Just in case anyone tries your solution, after I tried it, be careful that the second parameter of

.getOrPut()

is a lambda, so it must go between

{}

, as well as expressing the type of the content inside the MutableSet. So, in this example: [CHANGES DONE MARKED IN BOLD]

myMap.getOrPut("another key", *{* mutableSetOf~*<String>*~() *}* ).add("value 1 of second entry")

(Mike was good at his solution in the way that

getOrPut()

returns the value of the entry, even if it is the one entry just created because it didn't exist previously). 2019-11-26 EDIT: the

<String>

part is not needed, the Kotlin type inference engine (on actual release version) is able to run it. It is more simple this way, only adding the

{}

that were missing on the first version wrote by Mike, as it is a lamba function. =)

Using 1.3.50, and the following compiles as expected:

val test = mutableMapOf<String, MutableSet<String>>()

test.getOrPut("value", { mutableSetOf() }).add("valueentry")

Are you using 1.3.41 or earlier?