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Kotlin - Conditional Statements
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  When-statement argument matching

When given an argument, the when-statement matches the argument against the branches in sequence. The matching is done using the == operator which performs null checks and compares the operands using the equals function. The first matching one will be executed.

when (x) {
    "English" -> print("How are you?")
    "German" -> print("Wie geht es dir?")
    else -> print("I don't know that language yet :(")
}

The when statement also knows some more advanced matching options:

val names = listOf("John", "Sarah", "Tim", "Maggie")
when (x) {
    in names -> print("I know that name!")
    !in 1..10 -> print("Argument was not in the range from 1 to 10")
    is String -> print(x.length) // Due to smart casting, you can use String-functions here
}
  When-statement as expression

Like if, when can also be used as an expression:

val greeting = when (x) {
    "English" -> "How are you?"
    "German" -> "Wie geht es dir?"
    else -> "I don't know that language yet :("
}
print(greeting)

To be used as an expression, the when-statement must be exhaustive, i.e. either have an else branch or cover all possibilities with the branches in another way.
  Standard if-statement 
val str = "Hello!"
if (str.length == 0) {
    print("The string is empty!")
} else if (str.length > 5) {
    print("The string is short!")
} else {
    print("The string is long!")
}

The else-branches are optional in normal if-statements.
  If-statement as an expression

If-statements can be expressions:

val str = if (condition) "Condition met!" else "Condition not met!"

Note that the else-branch is not optional if the if-statement is used as an expression.

This can also been done with a multi-line variant with curly brackets and multiple else if statements.

val str = if (condition1){
    "Condition1 met!" 
   } else if (condition2) {
    "Condition2 met!" 
   } else {
    "Conditions not met!"
   }

TIP: Kotlin can infer the type of the variable for you but if you want to be sure of the type just annotate it on the variable like: val str: String = this will enforce the type and will make it easier to read.
  When-statement instead of if-else-if chains

The when-statement is an alternative to an if-statement with multiple else-if-branches:

when {
    str.length == 0 -> print("The string is empty!")
    str.length > 5  -> print("The string is short!")
    else            -> print("The string is long!")
}

Same code written using an if-else-if chain:

if (str.length == 0) {
    print("The string is empty!")
} else if (str.length > 5) {
    print("The string is short!")
} else {
    print("The string is long!")
}

Just like with the if-statement, the else-branch is optional, and you can add as many or as few branches as you like. You can also have multiline-branches:

when {
    condition -> {
        doSomething()
        doSomeMore()
    }
    else -> doSomethingElse()
}
  When-statement with enums

when can be used to match enum values:

enum class Day {
    Sunday,
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday
}

fun doOnDay(day: Day) {
    when(day) {
        Day.Sunday ->     // Do something
        Day.Monday, Day.Tuesday ->     // Do other thing
        Day.Wednesday ->  // ...
        Day.Thursday ->   // ...
        Day.Friday ->     // ...
        Day.Saturday ->   // ...
    }
}

As you can see in second case line (Monday and Tuedsay) it is also possible to combine two or more enum values.

If your cases are not exhaustive the compile will show an error. You can use else to handle default cases:

fun doOnDay(day: Day) {
    when(day) {
        Day.Monday ->     // Work
        Day.Tuesday ->    // Work hard
        Day.Wednesday ->  // ...
        Day.Thursday ->   //
        Day.Friday ->     //
        else ->           // Party on weekend
    }
}

Though the same can be done using if-then-else construct, when takes care of missing enum values and makes it more natural.

Check here (opens new window) for more information about kotlin enum

Remarks

In contrast to Java's switch, the when statement has no fall-through behavior. This means, that if a branch is matched, the control flow returns after its execution and no break statement is required. If you want to combine the bahaviors for multiple arguments, you can write multiple arguments separated by commas:

when (x) {
    "foo", "bar" -> println("either foo or bar")
    else -> println("didn't match anything")
}



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