Default Parameters in Kotlin

Introduction

Default Parameters 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.

Function parameters can declare default values so callers may omit them. 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 parameters can declare default values so callers may omit them.
• Default arguments reduce overloads and keep function signatures flexible.
• Parameters with defaults should usually appear after required parameters.

Syntax

fun greet(name: String, prefix: String = "Hello") { }

Default Parameters in Kotlin Example Program in Kotlin

fun showMessage(title: String, level: String = "INFO") {
    println("[$level] $title")
}

fun main() {
    showMessage("Build complete")
    showMessage("Warning found", "WARN")
}

Sample Output

[INFO] Build complete
[WARN] Warning found

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. Function parameters can declare default values so callers may omit them.

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: function parameters can declare default values so callers may omit them.
• 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 parameters can declare default values so callers may omit them.
• Default arguments reduce overloads and keep function signatures flexible.
• Parameters with defaults should usually appear after required parameters.
• 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.