Annotation Declaration in Kotlin

Introduction

Annotation Declaration is a fundamental concept every Kotlin developer should understand. Annotations attach metadata to declarations. Frameworks like Spring, Retrofit, and Room read them to configure behaviour at compile or runtime.

Annotations add metadata to classes, functions and properties. 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

• Annotations add metadata to classes, functions and properties.
• Declared using annotation class keyword.
• Annotation parameters must be compile-time constants.

Syntax

annotation class MyTag

Annotation Declaration in Kotlin Example Program in Kotlin

annotation class Version(val number: Int)

@Version(1)
class ApiService

fun main(args: Array<String>) {
    println("Annotation example loaded")
}

Sample Output

Annotation example loaded

When to use

Use annotations to declare framework configuration declaratively instead of wiring everything manually.

How it works

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

2. The println("Annotation example loaded") statement writes a line to the console — this produces part of the sample output below.

3. Annotations add metadata to classes, functions and properties.

4. 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: annotations add metadata to classes, functions and properties.
• 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

• Annotations add metadata to classes, functions and properties.
• Declared using annotation class keyword.
• Annotation parameters must be compile-time constants.
• 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.