Reflection Basics in Kotlin

Introduction

Reflection Basics 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.

Reflection inspects program structure at runtime. 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

• Reflection inspects program structure at runtime.
• KClass and KFunction provide class and function metadata.
• Reflection has runtime cost; use when necessary.

Syntax

obj::class.members

Reflection Basics in Kotlin Example Program in Kotlin

class User(val id: Int, val name: String)

fun main(args: Array<String>) {
    val user = User(1, "Riya")
    val properties = user::class.members.filterIsInstance<kotlin.reflect.KProperty<*>>()
    properties.forEach { println("${it.name} = ${it.get(user)}") }
}

Sample Output

id = 1
name = Riya

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. val user = User(1, "Riya") assigns or updates a value used later in the program.

3. val properties = user::class.members.filterIsInstance<kotlin.reflect.KProperty<*>>() assigns or updates a value used later in the program.

4. The println("${it.name} = ${it.get(user)}") statement writes a line to the console — this produces part of the sample output below.

5. Reflection inspects program structure at runtime.

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

Common Mistakes

Key Points

• Reflection inspects program structure at runtime.
• KClass and KFunction provide class and function metadata.
• Reflection has runtime cost; use when necessary.
• 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.