In Java, I can create a map using the toMap collec...

# announcements

[Quincy]

In Java, I can create a map using the toMap collector, like this:

Stream.of(...)
    .toMap(
        keySelector,
        valueTransform,
        (x, y) -> {
            if (x.getTimestamp().isBefore(y.getTimestamp())) return x;
            else return y;
        });

If there would be duplicate values then the third argument determines what to do about it. In Kotlin, the associateBy function always keeps the most recent value. Is there a way to create a map the way Java does, where I get to decide what to do with duplicates?

[Nir]

Yeah, you can use a groupingBy

[Nir]

associate (and friends) is intended to be the simple function for the common use case, the equivalent of map basically

[Roukanken]

for example

listOf(1, 2, 3, 4, 5)
    .groupingBy { it % 2 }
    .reduce { key, accumulator, element -> accumulator * element }

gets you

{1=15, 0=8}

(in reduce , accumulator is the value currently associated with key and element is the one that is trying to be new one)

there is also fold when you want to also do some type changing mapping, not just selecting or simple operations like I did

[Quincy]

Thank you. I'd used groupingBy previously, but not much, and didn't think about it here. My example becomes:

listOf(...)
    .groupingBy(keySelector)
    .reduce { _, x, y -> if (x.timestamp.isBefore(y.timestamp)) x else y }

[Nir]

yeah, groupingBy is very nice, I miss it in other languages

[Nir]

if you don't care too much about the efficiency/laziness of it all then it's also possible to do .associate followed by .mapValues, probably would be a little more readable arguably

[Nir]

since you'd just be able to call min

[Nir]

minBy rather

[Nir]

maybe groupBy followed by mapValues, actually. anyhow, doesn't matter.

[Quincy]

I'll play with those suggestions and see if anything strikes as preferable. Thanks again!