Top 10 Mistakes in Kotlin and How to Avoid Them

Kotlin is a modern, concise, and powerful programming language, widely used for Android development, backend services, and multiplatform applications. However, developers—especially those coming from Java—often make common mistakes that can lead to performance issues, unexpected behavior, and maintainability problems.

In this article, we’ll explore the Top 10 Mistakes in Kotlin and provide best practices to avoid them, with bad and good examples.

1️⃣ Ignoring Null Safety (NullPointerException) 💥

❌ Mistake: Not Handling Null Values Properly

fun printLength(str: String) {
    println("Length: ${str.length}") // ❌ May throw NullPointerException
}

fun main() {
    val name: String? = null
    printLength(name!!) // ❌ Throws NullPointerException
}

✔ Issue: Using !! (force unwrap) ignores null safety, leading to crashes.

✅ Solution: Use Safe Calls and the Elvis Operator

fun printLength(str: String?) {
    println("Length: ${str?.length ?: "Unknown"}") // ✅ Safe handling of null
}

fun main() {
    val name: String? = null
    printLength(name) // ✅ No crash
}

✔ Best Practices:

• Use ?. (safe calls) to avoid null checks.
• Use ?: (Elvis operator) to provide default values.

2️⃣ Using var Instead of val Unnecessarily ⚠️

❌ Mistake: Using var for Immutable Variables

var username = "Alice"
username = "Bob" // ❌ Allowed, but unnecessary

✔ Issue: Using var when the value should not change.

✅ Solution: Use val for Immutable Variables

val username = "Alice" // ✅ Now `username` cannot be modified

✔ Best Practices:

• Use val by default.
• Use var only when reassignment is necessary.

3️⃣ Using Traditional if-else Instead of Expression Syntax 🎯

❌ Mistake: Using if-else Like Java

fun max(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}

✔ Issue: More verbose than needed.

✅ Solution: Use Expression Syntax for Conciseness

fun max(a: Int, b: Int) = if (a > b) a else b // ✅ More readable

✔ Best Practices:

• Use expression syntax for simple conditions.
• Makes the code shorter and cleaner.

4️⃣ Ignoring when Instead of if-else Chains 🔄

❌ Mistake: Using if-else Chains Instead of when

fun getType(value: Any): String {
    if (value is Int) return "Integer"
    else if (value is String) return "String"
    else return "Unknown"
}

✔ Issue: if-else chains reduce readability.

✅ Solution: Use when for Better Readability

fun getType(value: Any) = when (value) {
    is Int -> "Integer"
    is String -> "String"
    else -> "Unknown"
}

✔ Best Practices:

• Use when for multiple conditions.
• Improves code clarity and readability.

5️⃣ Not Using Data Classes for Simple Models 🏗️

❌ Mistake: Writing Boilerplate for Simple Models

class User(val name: String, val age: Int) {
    override fun toString() = "User(name=$name, age=$age)"
}

✔ Issue: Manually implementing toString(), equals(), and hashCode().

✅ Solution: Use data class for Automatic Implementations

data class User(val name: String, val age: Int) // ✅ Less code, more features

✔ Best Practices:

• Use data class for value objects.
• Automatically provides toString(), equals(), and copy().

6️⃣ Using List Instead of MutableList When Needed 🔄

❌ Mistake: Using Immutable List for Modification

fun addElement(list: List<String>) {
    list.add("New Item") // ❌ Compilation Error
}

✔ Issue: List in Kotlin is immutable by default.

✅ Solution: Use MutableList for Modifiable Lists

fun addElement(list: MutableList<String>) {
    list.add("New Item") // ✅ Allowed
}

✔ Best Practices:

• Use List for read-only collections.
• Use MutableList for modifiable collections.

7️⃣ Not Using apply, let, run, and also for Clean Code ✨

❌ Mistake: Repeating Object References

val user = User("Alice", 25)
user.name = "Bob"
user.age = 30

✔ Issue: Code is less readable.

✅ Solution: Use apply for Object Initialization

val user = User("Alice", 25).apply {
    name = "Bob"
    age = 30
}

✔ Best Practices:

• Use apply for object initialization.
• Use let, run, also for chaining operations.

8️⃣ Overlooking Default Arguments in Functions 🚀

❌ Mistake: Writing Multiple Overloaded Functions

fun greet(name: String) {
    println("Hello, $name!")
}

fun greet() {
    println("Hello, Guest!")
}

✔ Issue: Unnecessary overloaded functions.

✅ Solution: Use Default Arguments

fun greet(name: String = "Guest") {
    println("Hello, $name!")
}

✔ Best Practices:

• Use default arguments instead of function overloading.

9️⃣ Forgetting const for Compile-Time Constants 📌

❌ Mistake: Using val Instead of const

val PI = 3.1415 // ❌ Not optimized at compile time

✔ Issue: val is evaluated at runtime.

✅ Solution: Use const val for Compile-Time Constants

const val PI = 3.1415 // ✅ Optimized at compile time

✔ Best Practices:

• Use const for compile-time constants.

🔟 Not Using Coroutines for Asynchronous Operations ⏳

❌ Mistake: Using Threads Instead of Coroutines

fun main() {
    Thread {
        println("Running on another thread")
    }.start()
}

✔ Issue: Threads are heavyweight and less efficient.

✅ Solution: Use Kotlin Coroutines

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        println("Running on another coroutine")
    }
}

✔ Best Practices:

• Use launch for fire-and-forget coroutines.
• Use async for parallel computations.

🎯 Conclusion

Avoiding these common Kotlin mistakes will help you write clean, concise, and efficient code.

Quick Recap

✔ Avoid !! operator (use ?. and ?: instead)
✔ Use val by default (only use var when needed)
✔ Prefer expression syntax (if-else → when, max())
✔ Use data class for models
✔ Enable Kotlin’s coroutine features for async tasks

Keywords

Kotlin best practices, Kotlin mistakes, Kotlin errors, Kotlin coding guide, Kotlin tutorial.