How to Read File Line by Line in Kotlin Easily

In Kotlin, you can read a file line by line using File(path).forEachLine {} or File(path).readLines(). These methods let you process each line one at a time or get all lines as a list.

Syntax

Here are two common ways to read a file line by line in Kotlin:

• File(path).forEachLine { line -> }: Processes each line one by one inside the lambda.
• File(path).readLines(): Reads all lines into a list of strings.

You need to import java.io.File to use these.

kotlin

import java.io.File

// Using forEachLine
File("filename.txt").forEachLine { line ->
    println(line)
}

// Using readLines
val lines = File("filename.txt").readLines()
for (line in lines) {
    println(line)
}

Example

This example shows how to read a file named example.txt line by line and print each line to the console.

kotlin

import java.io.File

fun main() {
    val fileName = "example.txt"
    File(fileName).forEachLine { line ->
        println(line)
    }
}

Output

Hello, world!
This is line 2.
This is line 3.

Common Pitfalls

Common mistakes when reading files line by line in Kotlin include:

• Not importing java.io.File.
• Using a wrong file path or missing the file, causing exceptions.
• Reading large files with readLines() which loads all lines into memory at once, possibly causing memory issues.
• Not handling exceptions like FileNotFoundException.

Always ensure the file exists and handle exceptions if needed.

kotlin

import java.io.File
import java.io.FileNotFoundException

fun main() {
    val fileName = "missing.txt"
    try {
        File(fileName).forEachLine { line ->
            println(line)
        }
    } catch (e: FileNotFoundException) {
        println("File not found: $fileName")
    }
}

Output

File not found: missing.txt

Quick Reference

Summary tips for reading files line by line in Kotlin:

• Use forEachLine for memory-efficient line processing.
• Use readLines if you need all lines at once.
• Always import java.io.File.
• Handle exceptions for missing or unreadable files.

Key Takeaways

Use File(path).forEachLine { } to read and process each line efficiently.

readLines() returns all lines as a list but uses more memory.

Always import java.io.File to work with files in Kotlin.

Handle exceptions to avoid crashes when files are missing.

Check your file path carefully to ensure the file can be found.