Priority Queue sorted by timestamp in message. Will guarantee order for messages delayed upto 3 sec.

PriorityQueue.kt

package io.github.vihangpatil.sandbox

import java.time.Instant
import java.util.*
import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit.MILLISECONDS
import kotlin.Comparator

data class Message(
        val timestamp: Long,
        val name: String)

val comparator = Comparator<Message> { msg1, msg2 -> (msg1.timestamp - msg2.timestamp).toInt() }

val queue: PriorityQueue<Message> = PriorityQueue(10, comparator)

const val allowedDelay = 3 //sec

fun main(args: Array<String>) {

    // images these 5 messages are received in reverse order,
    // but were sent in order with 1 sec interval
    val now = Instant.now().epochSecond
    queue.offer(Message(now - 1L, "Five")) // 1 sec old
    queue.offer(Message(now - 2L, "Four")) // 2 sec old
    queue.offer(Message(now - 3L, "Three")) // 3 sec old
    queue.offer(Message(now - 4L, "Two")) // 4 sec old
    queue.offer(Message(now - 5L, "One")) // 5 sec old

    val stse = Executors.newSingleThreadScheduledExecutor()
    stse.scheduleWithFixedDelay({
        if (queue.peek().timestamp < Instant.now().epochSecond - allowedDelay) {
            println(queue.poll())
        }
    },
            0, // initial delay
            300, // delay
            MILLISECONDS) // timeunit


    // stop everything after 4 sec
    stse.schedule({stse.shutdown()}, 4500, MILLISECONDS)
}