The Kotlin REPL

The Kotlin REPL (“Read-Evaluate-Print-Loop”) is a command-line interpreter that you use as a “playground” area to test your Kotlin code. To start a REPL session, just type kotlinc at your operating system command line, and you’ll see this:

> kotlinc
Welcome to Kotlin version 1.2.51 (JRE 1.8.0_181-b13)
Type :help for help, :quit for quit
>>> _

The prompt for the Kotlin REPL is >>>, but because I don’t like that I only show one > character for the prompt in this book.

Because the REPL is a command-line interpreter, it just sits there waiting for you to type something. Once you’re in the REPL, you can type Kotlin expressions to see how they work:

> 1 + 1

> 4 / 2

> 4 % 2

As those examples show, when you type simple expressions, the Kotlin REPL shows the result of the expression on the line after the prompt. For other expressions you may have to type the variable name you created to see its value:

> val x = 1 + 1

> x

You can also try Kotlin online at

Take the REPL for a spin

You’re going to use the Kotlin REPL a lot in this book, so go ahead and start it (with kotlinc) and experiment with it. Here are a few expressions you can try to see how it all works:

1 + 1
4 / 2
5 / 2
4 % 2
5 % 2
5 / 2.0
1 + 2 * 3
(1 + 2) * 3
val name = "John Doe"
"hello, world".take(5)
if (2 > 1) println("greater") else println("lesser")
for (i in 1..3) println(i)
listOf(1,2,3).forEach { println(it) }

As a little more advanced exercise, here’s how to define a class named Person, create an instance of it, and then show the value of the name field:

class Person(var name: String)
val p = Person("Kim")

Notice that you don’t need the new keyword when creating an instance of a class.

Use javaClass to see an object’s type

To see an object’s type in the REPL, call .javaClass on the instance:

> "foo".javaClass
class java.lang.String

> 1.javaClass

> 1.0.javaClass

Here’s a map:

// paste this into the repl
val map = mapOf(
    1 to "one",
    2 to "two"

> map.javaClass
class java.util.LinkedHashMap

Here’s a list:

> listOf(1,2,3).javaClass
class java.util.Arrays$ArrayList

The following examples on the result of intArrayOf show other calls you can make after javaClass:

> val x = intArrayOf(1,2,3)

> println(

> println(x.javaClass.kotlin)
class kotlin.IntArray

> println(x.javaClass.kotlin.qualifiedName)

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