🧭 Kotlin – When Expression: The Modern Switch Statement

🧲 Introduction – Why Learn Kotlin when Expression?

The traditional switch statement in Java is limited and verbose. Kotlin replaces it with the much more powerful and flexible when expression. It allows pattern matching, range checks, type checking, and value returns—all in a clean, readable way.

🎯 In this guide, you’ll learn:

• How to use when as a switch alternative
• Match values, ranges, and conditions
• Return values from when expressions
• Handle type checks with smart casting

---

🔀 What Is a when Expression in Kotlin?

The when expression checks a value against multiple conditions and executes the matching block. It’s Kotlin’s answer to the Java switch.

✅ Basic Syntax:

val day = 3

when (day) {
    1 -> println("Monday")
    2 -> println("Tuesday")
    3 -> println("Wednesday")
    else -> println("Invalid day")
}

🟢 Output:

Wednesday

---

🔁 when as an Expression (Return Value)

You can assign the result of a when to a variable:

val number = 2

val result = when (number) {
    1 -> "One"
    2 -> "Two"
    3 -> "Three"
    else -> "Unknown"
}

println(result)

🟢 Output:

Two

---

🔢 Matching Multiple Values in a Case

val letter = 'A'

when (letter) {
    'A', 'E', 'I', 'O', 'U' -> println("Vowel")
    else -> println("Consonant")
}

🟢 Output:

Vowel

---

🧮 Using Ranges and Collections

val age = 25

when (age) {
    in 0..12 -> println("Child")
    in 13..19 -> println("Teenager")
    in 20..59 -> println("Adult")
    else -> println("Senior")
}

✅ Use in keyword to check if a value lies within a range.

---

🔍 Type Checking in when

You can use is to perform type checks and automatically smart-cast.

fun checkType(x: Any) {
    when (x) {
        is Int -> println("Integer: ${x + 1}")
        is String -> println("String of length ${x.length}")
        else -> println("Unknown type")
    }
}

🟢 Output:

String of length 6 // if you pass "Kotlin"

---

⚠️ When Without Argument (Condition-Based)

You can omit the input and write conditions directly:

val score = 85

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

✅ Great when multiple unrelated conditions are needed.

---

❌ Fallthrough Doesn’t Happen in Kotlin

Each when case is isolated—no fallthrough like in Java switch.

val x = 1
when (x) {
    1 -> println("One") // only this runs
    2 -> println("Two")
}

---

🧠 Best Practices

Practice	Benefit
Prefer when over nested if	Cleaner and more readable
Use else block	Handle all unexpected inputs
Return value directly	Makes code concise and expressive
Avoid fallthrough logic	Kotlin doesn’t allow it

---

🚫 Common Mistakes

❌ Mistake	✅ Solution
Missing else in expression use	Always include else if used as value
Using break like in Java	Not needed; each case is isolated
Using when on incompatible types	Match on compatible values or use type checks

---

📌 Summary – Recap & Next Steps

The when expression is Kotlin’s powerful alternative to Java’s switch. It supports value matching, condition checks, ranges, types, and acts as an expression returning results—making it a clean, smart, and safe tool in your Kotlin toolkit.

🔍 Key Takeaways:

• when replaces switch and supports multiple conditions.
• Can be used as a statement or to return a value (expression).
• Works with values, ranges, collections, and type checks.
• No fallthrough behavior—each case is self-contained.

⚙️ Practical Use:
Used in UI control, API status handling, type-based logic, and form validation in Android, backend, and multiplatform Kotlin applications.

---

❓ FAQs – Kotlin When Expression

❓ How is when different from Java’s switch?
✅ when supports value matching, range checking, type checking, and condition evaluation—without fallthrough or break.

---

❓ Can I return a value from a when expression?
✅ Yes. Use when as an expression to assign a value:

val result = when (x) { 1 -> "One" else -> "Other" }

---

❓ Is the else block mandatory in Kotlin when?
✅ It’s required if the when is used as an expression (to return a value), but optional if used as a statement.

---

❓ Can I check for multiple values in one case?
✅ Yes. Use commas to separate values:

when (letter) {
    'A', 'E', 'I' -> println("Vowel")
}

---

❓ Can I use when without a subject (input)?
✅ Yes. Use it for complex boolean conditions:

when {
    x > 10 -> ...
    x < 0 -> ...
}

---

Share Now :