Takes two directory names as arguments, iterates through the files in those directories, adds their content to lists of lists and prints what it has found.

gottaReadEmAll.kt

import java.io.File

var file: MutableList<List<String>> = ArrayList() //list of lists
var file2: MutableList<List<String>> = ArrayList() //list of lists

fun main(args: Array<String>) {
    file  = readDir(args[0])
    file2 = readDir(args[1])

    for (e in file) println(e)
    for (e in file2) println(e)
}

fun readDir(dir: String): MutableList<List<String>> {
    val readFiles: MutableList<List<String>> = ArrayList()
    File(dir).walkTopDown().forEach {
        println(it.toString())
        if (File(it.toString()).extension == "txt") { //balks at directory names
            readFiles.add(File(it.toString()).readLines()) //add list of lines to list of lists
        }
    }
    return readFiles
}