Sorting data with custom comparators

In this recipe, we are going to explore the support for sorting collections' elements by their properties.

Getting started

Let's assume we are dealing with the two collections of the Message type declared as follows:

data class Message(val text: String,
                   val sender: String,
                   val receiver: String,
                   val time: Instant = Instant.now())

These are provided by the allMessages variable:

val sentMessages = listOf(
        Message("I'm programming in Kotlin, of course", 
"Samuel", 
"Agat", 
Instant.parse("2018-12-18T10:13:35Z")),
Message("Sure!", 
"Samuel", 
"Agat", 
Instant.parse("2018-12-18T10:15:35Z"))
)
val inboxMessages = mutableListOf(
        Message("Hey Sam, any plans for the evening?", 
"Samuel", 
"Agat", 
Instant.parse("2018-12-18T10:12:35Z")),
Message("That's cool, can I join you?", 
"Samuel", 
"Agat",
Instant.parse("2018-12-18T10:14:35Z"))
)
val allMessages = sentMessages + inboxMessages

If we print the...