Single Expression Functions in Kotlin

Introduction

Single Expression Functions is a fundamental concept every Kotlin developer should understand. Functions are first-class in Kotlin — parameters, return types, and concise syntax make APIs easy to read and reuse.

When a function body is a single expression, you can use the = syntax. 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

• When a function body is a single expression, you can use the = syntax.
• The compiler can infer the return type for single-expression functions.
• This style keeps small utility functions concise and readable.

Syntax

fun double(x: Int) = x * 2

Single Expression Functions in Kotlin Example Program in Kotlin

fun square(n: Int) = n * n
fun isEven(n: Int) = n % 2 == 0

fun main() {
    println("Square of 5: ${square(5)}")
    println("Is 8 even? ${isEven(8)}")
}

Sample Output

Square of 5: 25
Is 8 even? true

When to use

Use default and named parameters to reduce overloads; use single-expression functions for small pure helpers.

How it works

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

2. When a function body is a single expression, you can use the = syntax.

3. 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: when a function body is a single expression, you can use the = syntax.
• 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

• When a function body is a single expression, you can use the = syntax.
• The compiler can infer the return type for single-expression functions.
• This style keeps small utility functions concise and readable.
• 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.