Map<K, V>

Map associates unique keys with values. Both key and value types must be fully declared.

Map<String, Integer> counts = Map<>()
counts.put(key: "open", value: 3)

if counts.containsKey("open") {
    Integer count = counts["open"]
}

Missing values

map[key] requires the key to exist. get(key:) returns a nullable value when it does not. Use containsKey(key:) to distinguish a missing key from a stored nullable value:

Integer value = 0
if counts.containsKey("closed") {
    value = counts["closed"]
}

Integer? optionalValue = counts.get(key: "closed")

Key rules

Map uses the language's consistent built-in equality and hash rules: value keys are computed recursively by structure, while class keys use object identity. Equatable and Hashable do not replace these rules. Inserting keys and values follows the semantics of their respective data categories.

A general Map does not promise iteration order. Use OrderedMap or SortedMap with an explicit comparator when insertion or sorted order is needed. Map itself has value semantics.