Kotlin stands out as a modern JVM language that emphasizes expressiveness, readability, and interoperability with Java. One of the most powerful design choices in Kotlin is its use of conventions — special function names that unlock specific language constructs.

If you’ve ever used operator overloading, destructuring declarations, or function-like objects in Kotlin, you’ve already seen conventions in action. In this article, we’ll explore what Kotlin conventions are, why they exist, and how developers can leverage them to write clean, idiomatic, and concise code.

What Are Kotlin Conventions?

In Java, many language features depend on specific interfaces. For example:

• Objects implementing java.lang.Iterable can be used in for loops.
• Objects implementing java.lang.AutoCloseable can be used in try-with-resources.

Kotlin takes a different approach. Instead of tying behavior to types, Kotlin ties behavior to function names.

• If your class defines a function named plus, you can use the + operator on its instances.
• If you implement compareTo, you can use <, <=, >, and >=.

This technique is called conventions because developers agree on certain function names that the compiler looks for when applying language features.

Why Kotlin Uses Conventions Instead of Interfaces

Unlike Java, Kotlin cannot modify existing classes to implement new interfaces. The set of interfaces a class implements is fixed at compile time.

However, Kotlin provides extension functions, which allow you to add new functionality to existing classes — including Java classes — without modifying their source code.

This flexibility means you can “teach” any class to work with Kotlin’s language constructs simply by defining convention methods, either directly in the class or as extensions.

Common Kotlin Conventions Every Developer Should Know

Kotlin conventions go beyond operator overloading. Here are the most commonly used ones:

1. iterator()

• Enables for loops on your class.

class MyCollection(private val items: List<String>) {
    operator fun iterator(): Iterator<String> = items.iterator()
}

fun main() {
    val collection = MyCollection(listOf("A", "B", "C"))
    for (item in collection) {
        println(item)
    }
}

2. invoke()

• Makes your class behave like a function.

class Greeter(val greeting: String) {
    operator fun invoke(name: String) = "$greeting, $name!"
}

fun main() {
    val hello = Greeter("Hello")
    println(hello("Kotlin")) // "Hello, Kotlin!"
}

3. compareTo()

• Enables natural ordering with comparison operators.

class Version(val major: Int, val minor: Int) : Comparable<Version> {
    override operator fun compareTo(other: Version): Int {
        return if (this.major != other.major) {
            this.major - other.major
        } else {
            this.minor - other.minor
        }
    }
}

fun main() {
    println(Version(1, 2) < Version(1, 3)) // true
}

4. Destructuring Declarations (componentN())

• Allows breaking objects into multiple variables.

data class User(val name: String, val age: Int)

fun main() {
    val user = User("amol", 30)
    val (name, age) = user
    println("$name is $age years old")
}

Benefits of Using Kotlin Conventions

• Expressive code → Write natural, domain-specific APIs.
• Conciseness → Reduce boilerplate compared to Java.
• Interoperability → Adapt Java classes without modification.
• Readability → Operators and constructs feel intuitive.

FAQs on Kotlin Conventions

Q1: Are Kotlin conventions the same as operator overloading?
 Not exactly. Operator overloading is one type of convention. Conventions also include invoke(), iterator(), and componentN() functions.

Q2: Can I define convention functions as extension functions?
 Yes. You can add plus, compareTo, or even componentN functions to existing Java or Kotlin classes via extensions.

Q3: Do Kotlin conventions impact runtime performance?
 No. They are syntactic sugar — the compiler translates them into regular function calls.

Q4: Are Kotlin conventions required or optional?
 They are optional. You only implement them when you want your class to support certain language constructs.

Conclusion

Kotlin conventions are a cornerstone of the language’s design, allowing developers to unlock powerful language features with nothing more than function names. From + operators to destructuring declarations, these conventions make code cleaner, more intuitive, and more interoperable with Java.

If you’re building libraries or frameworks in Kotlin, embracing conventions is one of the best ways to make your APIs feel natural to other developers.