Kotlin is a powerful and expressive language, but it introduces some challenges when dealing with mutable variables inside lambdas. If you’ve ever encountered issues while capturing mutable variables in Kotlin lambdas, you’re not alone. Many developers struggle with this concept, leading to unexpected behavior, performance concerns, and even compiler errors.

In this blog post, we’ll dive deep into capturing mutable variables in Kotlin lambdas, explore why developers often face difficulties, and discuss best practices to handle them effectively.

Before we dive into the struggles, let’s first understand what it means to capture a mutable variable in a lambda.

Understanding Capturing Mutable Variables in Kotlin Lambdas

A lambda expression in Kotlin can access and “capture” variables from its surrounding scope. This is a feature known as closures. However, how a variable is captured depends on whether it is mutable or immutable.

When a lambda function is defined inside another function, it can access variables declared in the outer function. If the lambda captures a mutable variable (a var), it essentially maintains a reference to that variable rather than making a copy of its value.

fun main() {
    var count = 0 // Mutable variable
    val increment = { count++ } // Lambda capturing 'count'
    
    increment()
    increment()
    
    println(count) // Output: 2
}

In this case, the lambda increment captures the mutable variable count, modifying it every time it is called.

Why Kotlin Developers Struggle with Capturing Mutable Variables

In above example, the lambda captures count, allowing it to be modified inside the lambda. But there’s a catch: Kotlin captures mutable variables by reference. This means any change inside the lambda affects the original variable.

This is where developers often struggle, especially when working with concurrency or multi-threading.

Concurrency Issues with Mutable Variables

When working in multi-threaded applications, capturing mutable variables in Kotlin lambdas can lead to unpredictable behavior due to race conditions.

var sharedCount = 0

fun main() {
    val workers = List(1000) {
        Thread { sharedCount++ }
    }
    
    workers.forEach { it.start() }
    workers.forEach { it.join() }
    
    println(sharedCount) // Unpredictable result
}

Since sharedCount is modified by multiple threads, the final value is unpredictable. The correct way to handle this in Kotlin is by using Atomic variables:

import java.util.concurrent.atomic.AtomicInteger

val sharedCount = AtomicInteger(0)

fun main() {
    val workers = List(1000) {
        Thread { sharedCount.incrementAndGet() }
    }
    
    workers.forEach { it.start() }
    workers.forEach { it.join() }
    
    println(sharedCount.get()) // Always consistent
}

Note: When you run both programs, you might think they give the same result. So, what’s the difference? The catch is that in the second case, the result is always consistent, no matter what. But in the first case, it’s unpredictable—even if it looks correct sometimes. Try stress testing it, and you’ll see the difference.

Detecting This Issue with a Stress Test

To reliably expose the race condition, increase the number of threads and iterations:

var sharedCount = 0

fun main() {
    val workers = List(10000) {
        Thread {
            repeat(100) { sharedCount++ }
        }
    }
    
    workers.forEach { it.start() }
    workers.forEach { it.join() }
    
    println(sharedCount) // Unpredictable, usually much less than 1,000,000
}

To really see the difference, try pushing the program harder — bump up the number of threads and iterations. Run the test, and you’ll notice the final count is all over the place, much lower than expected. That’s the unpredictability we’re talking about. Hope it’s clear now..!

Conclusion

Capturing mutable variables in Kotlin lambdas can be tricky due to variable scoping, reference capturing, and concurrency issues. By understanding these challenges and following best practices, you can write safer and more predictable Kotlin code.

If you’re struggling with such issues in your Kotlin projects, try applying the techniques discussed here. 

Happy Capturing..!