Simpler way to define a multi nested/hierarchy map in Kotlin

Question

Presently if I have to define a nested/hierarchal Map something like Map<String, Map<String, Map<String, String>>>, I have to separately define each nested map separately and use them to manually build the hierarchy. Is there a simpler way to declare and define such maps? Something similar to what I can do in Javascript.

PS: new to Kotlin

Javascript example (Hypothetical Example).

var schema = {
    "US": {
        "WA": {
            "Seattle": "Pike Street"
        }
    },
    "IN" : {
        "Karnataka": {
            "Bangalore" : "MG Road"
        }
    }
};

Kotlin:

val WA_CITY_TO_ST_MAP = mapOf(
    "Seattle" to "",
    "Redmond" to "1st Ave"
)
val KARNATAKA_CITY_TO_ST_MAP = mapOf("Bangalore" to "MG Road")

val US_STATE_TO_CITIES_MAP = mapOf("WA" to WA_CITY_TO_ST_MAP)
val IN_STATES_TO_CITIES_MAP = mapOf("Karnataka" to KARNATAKA_CITY_TO_ST_MAP)

val schema = mapOf("US" to US_STATE_TO_CITIES_MAP, "IN" to IN_STATES_TO_CITIES_MAP)

Added what I trying to achieve. I am also not sure why kotlin chose to have "to" keyword when ":" could have done the same job. — Keen Sage, Jul 26 '20 at 19:34
[`to`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/to.html) is not a keyword, not part of the language; it's just an ordinary extension function defined in the standard library. (You couldn't call a function `:`, so that _would_ need to be defined in the language itself.) — gidds, Jul 26 '20 at 20:59
The language design avoids using symbols across different domains. `:` is already used for the domain of type. Part of the reason they haven’t added the ternary operator is that `?` has to do with nullability any time you see it in Kotlin and the ternary operator would break that rule. — Tenfour04, Jul 26 '20 at 21:15

score 1 · Accepted Answer · answered Jul 26 '20 at 20:35

The nested maps can be built inline, there's no need to initialise them as separate vals:

val schema = mapOf(
            "US" to mapOf(
                    "WA" to mapOf(
                            "Seattle" to "",
                            "Redmond" to "1st Ave"
                    )
            ),
            "IN" to mapOf(
                    "Karnataka" to mapOf(
                            "Bangalore" to "MG Road"
                    )
            )
    )

If your structure doesn't always have the same level of nesting (much like JSON), you might want to look at handling your data with an appropriate JSON library.

Simpler way to define a multi nested/hierarchy map in Kotlin

1 Answers1