read Lines from File in Kotlin Example Program in Kotlin

import java.io.File

fun main(args: Array<String>) {
    val file = File.createTempFile("lines", ".txt")
    file.writeText("One\
Two\
Three")
    file.readLines().forEach { println(it) }
    file.deleteOnExit()
}

Sample Output

One
Two
Three

When to use

Use file I/O for reading config files, writing logs, or processing CSV/JSON stored on disk.

How it works

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

2. val file = File.createTempFile("lines", ".txt") assigns or updates a value used later in the program.

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

4. ReadLines returns list of lines from a text file.

5. 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: readLines returns list of lines from a text file.
• 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

• readLines returns list of lines from a text file.
• forEachLine reads file line by line without loading all lines at once.
• Useful for logs and large text files.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• File examples assume a writable working directory; adjust paths for your environment.
• Semicolons at the end of statements are optional in Kotlin.