Control Flow in Kotlin: When Expression

1. Why Use When?

The when expression in Kotlin is a powerful alternative to the traditional if-else-if ladder. It is used to:

Handle multiple conditional branches more clearly and concisely.
Improve readability compared to long nested if-else statements.
Match values, ranges, types, or conditions in a single construct.
Make code more maintainable and less error-prone.

2. How to Use When? (With Example)

The when expression evaluates a value or condition and executes the corresponding block.

fun main() {
    val day = 3
    val dayName = when(day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3 -> "Wednesday"
        4 -> "Thursday"
        5 -> "Friday"
        6 -> "Saturday"
        7 -> "Sunday"
        else -> "Invalid day"
    }
    println("Day: $dayName")
}

Output:

Day: Wednesday

3. Explain Each Form

3.1 Basic When

Matches a value against constants:

val color = "Red"
when (color) {
    "Red" -> println("Stop")
    "Green" -> println("Go")
    "Yellow" -> println("Caution")
    else -> println("Unknown color")
}

3.2 When with Multiple Conditions

You can group multiple conditions with commas:

val number = 3
when (number) {
    1, 3, 5, 7, 9 -> println("Odd number")
    2, 4, 6, 8, 10 -> println("Even number")
    else -> println("Out of range")
}

3.3 When as an Expression

When can return a value:

val score = 85
val grade = when {
    score >= 90 -> "A"
    score >= 75 -> "B"
    score >= 60 -> "C"
    else -> "Fail"
}
println("Grade: $grade")

3.4 When with Ranges and Types

You can check for ranges or types:

val x: Any = 42
when (x) {
    in 1..50 -> println("x is between 1 and 50")
    is String -> println("x is a string")
    else -> println("Unknown type or value")
}

4. Key Points

When is more readable and concise than long if-else-if chains.
It can be used both as a statement (side-effects) or an expression (returns value).
The else branch is optional if all possible values are covered.
Supports constants, ranges, types, and arbitrary boolean conditions.

5. Interview Questions

Why Use When?

Why would you use a when expression instead of multiple if-else statements?
How does using when improve code readability and maintainability?

How When Works?

Explain how Kotlin matches values or conditions in a when expression.
What is the difference between using when as a statement and when as an expression?
How can you use ranges or types in a when expression?