Function Reference in Kotlin

Introduction

Function Reference is a fundamental concept every Kotlin developer should understand. Reflection inspects types, properties, and functions at runtime — useful for serialization libraries, DI frameworks, and testing utilities.

Function reference points to existing function using ::functionName. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Function reference points to existing function using ::functionName.
• Can be passed where functional type is expected.
• Bound references include receiver using obj::method.

Syntax

val fn = ::println

Function Reference in Kotlin Example Program in Kotlin

fun greet(name: String) = "Hello, $name"

fun main(args: Array<String>) {
    val reference: (String) -> String = ::greet
    println(reference("Kotlin"))
}

Sample Output

Hello, Kotlin

When to use

Use reflection sparingly for frameworks and generic utilities; prefer compile-time type safety when possible.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. fun greet(name: String) = "Hello, $name" assigns or updates a value used later in the program.

3. val reference: (String) -> String = ::greet assigns or updates a value used later in the program.

4. The println(reference("Kotlin")) statement writes a line to the console — this produces part of the sample output below.

5. Function reference points to existing function using ::functionName.

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: function reference points to existing function using ::functionName.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Function reference points to existing function using ::functionName.
• Can be passed where functional type is expected.
• Bound references include receiver using obj::method.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.