Kotlin when Expression Explained (2025 Guide to Smarter Conditional Logic)
Replace clunky if..else chains with Kotlin’s when expression for cleaner, more readable conditional logic! 🌟 It’s a powerful, flexible tool that selects a code path based on conditions, acting like a modernized switch with superpowers. 🎉
- 🔍 Basic When: Match values for simple branching. ✅
- 📊 When as Expression: Return values directly. 📦
- ⚖️ Advanced Features: Ranges, type checks, and more. 🌈
Basic When Expression 🔍
The when expression matches a value against branches, executing the first matching block. It’s Kotlin’s sleek alternative to Java’s switch! 🌟
fun main() {
val day = 4 // 🔢 Day number
val result = when (day) { // 🌟 Match day
1 -> "Monday" // 📅
2 -> "Tuesday"
3 -> "Wednesday"
4 -> "Thursday"
5 -> "Friday"
6 -> "Saturday"
7 -> "Sunday"
else -> "Invalid day." // 🚫 Fallback
}
println(result) // Outputs: Thursday 📊
}
How It Works:
- 🔍 Single Check: The input (day) is evaluated once. ⚖️
- ➡️ Arrow Syntax: -> links conditions to results. 📌
- ✅ Match and Run: Executes the matching branch. 🚀
- 🚫 else: Handles unmatched cases, required for expressions. 📦
When as an Expression 📦
Like if, when can return a value when assigned to a variable, making it ultra-concise! 🎯 Use else to ensure all cases are covered.
fun main() {
val x: Any = 13.37 // 🔢 Any type
val result = when (x) { // 🌟 Type checking
is Int -> "is an Int" // ✅ Int match
!is Double -> "is not Double" // ❌ Not Double
is String -> "is a String" // 📜 String match
else -> "is none of the above" // 🚫 Fallback
}
println("$x $result") // Outputs: 13.37 is none of the above ✨
}
Expression Benefits:
- 📦 Return Value: Assigns results directly to variables. ✅
- ⚡ Compact: Replaces verbose conditionals. 🧹
- 🔍 Versatile: Handles types, values, and conditions. 🌈
Advanced When Features: Ranges, Multiple Values, and More 💡
The when expression shines with advanced capabilities like ranges, multiple value matching, and type checks. 🌟
- 📋 Multiple Values: Match multiple options (e.g., 1, 2, 3). 🔢
- 📏 Ranges: Use in or !in for ranges (e.g., in 1..10). 📊
- 🔎 Type Checks: Use is or !is for type matching. 🧩
- ⚡ Condition Blocks: Evaluate complex conditions without an argument. 🧠
fun main() {
val month = 3 // 🔢 Month number
when (month) { // 🌟 Match with ranges
1, 2, 3 -> println("Spring 🌸") // 📋 Multiple values
in 4..6 -> println("Summer ☀️") // 📏 Range
in 7..9 -> println("Fall 🍂")
in 10..12 -> println("Winter ❄️")
else -> println("Invalid Month 🚫")
} // Outputs: Spring 🌸 📊
val value: Any = "Kotlin" // 🔢 Any type
when (value) { // 🌟 Type checking
is String -> println("String of length ${value.length}") // 📜
is Int -> println("Integer: $value")
else -> println("Unknown type")
} // Outputs: String of length 6 📊
}
Advanced Tip: Combine ranges and type checks for powerful, readable logic! 🚀
When Without an Argument 🧠
Use when without an argument to evaluate arbitrary conditions, replacing complex if-else ladders. ⚖️
fun main() {
val score = 85 // 🔢 Student score
val grade = when { // 🌟 No argument
score >= 90 -> "A" // 🥇
score >= 80 -> "B" // 🥈
score >= 70 -> "C" // 🥉
else -> "D" // 📉
}
println("Grade: $grade") // Outputs: Grade: B 📊
}
No-Argument Benefit: Simplifies condition-heavy logic into a clean, expressive format. 🧹
When with Null Safety 🚨
Kotlin’s null safety integrates seamlessly with when to handle nullable types safely. 🛡️
fun main() {
val name: String? = null // 🌈 Nullable string
when (name) { // 🔍 Check for null
null -> println("Name is null") // 🚫
else -> println("Name: $name, Length: ${name.length}") // ✅
} // Outputs: Name is null 📊
}
Null Safety Tip: Use when with null checks or combine with ?. and ?: for robust handling. ⚡
Provided Example: If vs. When 📝
The provided code compares if and when, showcasing when’s elegance. Here’s an enhanced version with detailed comments:
fun main() {
// 🛠️ If Statements
var age = 17 // 🔢 Age input
if (age >= 21) { // 🌟 Check drinking age
print("Now you may drink in the US 🍺") // 🥂
} else if (age >= 18) { // 🔍 Check voting age
print("Now you may vote 🗳️") // 🗳️
} else if (age >= 16) { // 🔍 Check driving age
print("You now may drive 🚗") // 🚗
} else { // 🛑 Fallback
print("You're too young 👶") // 👶
} // Outputs: You now may drive 🚗 📊
// 🌈 When Expression
var season = 3 // 🔢 Season number
when (season) { // 📅 Match season
1 -> println("Spring 🌸")
2 -> println("Summer ☀️")
3 -> println("Fall 🍂")
4 -> println("Winter ❄️")
else -> println("Invalid Season 🚫")
} // Outputs: Fall 🍂 📊
// 📏 When with ranges
var month = 3 // 🔢 Month number
when (month) { // 🌟 Match with ranges
1, 2, 3 -> println("Spring 🌸") // 📋 Multiple values
in 4..6 -> println("Summer ☀️") // 📏 Range
in 7..9 -> println("Fall 🍂")
in 10..12 -> println("Winter ❄️")
else -> println("Invalid Month 🚫")
} // Outputs: Spring 🌸 📊
// 🎯 If-to-When Challenge
when (age) { // 🌟 Convert if to when
!in 0..20 -> print("Now you may drink in the US 🍺") // 🥂
in 18..20 -> print("Now you may vote 🗳️") // 🗳️
16, 17 -> print("You now may drive 🚗") // 🚗
else -> print("You're too young 👶") // 👶
} // Outputs: You now may drive 🚗 📊
}
Comparison Insight: when is more concise and readable for value matching or range checks compared to if. ⚖️
Practical Use Case: Menu System 🍽️
Use when to build a user-friendly menu system for apps or scripts. 🛠️
fun main() {
val choice = 2 // 🔢 User menu selection
val action = when (choice) { // 🌟 Match menu option
1 -> "Start Game 🎮"
2 -> "View Scores 🏆"
3 -> "Settings ⚙️"
4 -> "Exit 🚪"
else -> "Invalid Option 🚫"
}
println(action) // Outputs: View Scores 🏆 📊
}
Menu Tip: Combine when with user input for interactive, dynamic systems. 🎯
when vs. if vs. Java switch ⚖️
Compare when with if and Java’s switch to choose the right tool. 📊
fun main() {
val code = 2 // 🔢 Input code
// Using if
val ifResult = if (code == 1) "Low" else if (code == 2) "Medium" else "High"
// Using when
val whenResult = when (code) { // 🌟 Cleaner
1 -> "Low"
2 -> "Medium"
else -> "High"
}
println("if: $ifResult") // Outputs: if: Medium 📊
println("when: $whenResult") // Outputs: when: Medium 📊
}
- 🔍 when: Best for value matching, ranges, or type checks. 🌈
- 🛠️ if: Ideal for complex or nested conditions. 🧩
- 🚫 Java switch: Less flexible, no ranges or type checks, requires break. 📜
Comparison Tip: Use when for clean, multi-branch logic; reserve if for intricate conditions. ⚖️
Best Practices for When Expressions ✅
- 🧹 Keep It Simple: Use when for value matching or ranges to avoid clutter. 🌈
- ✅ Include else: Ensure all cases are covered when assigning to variables. 🚫
- 📏 Use Ranges: Leverage in and !in for numeric or enum ranges. 🔢
- 🔎 Type Safety: Use is for type checks with smart casts. 🧩
- 🛡️ Null Handling: Check for null explicitly in when branches. 🚨
- 📝 Readable Code: Add comments for complex when logic to aid maintenance. 🧠
Frequently Asked Questions (FAQ) ❓
- Why use when instead of if? 🤔
when is more concise for value matching, ranges, or type checks, reducing boilerplate compared to if-else ladders. 🌈 - Is else mandatory in when expressions? 🚫
Yes, when assigning when to a variable to ensure all paths return a value; optional otherwise. 📦 - Can when handle ranges and types? 📊
Yes, use in for ranges (e.g., in 1..10) and is for type checks (e.g., is String). 🔎 - How does when differ from Java’s switch? ⚖️
when supports ranges, type checks, and expressions, doesn’t need break, and is more flexible than switch. 📜 - Can when be used without an argument? 🧠
Yes, when without an argument evaluates conditions like if-else, ideal for complex logic. ⚡ - How does when handle nulls? 🚨
Check null explicitly in a branch or use null-safe operators (?., ?:) for nullable inputs. 🛡️
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