Building Maps in Kotlin

Wednesday, Apr 8, 2020 |

Over the years, I've found myself processing a set of data and storing it in a Map, say, something like Map<Long, List<String>> (think something like a list of Room objects, keyed by a building id). I have found myself writing it something like this (in non-idiomatic Kotlin):

val foo = map.get(key)
if (foo == null) {
    foo = MutableList<String>()
    map.put(key, foo)
}
foo.add(bar)

Fortunately, the Kotlin standard library has a better way:

map.getOrPut(key) { mutableListOf() }
    .add(bar)

If key is not found, the lambda is run, adding the result to Map and return to use the value, new or otherwise, to which we add bar. Much more concise. :) Generally speaking, any time you can let the language/compiler do the work for you, you're going to be better off.