Example3.kt

import kotlinx.coroutines.future.await
import java.util.concurrent.CompletableFuture

internal class Example3(private val database: Database) {
    suspend fun activateUser(userId: Long?): Boolean {
        val user = database.findUser(userId).await()
        if (user != null && !user.isActive) {
            database.activateUser(userId).await()
            return true
        } else {
            return false
        }
    }
}

internal interface Database {
    fun findUser(id: Long?): CompletableFuture<User?>
    fun activateUser(id: Long?): CompletableFuture<Void?>
}

internal interface User {
    val isActive: Boolean
}