Challenge 2: A Custom Annotation with a Reader — Solution

import kotlin.reflect.KClass

@Target(AnnotationTarget.CLASS)
@Retention(AnnotationRetention.RUNTIME)
annotation class Author(val name: String)

@Author("Philip Osztromok")
class Report

class UnattributedNotes

fun printAuthor(kClass: KClass<*>) {
    val author = kClass.annotations.filterIsInstance<Author>().firstOrNull()
    if (author != null) {
        println("${kClass.simpleName} written by ${author.name}")
    } else {
        println("${kClass.simpleName}: No author")
    }
}

fun main() {
    printAuthor(Report::class)
    printAuthor(UnattributedNotes::class)
}

/*
Output:
Report written by Philip Osztromok
UnattributedNotes: No author

Notes:
- @Retention(AnnotationRetention.RUNTIME) is required — without it, the
  annotation would compile fine but be invisible to reflection at
  runtime, and kClass.annotations would never find it.
- kClass.annotations returns every annotation actually present on the
  class; filterIsInstance<Author>() narrows that list down to just
  Author annotations (there could in principle be others), and
  firstOrNull() safely handles the case where none is present.
- Report has @Author applied, so its name comes back; UnattributedNotes
  has no such annotation, so the function falls through to the
  "No author" branch.
*/