---
title: "If Not Now, When?"
url: "https://www.bekk.christmas/post/2019/03/if-not-now-when"
authors: ["Øyvind Midtbø"]
published: "2019-12-03"
updated: "2024-11-07T13:05:28Z"
language: "en-US"
type: "article"
tags: ["Kotlin"]
---

# If Not Now, When?

This article will give you a brief introduction to the Kotlin `when` expression and how to use it.

If you're used to Java, you’re probably used to the `switch` statement:

```
switch (number) { case 0: System.out.println("zero"); break; case 1: System.out.println("one"); break; case 2: System.out.println("two"); break; default: System.out.println("something else"); break; } 
```

In Kotlin we have the `when` statement – which can be described as `switch` on steroids. `when` can be used both as an expression and as a statement.

The first example is a basic `when` statement:

```
when (number) { 0 -> println("zero") 1 -> println("one") 2 -> println("two") else -> println("something else") } 
```

You can already see that it’s cleaner than the `switch` statement.

It's possible to specify ranges as well:

```
when (number) { 0 -> println("zero") 1, 2 -> println("one or two") in 3..10 -> println("between three and ten") !in 11..20 -> println("not between eleven and twenty") else -> println("something else") } 
```

You can also check the type of the parameter with the `is` expression. Here you’ll also benefit from the _smart cast_ in Kotlin, which means that the `is` keyword implicitly casts `value` to the given type.

```
when (value) { is String -> println(value.replaceFirst("a", "b")) is Int -> println(value * 2) is Boolean -> println(!value) } 
```

In these examples we've seen the use of `when` as a statement. As mentioned earlier, `when` can also be used as an expression, as we can see in this example:

```
val numberAsString = when (number) { 1 -> "one" 2 -> "two" else -> "something else" } println("The number is $numberAsString") 
```

If the compiler can guarantee that `when` always returns a value there is no need for `else`:

```
fun example(trueOrFalse: Boolean) { when (trueOrFalse) { true -> println("true") false -> println("false") } } 
```

You can even use `when` without an argument. When used without an argument it acts more like an if-else chain:

```
when { carType == "Porsche" -> println("fast car") carType == "Jeep" -> println("beach car") numberOfCars > 5 -> println("many cars, but not Porsche or Jeep") } 
```

As you can see, the Kotlin `when` statement can be used in many different ways. Happy coding!

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