Annotations are a way to attach meta info to your code (such as documentation, configuration, and others).

Let's look at the following example code:

annotation class Tasty

An annotation itself can be annotated to modify its behavior:

@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
annotation class Tasty

In this case, the Tasty annotation can be set on classes, interfaces, and objects, and it can be queried at runtime.

For a complete list of options, check the Kotlin documentation.

Annotations can have parameters with one limitation, they can't be nullable:

@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
annotation class Tasty(val tasty:Boolean = true)

@Tasty(false)
object ElectricOven : Oven {
  override fun process(product: Bakeable) {
    println(product.bake())
  }
}

@Tasty
class CinnamonRoll : Roll("Cinnamon")

@Tasty
interface Fried {
  fun fry(): String
}

To query annotation values at runtime, we must use the reflection API (kotlin-reflect.jar must be in your classpath):

fun main(args: Array<String>) {
    val annotations: List<Annotation> = ElectricOven::class.annotations

    for (annotation in annotations) {
        when (annotation) {
            is Tasty -> println("Is it tasty? ${annotation.tasty}")
            else -> println(annotation)
        }
    }
}