Get File Extension in Kotlin

In Kotlin, to extract or get the file extension, use extension property of the java.io.File class. File.extension is a String value. It returns the extension if the file has an extension, or an empty string if the file does not have an extension.

Examples

1. Get File Extension

In this example, we will take a file and get its extension.

Main.kt

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import java.io.File

fun main(args: Array<String>) {
    var f = File("file.txt")
    print(f.extension)
}

Make sure file.txt is in the project root directory.

Output

txt

2. Get File Extensions of All Files in a Folder

In this example, we will walk through all the files in the project root directory (if you are running the program from IDE) or all the files in the directory from which you are running the Kotlin program in command line, and print the extension of each file.

Also, we are checking if the current File object is a file using File.isFile() method.

Main.kt

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import java.io.File

fun main(args: Array<String>) {
    val fileName = "data.txt"
    var file = File(fileName)

    File("./").walk().forEach {
        println(it.extension + " is the extension of " + it.name)
    }
}

Output

name is the extension of .name
xml is the extension of codeStyleConfig.xml
xml is the extension of kotlinc.xml
xml is the extension of KotlinJavaRuntime.xml
xml is the extension of misc.xml
xml is the extension of modules.xml
xml is the extension of workspace.xml
txt is the extension of file.txt
iml is the extension of Kotlin Basics.iml
jar is the extension of kotlin-reflect-sources.jar
jar is the extension of kotlin-reflect.jar
jar is the extension of kotlin-stdlib-jdk7-sources.jar
jar is the extension of kotlin-stdlib-jdk7.jar
jar is the extension of kotlin-stdlib-jdk8-sources.jar
jar is the extension of kotlin-stdlib-jdk8.jar
jar is the extension of kotlin-stdlib-sources.jar
jar is the extension of kotlin-stdlib.jar
jar is the extension of kotlin-test-sources.jar
jar is the extension of kotlin-test.jar
class is the extension of HelloWorldKt.class
class is the extension of KotlinExampleKt.class
class is the extension of KotlinForLoopKt.class
class is the extension of KotlinMainKt.class
class is the extension of KotlinStringLengthKt.class
class is the extension of KotlinStringToIntegerKt.class
kotlin_module is the extension of Kotlin Basics.kotlin_module
kt is the extension of HelloWorld.kt
kt is the extension of KotlinExample.kt
kt is the extension of KotlinForLoop.kt
kt is the extension of KotlinMain.kt
kt is the extension of KotlinStringLength.kt
kt is the extension of KotlinStringToInteger.kt

Conclusion

In this Kotlin Tutorial, we learned how to get the extension for a file, or for all files in a folder, using File.extension.