Best Practices for Kotlin: Common Mistakes to Avoid

Kotlin is a powerful, expressive language that simplifies complex tasks, but its flexibility can lead to anti-patterns if not handled with care. Developers transitioning from Java often carry over habits that don't leverage Kotlin's unique features, while newer developers might struggle with the language's more advanced idioms. This article explores common mistakes in Kotlin development and provides actionable best practices to ensure your code is maintainable, performant, and idiomatic.

The Null Safety Trap

One of Kotlin's flagship features is null safety. However, developers frequently bypass this by using the not-null assertion operator (!!).

Avoiding the !! Operator

The !! operator forces a value to be non-null. If the value is null, it throws a NullPointerException, effectively defeating the purpose of Kotlin's type system.

Instead of using !!, prefer safe calls, the Elvis operator (?:), or let blocks. These approaches handle nullability gracefully without risking runtime crashes.

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

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

Efficient Collection Processing

Kotlin provides a rich set of collection operations, but using them inefficiently can lead to performance bottlenecks, especially with large datasets.

Chaining Operations Wisely

When processing collections, avoid multiple intermediate steps that create unnecessary lists. For example, chaining map and filter multiple times on a large list creates a new collection at every step. If you need to perform multiple operations, consider using Sequence to process items lazily.

// Inefficient for large lists
val result = list.filter { it > 10 }.map { it * 2 }

// More efficient for large datasets
val result = list.asSequence().filter { it > 10 }.map { it * 2 }.toList()

Scope Function Misuse

Kotlin's scope functions (let, run, apply, also, with) are powerful tools, but they are often misused, leading to code that is harder to read than standard procedural blocks.

When to Use Which Function

• apply: Use for object configuration.
• let: Use for null checks or executing a block on a non-null object.
• also: Use for side effects, like logging.

Avoid nesting scope functions. If you find yourself nesting let inside run, you have likely over-engineered the logic. A simple if statement or a standard variable assignment is often more readable.

Embracing Immutability

Immutability is a cornerstone of robust software. Kotlin makes it easy to declare immutable variables with val and immutable collections with listOf.

Prefer val over var

Default to val for all variables. Only use var when you explicitly need to reassign a value. This reduces the surface area for bugs caused by unexpected state changes. Furthermore, when working with collections, prefer the immutable interfaces (List, Set, Map) over their mutable counterparts (MutableList, etc.) whenever possible.

Structured Concurrency with Coroutines

Kotlin Coroutines are excellent for asynchronous programming, but they require careful management to avoid memory leaks and orphaned tasks.

Avoid GlobalScope

Never use GlobalScope in production code. It creates coroutines that are not bound to any lifecycle, making them difficult to cancel and prone to memory leaks. Instead, use CoroutineScope tied to the lifecycle of your component, such as viewModelScope in Android or a custom scope in a backend service.

// Avoid
GlobalScope.launch { /* ... */ }

// Prefer
class MyViewModel : ViewModel() {
    fun fetchData() {
        viewModelScope.launch { /* ... */ }
    }
}

Conclusion

Writing high-quality Kotlin code is about more than just making it work; it is about leveraging the language's features to write code that is safe, concise, and easy to maintain. By avoiding the !! operator, using sequences for large collections, picking the right scope functions, prioritizing immutability, and following structured concurrency, you can significantly improve your codebase. Start by auditing your current project for these common mistakes, and you will see immediate improvements in stability and readability.

FAQ

Why is the !! operator considered bad practice?

It forces a null check that bypasses the compiler's safety guarantees, leading to potential NullPointerExceptions at runtime.

When should I use a Sequence instead of a List?

Use a Sequence when you have a large collection and are performing multiple chained operations, as it processes elements lazily rather than creating new intermediate collections.

What is the main benefit of using val?

It enforces immutability, which makes your code easier to reason about, safer in multi-threaded environments, and less prone to accidental state changes.

Is it ever okay to use GlobalScope?

Only in very specific, limited use cases, such as top-level application-wide background tasks that must outlive any specific component lifecycle, though this is rarely necessary.