use Function for Resources in Kotlin

Introduction

use Function for Resources 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.

The use extension closes Closeable resources automatically, even when exceptions occur. 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

• The use extension closes Closeable resources automatically, even when exceptions occur.
• It is Kotlin’s equivalent of Java’s try-with-resources.
• Always prefer use when working with files, streams, and database connections.

Syntax

File("data.txt").bufferedReader().use { reader ->
    reader.readLine()
}

use Function for Resources in Kotlin Example Program in Kotlin

import java.io.File

fun main() {
    val file = File("sample.txt")
    file.writeText("Line 1
Line 2")
    file.bufferedReader().use { reader ->
        reader.forEachLine { line -> println(line) }
    }
    file.delete()
}

Sample Output

Line 1
Line 2

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. The use extension closes Closeable resources automatically, even when exceptions occur.

3. 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

Common Mistakes

Key Points

• The use extension closes Closeable resources automatically, even when exceptions occur.
• It is Kotlin’s equivalent of Java’s try-with-resources.
• Always prefer use when working with files, streams, and database connections.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• This example creates a temporary file in the working directory and deletes it at the end.
• File examples assume a writable working directory; adjust paths for your environment.