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Mastering Kotlin Essentials

Maps

Learn how to use maps, find values using keys, and add new elements to maps.

We'll cover the following...

Key-value mapping

We use maps to keep associations from keys to their values. For instance:

  • From user ID to an object representing the user

  • From a website to its IP address

  • From a configuration name to data stored in this configuration

Kotlin 1.5
class CachedApiArticleRepository(
    val articleApi: ArticleApi
) {
    val articleCache: MutableMap<String, String> =
        mutableMapOf()
    fun getContent(key: String) =
        articleCache.getOrPut(key) {
            articleApi.fetchContent(key)
        }
}
class DeliveryMethodsConfiguration(
    val deliveryMethods: Map<String, DeliveryMethod>
)
class TokenRepository {
    private var tokenToUser: Map<String, User> = mapOf()
    fun getUser(token: String) = tokenToUser[token]
    fun addToken(token: String, user: User) {
        tokenToUser[token] = user
    }
}

Creating maps

We can create a map using the mapOf function and then use key-value pairs as arguments to specify key-value associations. For instance, we might define a map that associates countries with their capital cities. Pairs can be defined using a constructor or the to function.

Kotlin 1.5
fun main() {
    val capitals = mapOf(
        "USA" to "Washington DC",
        "Poland" to "Warsaw",
    )
    println(capitals) // {USA=Washington DC, Poland=Warsaw}
}
...