Kotlin Language Reference

I/O

Output

stdout

print(...) to print something without a newline, and println(...) to print with a newline at the end. println() with no args to start a new line.

stderr

Nothing built in to Kotlin to print to stderr. Use Java functions when running on the JVM.

System.err.println("Error!")

Logging

Use available Java logging libraries. Generally will look like this:

class MyClass {
    companion object {
        val LOG = Logger.getLogger(MyClass::class.java.name) 
    }

    fun foo() {
        LOG.warning("Uh-oh")
    }
}

In Android:

Log.w("Something bad")

Command Line Arguments

fun main(args: Array<String>) {
    println(args.size)
    println(args.contentToString())
}

Output from input “x y z”:

3
[x, y, z]

Text Input

val input = readln()
val i = readln().toIntOrNull()

Use readlnOrNull() to return null if end of input is reached, otherwise readln() throws an exception.

Scanner

Usually it is best to use a Java Scanner.

import java.util.Scanner

val scanner = Scanner(System.`in`)

val line = scanner.nextLine()
val i = scanner.nextInt()

scanner.close()

Call close when done or take advantage of AutoCloseable resource.

Scanner(System.`in`).use { scanner ->
    //...
}