How can I get the last key or value in a Kotlin Map collection? It seems like it cannot be done by using an index value.

CodePudding user response：

There's a couple ways it can be done. While you can't elegantly print a map directly, you may print it's entry set.

The first way, and the way that I DO NOT recommend, is by calling the .last() function on the entry set. This can be accomplished with testMap.entries.last(). The reason I don't recommend this method is because in real data this method is non-deterministic -- meaning there's no way to guarantee the characteristics of the value returned.

While I don't recommend this method, I don't know your application and this may be sufficient.

I DO recommend using the .sortedBy() function on your entry set, and then calling .last() on it. This allows you to make some sort of assumption about the results returned, something that is typically necessary, otherwise why do you want the last?

See this example comparing the two methods and then comparing the method against the order you would get if you iterate with the .forEach function:

fun main(args: Array<String>) {
    val testMap = mutableMapOf<Long, String>()

    testMap[1] = "Hello"
    testMap[5] = "World"
    testMap[3] = "Foobar"

    println(testMap.entries.last())
    println(testMap.entries.sortedBy { it.key }.last())

    println("\norder via loop:")

    testMap
        .entries
        .forEach {
            println("\t$it")
        }
}

Take a look at the output:

3=Foobar
5=World

order via loop:
1=Hello
5=World
3=Foobar

Here we see that the value returned from .last(), is the last value that was inserted into the map - the same happens with .forEach. This is okay, but usually we want our map to have some sort of order. In this example, i've called for the entry set to be sorted by the key value, so that our call to .last() on the entry set returns the key/value pair with the largest key.