appendText

Appends text to the content of this file using UTF-8 or the specified charset.

fun File.appendText(text: String, charset: Charset = Charsets.UTF_8)(source)

import java.io.File
import java.nio.charset.Charset

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

    // Append a line using the default UTF-8 charset
    file.appendText("Hello, world!\n")

    // Append another line with a specified charset
    file.appendText("Goodbye, world!\n", Charset.forName("UTF-8"))
}

Source