KOTLIN Tutorial

🔄 when Expression in Kotlin

when is Kotlin’s version of the switch-case, but much more flexible and powerful. It can be used as a statement or an expression to return values. Let’s dive in!

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1️⃣ Basic Usage

Let’s see how when works with simple conditions:

fun main() {
    val day = 3

    when (day) {
        1 -> println("Monday 😴")
        2 -> println("Tuesday 💤")
        3 -> println("Wednesday 🐫")
        4 -> println("Thursday 📚")
        5 -> println("Friday 🥳")
        else -> println("Weekend! 🎉")
    }
}
  

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2️⃣ Multiple Options in a Single Case

You can group multiple values in one branch:

fun main() {
    val today = "Saturday"

    when (today) {
        "Saturday", "Sunday" -> println("Weekend Vibes! 😎")
        "Monday" -> println("Back to work 😩")
        else -> println("Regular day 😐")
    }
}
  

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3️⃣ when as an Expression

Just like if, when can also return a value:

fun main() {
    val score = 92

    val grade = when {
        score >= 90 -> "A 🌟"
        score >= 75 -> "B 👍"
        score >= 60 -> "C 🙂"
        else -> "D 😬"
    }

    println("Grade: $grade")
}
  

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4️⃣ Using when Without an Argument

You can use when without directly checking a variable:

fun main() {
    val age = 25

    when {
        age < 13 -> println("Child 👶")
        age in 13..19 -> println("Teenager 👦")
        age >= 20 -> println("Adult 🧑")
    }
}
  

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5️⃣ Real-World Use Case: Traffic Lights 🚦

Let’s simulate traffic signals:

fun main() {
    val signal = "Green"

    when (signal.lowercase()) {
        "red" -> println("STOP ❌")
        "yellow" -> println("SLOW DOWN ⚠️")
        "green" -> println("GO ✅")
        else -> println("Unknown Signal 🚧")
    }
}
  

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6️⃣ Summary

• ✔️ when replaces switch and supports ranges, types, expressions.
• ✔️ Use it with or without a subject.
• ✔️ Cleaner, smarter, and Kotlin-idiomatic.