kotlin read a file line by line (readLines, useLines, forEachLine)

In Kotlin/JVM, File("data.txt").readLines() returns all lines of a file as a List<String>, and File("data.txt").forEachLine { println(it) } processes them one at a time. For large files use useLines { lines -> ... }, which gives you a lazy Sequence<String> and closes the file when the block ends. All of them read UTF-8 by default and drop the line terminators.

Examples

import java.io.File

fun main() {
    val file = File("data.txt")

    // Small files: everything in memory as List<String>
    val lines = file.readLines()
    println("${lines.size} lines, first: ${lines.firstOrNull()}")

    // Any size: one line at a time
    file.forEachLine { line -> println(line.uppercase()) }

    // Any size, with sequence operations; the file is closed after the block
    val errors = file.useLines { seq -> seq.filter { "ERROR" in it }.count() }
    println("$errors error lines")

    // Whole file as one String
    val text = file.readText()
    println(text.length)
}

Choosing a function

Pitfalls

• Escaping the sequence. The sequence from useLines is only valid inside the block: returning it and iterating later fails because the reader is already closed. Turn it into a list (seq.toList()) or compute the result inside the block.
• Missing files. File("missing.txt").readLines() throws java.io.FileNotFoundException. Check file.exists() first or catch the exception.
• Relative paths. A relative path is resolved against the working directory of the JVM, which for ./gradlew run is the subproject folder, not necessarily the folder you expect. Print file.absolutePath when a file is not found.
• Kotlin Multiplatform. java.io.File exists only on the JVM (and Android); common code needs a library such as kotlinx-io or Okio.
• Numbers in lines must be parsed, see convert a string to an Int.

Confirm it worked

Print lines.size and compare it with the line count your editor shows for the file. A final newline does not add an empty entry, but blank lines inside the file come back as empty strings; drop them with lines.filter { it.isNotBlank() } if they do not matter.

FAQ