Passing Function as Parameter in Kotlin

Introduction

Passing Function as Parameter is a fundamental concept every Kotlin developer should understand. Higher-order functions accept or return other functions, enabling reusable patterns like map, filter, and custom control abstractions.

Function references can be passed using :: operator. 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 references can be passed using :: operator.
• This avoids rewriting lambda when existing function can be reused.
• Improves readability for named functions.

Syntax

list.map(::transform)

Passing Function as Parameter in Kotlin Example Program in Kotlin

fun toUpper(text: String) = text.uppercase()

fun main(args: Array<String>) {
    val words = listOf("kotlin", "java")
    val upper = words.map(::toUpper)
    println(upper)
}

Sample Output

[KOTLIN, JAVA]

When to use

Use higher-order functions to extract repeated control-flow patterns into reusable abstractions.

How it works

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

2. fun toUpper(text: String) = text.uppercase() assigns or updates a value used later in the program.

3. val words = listOf("kotlin", "java") assigns or updates a value used later in the program.

4. val upper = words.map(::toUpper) assigns or updates a value used later in the program.

5. The println(upper) statement writes a line to the console — this produces part of the sample output below.

6. Function references can be passed using :: operator.

7. 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 references can be passed using :: operator.
• 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 references can be passed using :: operator.
• This avoids rewriting lambda when existing function can be reused.
• Improves readability for named functions.
• 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.