How to read a file in Kotlin

1. Introduction

In this tutorial, we are going to present several ways to read a file in Kotlin. Kotlin is a cross-platform, general-purpose programming language that is becoming more and more popular since it is now the preferred language for Android app development.

Read a file in Java

2. Create a file

First, let's create a file for testing purposes with a name frontbackend.txt and the following content:

Tutorials:
    - one
    - two
    - three

We can do it using the touch command on Linux or with a Notepad on Windows. For a sake of simplicity let's put it into a folder with our sample Kotlin files.

3. Read a file using `InputStream`

3.1. Read all lines at once

The first approach use BufferedReader and use method:

import java.io.File
import java.io.InputStream

fun main(args: Array<String>) {
    val inputStream: InputStream = File("frontbackend.txt").inputStream()

    val content = inputStream.bufferedReader().use { it.readText() }

    println(content)
}

Here:

first, we get an InputStream from File object,
next, using inputStream.bufferedReader() we get a BufferedReader responsible for reading streams,
finally, to read the content we use the use{ it.readTest() }command which is the same as Closeable.use(Reader.readText()) in Java,
note that the last operation will automatically close the InputStream.

The output will be:

Tutorials:
    - one
    - two
    - three

3.2. Read line by line

In the following example we make use of Reader.useLines(...) method to read a file line by line:

import java.io.File
import java.io.InputStream

fun main(args: Array<String>) {
    val inputStream: InputStream = File("frontbackend.txt").inputStream()
    val out = mutableListOf<String>()

    inputStream.bufferedReader().useLines { lines -> lines.forEach { out.add(it) } }

    out.forEach { println(it) }
}

In this example:

first, we read an InputStream of a frontbackend.txt file,
next, we created a mutable list that will hold file content lines,
then, we read all lines and put them in our collection using the bufferedReader().useLines method,
the final step is to print all lines from our collection.

If everything is correct you should get the same result as in the first approach:

Tutorials:
    - one
    - two
    - three

4. Conclusion

In this short article, we presented ways to read a file in Kotlin. The first method read all file content at once, second read line by line.