When

Instead of the widely used switch statement, Kotlin provides the more flexible and clear when construction. It can be used either as a statement or as an expression.

When Statement

fun main() {
    cases("Hello")
    cases(1)
    cases(0L)
    cases(MyClass())
    cases("hello")
}

fun cases(obj: Any) {                                
    when (obj) {                                     // 1   
        1 -> println("One")                          // 2
        "Hello" -> println("Greeting")               // 3
        is Long -> println("Long")                   // 4
        !is String -> println("Not a string")        // 5
        else -> println("Unknown")                   // 6
    }   
}

class MyClass

1. This is a when statement.
2. Checks whether obj equals to 1.
3. Checks whether obj equals to "Hello".
4. Performs type checking.
5. Performs inverse type checking.
6. Default statement (might be omitted).

Note that all branch conditions are checked sequentially until one of them is satisfied. So, only the first suitable branch will be executed.

When Expression

fun main() {
    println(whenAssign("Hello"))
    println(whenAssign(3.4))
    println(whenAssign(1))
    println(whenAssign(MyClass()))
}

fun whenAssign(obj: Any): Any {
    val result = when (obj) {                   // 1
        1 -> "one"                              // 2
        "Hello" -> 1                            // 3
        is Long -> false                        // 4
        else -> 42                              // 5
    }
    return result
}

class MyClass

1. This is a when expression.
2. Sets the value to "one" if obj equals to 1.
3. Sets the value to one if obj equals to "Hello".
4. Sets the value to false if obj is an instance of Long.
5. Sets the value 42 if none of the previous conditions are satisfied. Unlike in when statement, the default branch is usually required in when expression, except the case when the compiler can check that other branches cover all possible cases.