Kotlin: Cannot put a key-value to ConcurrentHashMap

I try to put a key-value to ConcurrentHashMap in Kotlin buf failed. The compiler tells me: No set method providing array access.

class MysqlDataProviderProxy() {
    private val NULL: Any = Object()
    var unionMaps: Map<Long, Any> = ConcurrentHashMap()


    fun init() {
        unionMaps[1] = NULL // No set method providing array access
    }
}

I don't know what does it mean. Is ConcurrentHashMap in Kotlin unmutable?

As Sweeper says you have the wrong type on unionMap.

Is ConcurrentHashMap in Kotlin unmutable

No, but unlike Java Maps/Lists/Sets have mutable and immutable interfaces and the interfaces Map, List, Set are the immutable variation.

You want this:

    class MysqlDataProviderProxy() {
        private val NULL: Any = Object()
        var unionMaps: MutableMap<Long, Any> = ConcurrentHashMap()


       init {
            unionMaps[1] = NULL
        }
    }

And the other thing is that your original function init() does not get executed as the instance is constructed, you probably want init {... as I show above

but the problem with that approach is that now you have made the map mutable outside your proxy class which might not be your intention, in which case you could do this:

    class MysqlDataProviderProxy() {
        private val NULL: Any = Object()
        var unionMaps: Map<Long, Any> = ConcurrentHashMap()

        init {
            (unionMaps as MutableMap)[1] = NULL
        }
    }