Kotlin Comparator after converting from Java Comparator

Question

I converted my Java files to Kotlin. And I fixed most of them. However, I don't understand this part using Comparator.

wordItems.sortWith(Comparator { (_, word), (_, word) ->
    val size1 = word!!.length
    val size2 = word.length
    if (size1 < size2) {
        [email protected] - 1
    } else if (size1 == size2) {
        [email protected] 0
    } else {
        [email protected] 1
    }
})

And this one also, I don't understand.

Collections.sort(wordItems) { (_, word), (_, word) ->
    val size1 = word!!.length
    val size2 = word.length
    if (size1 < size2) {
        [email protected] - 1
    } else if (size1 == size2) {
        [email protected] 0
    } else {
        [email protected] 1
    }
}

How can I change this to make it work?

Your question is too broad. Please tell us exactly what it is you don't understand. — avolkmann
– avolkmann, Commented Jun 26, 2019 at 16:08
They already are using Comparator. Comparator is a functional interface and a lambda defines the compare() method. See also What does return@ mean?. What I don't understand is that it seems the lambda has two arguments and both of them are identical: (_, word), (_, word). Are you sure both are called word? — DodgyCodeException
– DodgyCodeException, Commented Jun 26, 2019 at 16:22

Ilya · Accepted Answer · 2019-06-27 00:23:34Z

2

Comparator is a SAM (single abstract method) interface in Java. An implementation of such interface can be instantiated in Kotlin using SAM-conversion technique, so instead of writing an anonymous object implementing Comparator like this:

val comparator = object : Comparator<WordItem> {
    override fun compare(item1: WordItem, item2: WordItem): Int {
        val size1 = item1.word.length
        val size2 = item2.word.length
        ...
    }
}

you can write

val comparator = Comparator<WordItem> { item1, item2 -> 
    val size1 = item1.word.length
    val size2 = item2.word.length
    ...
}

Here I assume that each WordItem has the word property.

It seems that you're comparing items by word length. In that case you can simplify such comparator even more with compareBy function:

wordItems.sortWith(compareBy { it.word.length })

or sortBy function

wordItems.sortBy { it.word.length }

answered Jun 27, 2019 at 0:23

Ilya

23.3k9 gold badges79 silver badges96 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

c-an Over a year ago

I see... So, return@Collections doesn't mean anything, right?

Ilya Over a year ago

Probably [email protected] is result of an incorrect Java-to-Kotlin conversion and was intended to return the value 0, 1 or -1 from the lambda function. The correct syntax of this labelled return would be return@Comparator 0. Here Comparator before lambda function body gives an implicit label with the same name to that lambda. See kotlinlang.org/docs/reference/returns.html#return-at-labels for the further reference.

Collectives™ on Stack Overflow

Kotlin Comparator after converting from Java Comparator

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related