Kotlin Best Practices: Essential Mistakes to Avoid Today

Kotlin has revolutionized JVM development by offering a concise, expressive, and safe alternative to Java. However, the transition from other languages often leads developers to carry over legacy habits that undermine Kotlin's core strengths. Whether you are building Android applications or server-side microservices, adhering to idiomatic patterns is crucial for performance and maintainability. This guide explores common Kotlin mistakes and provides actionable best practices to help you write professional, high-quality code.

Mastering Null Safety

One of Kotlin's primary selling points is its robust type system, which treats nullability as a first-class citizen. Despite this, many developers inadvertently bypass these protections, leading to runtime crashes.

The Problem with the Not-Null Assertion Operator

The !! operator is often called the "null-pointer exception operator" for a reason. Using it forces a value to be non-null, throwing a NullPointerException if it is actually null. This defeats the purpose of Kotlin's type system.

Instead of using !!, favor safe calls, the Elvis operator, or idiomatic scope functions:

// Avoid this
val length = name!!.length

// Use this
val length = name?.length ?: 0

Overusing Lateinit

While lateinit is useful for dependency injection or unit testing, it should not be a default choice. If you find yourself using lateinit frequently, consider whether your class design could be improved by using constructor injection or nullable types with a default value. If you must use it, always check isInitialized before accessing the property to avoid UninitializedPropertyAccessException.

Embracing Idiomatic Kotlin

Writing "Java in Kotlin" is a common trap. Kotlin provides powerful language features that make code more readable and less error-prone if used correctly.

Scope Functions for Clarity

Kotlin provides let, run, with, apply, and also. A common mistake is using these functions where a simple if statement or a variable assignment would be clearer. Use apply for object configuration, let for null checks, and also for side effects like logging.

// Clear and idiomatic
val user = User().apply {
    name = "Alice"
    age = 30
}

Prefer Immutable Collections

Mutable state is a frequent source of bugs in concurrent applications. Always prefer List over MutableList unless you have a specific reason to modify the collection. Use the copy() method for data classes to create modified versions of objects rather than mutating existing ones.

Efficient Coroutine Management

Coroutines are powerful, but they require careful handling to avoid memory leaks and performance bottlenecks.

Blocking the Main Thread

Never perform blocking operations (like I/O or heavy computation) inside a coroutine launched on Dispatchers.Main. Always use withContext(Dispatchers.IO) for blocking tasks. Failing to do so will freeze your application's UI or degrade server throughput.

Improper Scope Usage

GlobalScope is almost always the wrong choice because it creates coroutines that are not bound to any lifecycle, leading to potential memory leaks. Always use structured concurrency by defining a CoroutineScope tied to the lifecycle of your component, such as viewModelScope in Android.

Designing for Maintainability

Writing code that works is only half the battle; writing code that is easy to change is the other half.

Avoid Massive Data Classes

Data classes are excellent for holding state, but they should not become "god objects." If a data class has too many properties, it is likely doing too much. Break it down into smaller, cohesive classes to improve readability and testability.

Leverage Extension Functions Wisely

Extension functions are a powerful tool to clean up your API, but they can be abused. Do not use them to hide complex logic or to make private class members accessible. Use them to provide utility methods that improve the readability of existing types.

Conclusion

Writing excellent Kotlin code is about embracing the language's philosophy rather than just its syntax. By avoiding null-safety bypasses, utilizing idiomatic scope functions, managing coroutines responsibly, and favoring immutability, you can significantly improve the quality of your software. Start by refactoring one module at a time, focusing on removing !! operators and ensuring your coroutine scopes are correctly defined.

Frequently Asked Questions

Should I use 'var' or 'val' by default?

Always use val (immutable) by default. Only use var (mutable) when you have a clear, justifiable need to change the variable's value later.

Is it ever okay to use '!!'?

It is rarely recommended. The only acceptable use case is in testing code where you are certain a value is non-null and want the test to fail explicitly if it is not.

How do I decide between 'let' and 'run'?

Use let when you want to execute a block of code on a non-null object and return a result. Use run when you want to execute a block of code on an object and return the result of the block, often used for object initialization or transformation.

Why is structured concurrency important?

Structured concurrency ensures that coroutines are launched within a specific scope, meaning they are automatically cancelled when that scope is destroyed, preventing memory leaks and orphaned background tasks.