blob: 5aa9504d28f97590aaad87e5db6d1f8bdf32b6dc [file] [log] [blame]
// Copyright (c) 2013, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
part of dart.collection;
/// Default function for equality comparison in customized HashMaps.
bool _defaultEquals(Object? a, Object? b) => a == b;
/// Default function for hash-code computation in customized HashMaps.
int _defaultHashCode(Object? a) => a.hashCode;
/// Type of custom equality function.
typedef _Equality<K> = bool Function(K a, K b);
/// Type of custom hash code function.
typedef _Hasher<K> = int Function(K object);
/// A hash-table based implementation of [Map].
/// The keys of a `HashMap` must have consistent [Object.==]
/// and [Object.hashCode] implementations. This means that the `==` operator
/// must define a stable equivalence relation on the keys (reflexive,
/// symmetric, transitive, and consistent over time), and that `hashCode`
/// must be the same for objects that are considered equal by `==`.
/// Iterating the map's keys, values or entries (through [forEach])
/// may happen in any order.
/// The iteration order only changes when the map is modified.
/// Values are iterated in the same order as their associated keys,
/// so iterating the [keys] and [values] in parallel
/// will give matching key and value pairs.
abstract class HashMap<K, V> implements Map<K, V> {
/// Creates an unordered hash-table based [Map].
/// The created map is not ordered in any way. When iterating the keys or
/// values, the iteration order is unspecified except that it will stay the
/// same as long as the map isn't changed.
/// If [equals] is provided, it is used to compare the keys in the table with
/// new keys. If [equals] is omitted, the key's own [Object.==] is used
/// instead.
/// Similar, if [hashCode] is provided, it is used to produce a hash value
/// for keys in order to place them in the hash table. If it is omitted, the
/// key's own [Object.hashCode] is used.
/// If using methods like [operator []], [remove] and [containsKey] together
/// with a custom equality and hashcode, an extra `isValidKey` function
/// can be supplied. This function is called before calling [equals] or
/// [hashCode] with an argument that may not be a [K] instance, and if the
/// call returns false, the key is assumed to not be in the set.
/// The [isValidKey] function defaults to just testing if the object is a
/// [K] instance.
/// Example:
/// ```dart template:expression
/// HashMap<int,int>(equals: (int a, int b) => (b - a) % 5 == 0,
/// hashCode: (int e) => e % 5)
/// ```
/// This example map does not need an `isValidKey` function to be passed.
/// The default function accepts only `int` values, which can safely be
/// passed to both the `equals` and `hashCode` functions.
/// If neither `equals`, `hashCode`, nor `isValidKey` is provided,
/// the default `isValidKey` instead accepts all keys.
/// The default equality and hashcode operations are assumed to work on all
/// objects.
/// Likewise, if `equals` is [identical], `hashCode` is [identityHashCode]
/// and `isValidKey` is omitted, the resulting map is identity based,
/// and the `isValidKey` defaults to accepting all keys.
/// Such a map can be created directly using [HashMap.identity].
/// The used `equals` and `hashCode` method should always be consistent,
/// so that if `equals(a, b)` then `hashCode(a) == hashCode(b)`. The hash
/// of an object, or what it compares equal to, should not change while the
/// object is a key in the map. If it does change, the result is
/// unpredictable.
/// If you supply one of [equals] and [hashCode],
/// you should generally also to supply the other.
external factory HashMap(
{bool Function(K, K)? equals,
int Function(K)? hashCode,
bool Function(dynamic)? isValidKey});
/// Creates an unordered identity-based map.
/// Effectively a shorthand for:
/// ```dart
/// HashMap<K, V>(equals: identical, hashCode: identityHashCode)
/// ```
external factory HashMap.identity();
/// Creates a [HashMap] that contains all key/value pairs of [other].
/// The keys must all be instances of [K] and the values of [V].
/// The [other] map itself can have any type.
factory HashMap.from(Map<dynamic, dynamic> other) {
HashMap<K, V> result = HashMap<K, V>();
other.forEach((dynamic k, dynamic v) {
result[k as K] = v as V;
return result;
/// Creates a [HashMap] that contains all key/value pairs of [other].
factory HashMap.of(Map<K, V> other) => HashMap<K, V>()..addAll(other);
/// Creates a [HashMap] where the keys and values are computed from the
/// [iterable].
/// For each element of the [iterable] this constructor computes a key/value
/// pair, by applying [key] and [value] respectively.
/// The keys of the key/value pairs do not need to be unique. The last
/// occurrence of a key will simply overwrite any previous value.
/// If no values are specified for [key] and [value] the default is the
/// identity function.
factory HashMap.fromIterable(Iterable iterable,
{K Function(dynamic element)? key, V Function(dynamic element)? value}) {
HashMap<K, V> map = HashMap<K, V>();
MapBase._fillMapWithMappedIterable(map, iterable, key, value);
return map;
/// Creates a [HashMap] associating the given [keys] to [values].
/// This constructor iterates over [keys] and [values] and maps each element
/// of [keys] to the corresponding element of [values].
/// If [keys] contains the same object multiple times, the last occurrence
/// overwrites the previous value.
/// It is an error if the two [Iterable]s don't have the same length.
factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
HashMap<K, V> map = HashMap<K, V>();
MapBase._fillMapWithIterables(map, keys, values);
return map;
/// Creates a [HashMap] containing the entries of [entries].
/// Returns a new `HashMap<K, V>` where all entries of [entries]
/// have been added in iteration order.
/// If multiple [entries] have the same key,
/// later occurrences overwrite the earlier ones.
factory HashMap.fromEntries(Iterable<MapEntry<K, V>> entries) =>
HashMap<K, V>()..addEntries(entries);