Retention and Inheritance in Kotlin

Introduction

Retention and Inheritance 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.

Retention policy controls whether annotation exists 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

• Retention policy controls whether annotation exists at runtime.
• ANNOTATION retention keeps metadata in compiled class files.
• RUNTIME retention allows reflection access.

Syntax

@Retention(AnnotationRetention.RUNTIME)

Retention and Inheritance in Kotlin Example Program in Kotlin

@Retention(AnnotationRetention.RUNTIME)
annotation class RuntimeTag

@RuntimeTag
class Demo

fun main(args: Array<String>) {
    val hasTag = Demo::class.annotations.any { it is RuntimeTag }
    println("Has RuntimeTag: $hasTag")
}

Sample Output

Has RuntimeTag: true

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. val hasTag = Demo::class.annotations.any { it is RuntimeTag } assigns or updates a value used later in the program.

3. The println("Has RuntimeTag: $hasTag") statement writes a line to the console — this produces part of the sample output below.

4. Retention policy controls whether annotation exists at runtime.

5. 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: retention policy controls whether annotation exists at runtime.
• 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

• Retention policy controls whether annotation exists at runtime.
• ANNOTATION retention keeps metadata in compiled class files.
• RUNTIME retention allows reflection access.
• 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.