Kotlin is known for its modern and expressive syntax, making development smoother and more efficient. One of the lesser-known but powerful features of Kotlin is inline properties, which can significantly impact performance. In this article, we’ll explore how inline properties improve performance in Kotlin, understand their implementation with examples, and see why they matter in real-world applications.

What Are Inline Properties?

Inline properties in Kotlin allow property accessors (getters and setters) to be inlined at the call site. This eliminates method call overhead and improves execution speed. Essentially, when you mark a property accessor as inline, the compiler replaces the function call with its actual code during compilation, resulting in a more efficient bytecode.

Why Use Inline Properties?

When you define a property with custom getters or setters in Kotlin, the compiler generates a function behind the scenes. Every time the property is accessed, this function is called. While this is usually not a problem, it can introduce unnecessary method call overhead, especially in performance-critical applications. Inline properties help optimize performance by removing this overhead.

Syntax of Inline Properties

To make a property’s accessor inline, you use the inline modifier with the getter or setter:

class User(val firstName: String, val lastName: String) {
    val fullName: String
        inline get() = "$firstName $lastName"
}

fun main() {
    val user = User("amol", "pawar")
    println(user.fullName)  // Output: amol pawar
}

Here,

• The fullName property has a custom getter that concatenates firstName and lastName.
• The inline keyword ensures that when fullName is accessed, the compiler replaces the function call with its actual expression.
• This improves performance by avoiding a method call at runtime.

How Inline Properties Improve Performance

Reduces Method Call Overhead

When a property accessor is inlined, the function call is replaced with the actual code. This removes the overhead of method calls, reducing execution time.

Without Inline Properties

class Rectangle(val width: Int, val height: Int) {
    val area: Int
        get() = width * height
}

fun main() {
    val rect = Rectangle(5, 10)
    println(rect.area)  // Generates a method call
}

• Here, area is accessed using a generated getter function, which results in a method call.

With Inline Properties

class Rectangle(val width: Int, val height: Int) {
    val area: Int
        inline get() = width * height
}

fun main() {
    val rect = Rectangle(5, 10)
    println(rect.area)  // No method call, inlined at compile-time
}

• The getter is inlined, meaning the multiplication happens directly where area is accessed, eliminating an extra function call.

Better Performance in Loops

If a property is accessed multiple times in a loop, an inline getter prevents redundant function calls, optimizing performance.

class Person(val age: Int) {
    val isAdult: Boolean
        inline get() = age >= 18
}

fun main() {
    val people = List(1_000_000) { Person(it % 50) }
    val adults = people.count { it.isAdult }  // More efficient with inline properties
    println("Number of adults: $adults")
}

• With inline properties, isAdult is evaluated without generating function calls in each iteration, making large computations faster.

Reduces Bytecode Size

Inlining properties reduces the number of generated methods, resulting in smaller bytecode size and potentially lower memory usage.

When to Use Inline Properties

They are beneficial when:

• The property is frequently accessed and has a simple getter or setter.
• You want to optimize performance in high-frequency operations like loops.
• You need to eliminate function call overhead for small computations.

However, avoid using them when:

• The getter or setter contains complex logic.
• The property returns large objects (inlining could increase code size).
• The function involves recursion (since inline functions cannot be recursive).

Best Practices for Inline Properties

1. Use them for lightweight operations — Keep the logic simple to maximize performance gains.
2. Avoid inline properties with large return types — This can lead to increased bytecode size.
3. Test performance improvements — Profile your code to ensure that inlining provides actual benefits.
4. Be mindful of code readability — Excessive inlining can make debugging harder.

Explore the complete breakdown here: [Main Article URL]

Conclusion

Inline properties in Kotlin are a simple yet powerful feature that can improve performance by removing method call overhead, reducing bytecode size, and optimizing loops. While they should be used wisely, they offer significant benefits in performance-critical applications.

By understanding and leveraging inline properties, you can write more efficient Kotlin code, ensuring faster execution and better resource utilization.