HashMap<K, V> abstract final

abstract final class HashMap<K, V> implements Map<K, V>

A hash-table based implementation of Map.

The HashMap is unordered (the order of iteration is not guaranteed).

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.

Notice: Do not modify a map (add or remove keys) while an operation is being performed on that map, for example in functions called during a forEach or putIfAbsent call, or while iterating the map (keys, values or entries).

Do not modify keys in any way which changes their equality (and thus their hash code) while they are in the map. If a map key's Object.hashCode changes, it may cause future lookups for that key to fail.

Example:

final Map<int, String> planets = HashMap(); // Is a HashMap

To add data to a map, use operator[]=, addAll or addEntries.

planets[3] = 'Earth';
planets.addAll({4: 'Mars'});
final gasGiants = {6: 'Jupiter', 5: 'Saturn'};
planets.addEntries(gasGiants.entries);
print(planets); // fx {5: Saturn, 6: Jupiter, 3: Earth, 4: Mars}

To check if the map is empty, use isEmpty or isNotEmpty. To find the number of map entries, use length.

final isEmpty = planets.isEmpty; // false
final length = planets.length; // 4

The forEach iterates through all entries of a map.

planets.forEach((key, value) {
  print('$key \t $value');
  // 5        Saturn
  // 4        Mars
  // 3        Earth
  // 6        Jupiter
});

To check whether the map has an entry with a specific key, use containsKey.

final keyOneExists = planets.containsKey(4); // true
final keyFiveExists = planets.containsKey(1); // false

To check whether the map has an entry with a specific value, use containsValue.

final marsExists = planets.containsValue('Mars'); // true
final venusExists = planets.containsValue('Venus'); // false

To remove an entry with a specific key, use remove.

final removeValue = planets.remove(6);
print(removeValue); // Jupiter
print(planets); // fx {4: Mars, 3: Earth, 5: Saturn}

To remove multiple entries at the same time, based on their keys and values, use removeWhere.

planets.removeWhere((key, value) => key == 5);
print(planets); // fx {3: Earth, 4: Mars}

To conditionally add or modify a value for a specific key, depending on whether there already is an entry with that key, use putIfAbsent or update.

planets.update(4, (v) => 'Saturn');
planets.update(8, (v) => '', ifAbsent: () => 'Neptune');
planets.putIfAbsent(4, () => 'Another Saturn');
print(planets); // fx {4: Saturn, 8: Neptune, 3: Earth}

To update the values of all keys, based on the existing key and value, use updateAll.

planets.updateAll((key, value) => 'X');
print(planets); // fx {8: X, 3: X, 4: X}

To remove all entries and empty the map, use clear.

planets.clear();
print(planets); // {}
print(planets.isEmpty); // true

See also:

• Map, the general interface of key/value pair collections.
• LinkedHashMap iterates in key insertion order.
• SplayTreeMap iterates the keys in sorted order.

Implemented types

• Map<K, V>

Constructors

HashMap() factory

factory HashMap({
  (bool Function(K, K))? equals,
  (int Function(K))? hashCode,
  (bool Function(dynamic))? isValidKey,
})

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 map with new keys. If equals is omitted, the key's own Object.== is used instead. The equals function must not change the map it's used as an equality for. If it does, the resulting behavior is unspecified.

Similarly, if hashCode is provided, it is used to produce a hash value for keys in order to place them in the map. If hashCode is omitted, the key's own Object.hashCode is used. The hashCode function must not change the map it's used as a hash code for. If it does, the resulting behavior is unspecified.

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 the hash code or equality of an object does change, the resulting behavior is unspecified.

If you supply one of equals and hashCode, you should generally also supply the other.

Some equals or hashCode functions might not work for all objects. If isValidKey is supplied, it's used to check a potential key which is not necessarily an instance of K, like the arguments to operator [], remove and containsKey, which are typed as Object?. If isValidKey returns false, for an object, the equals and hashCode functions are not called, and no key equal to that object is assumed to be in the map. The isValidKey function defaults to just testing if the object is an instance of K.

Example:

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 precisely 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 hash code operations are known 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.

Implementation

external factory HashMap({
  bool Function(K, K)? equals,
  int Function(K)? hashCode,
  bool Function(dynamic)? isValidKey,
});

HashMap.from() factory

factory HashMap.from(Map<dynamic, dynamic> other)

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.

final baseMap = {1: 'A', 2: 'B', 3: 'C'};
final fromBaseMap = HashMap<int, String>.from(baseMap);
print(fromBaseMap); // {1: A, 2: B, 3: C}

Implementation

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;
}

HashMap.fromEntries() factory

factory HashMap.fromEntries(Iterable<MapEntry<K, V>> entries)

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.

Example:

final numbers = [11, 12, 13, 14];
final map = HashMap.fromEntries(numbers.map((i) => MapEntry(i, i * i)));
print(map); // {11: 121, 12: 144, 13: 169, 14: 196}

Implementation

factory HashMap.fromEntries(Iterable<MapEntry<K, V>> entries) =>
    HashMap<K, V>()..addEntries(entries);

HashMap.fromIterable() factory

factory HashMap.fromIterable(
  Iterable<dynamic> iterable, {
  (K Function(dynamic element))? key,
  (V Function(dynamic element))? value,
})

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. Example:

final numbers = [11, 12, 13, 14];
final mapFromIterable = HashMap<int, int>.fromIterable(numbers,
    key: (i) => i, value: (i) => i * i);
print(mapFromIterable); // {11: 121, 12: 144, 13: 169, 14: 196}

Implementation

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;
}

HashMap.fromIterables() factory

factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values)

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 Iterables don't have the same length. Example:

final keys = ['Mercury', 'Venus', 'Earth', 'Mars'];
final values = [0.06, 0.81, 1, 0.11];
final mapFromIterables = HashMap.fromIterables(keys, values);
print(mapFromIterables);
// {Earth: 1, Mercury: 0.06, Mars: 0.11, Venus: 0.81}

Implementation

factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
  HashMap<K, V> map = HashMap<K, V>();
  MapBase._fillMapWithIterables(map, keys, values);
  return map;
}

HashMap.identity() factory

factory HashMap.identity()

Creates an unordered identity-based map.

Keys of this map are considered equal only to the same object, and do not use Object.== at all.

Effectively shorthand for:

HashMap<K, V>(equals: identical, hashCode: identityHashCode)

Implementation

external factory HashMap.identity();

HashMap.of() factory

factory HashMap.of(Map<K, V> other)

Creates a HashMap that contains all key/value pairs of other. Example:

final baseMap = <int, String>{1: 'A', 2: 'B', 3: 'C'};
final mapOf = HashMap<num, Object>.of(baseMap);
print(mapOf); // {1: A, 2: B, 3: C}

Implementation

factory HashMap.of(Map<K, V> other) => HashMap<K, V>()..addAll(other);

Properties

entries no setter inherited

Iterable<MapEntry<K, V>> get entries

The map entries of this Map.

Inherited from Map.

Implementation

Iterable<MapEntry<K, V>> get entries;

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

external int get hashCode;

isEmpty no setter inherited

bool get isEmpty

Whether there is no key/value pair in the map.

Inherited from Map.

Implementation

bool get isEmpty;

isNotEmpty no setter inherited

bool get isNotEmpty

Whether there is at least one key/value pair in the map.

Inherited from Map.

Implementation

bool get isNotEmpty;

keys no setter inherited

Iterable<K> get keys

The keys of this Map.

The returned iterable has efficient length and contains operations, based on length and containsKey of the map.

The order of iteration is defined by the individual Map implementation, but must be consistent between changes to the map.

Modifying the map while iterating the keys may break the iteration.

Inherited from Map.

Implementation

Iterable<K> get keys;

length no setter inherited

int get length

The number of key/value pairs in the map.

Inherited from Map.

Implementation

int get length;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

values no setter inherited

Iterable<V> get values

The values of this Map.

The values are iterated in the order of their corresponding keys. This means that iterating keys and values in parallel will provide matching pairs of keys and values.

The returned iterable has an efficient length method based on the length of the map. Its Iterable.contains method is based on == comparison.

Modifying the map while iterating the values may break the iteration.

Inherited from Map.

Implementation

Iterable<V> get values;

Methods

addAll() inherited

void addAll(Map<K, V> other)

Adds all key/value pairs of other to this map.

If a key of other is already in this map, its value is overwritten.

The operation is equivalent to doing this[key] = value for each key and associated value in other. It iterates over other, which must therefore not change during the iteration.

final planets = <int, String>{1: 'Mercury', 2: 'Earth'};
planets.addAll({5: 'Jupiter', 6: 'Saturn'});
print(planets); // {1: Mercury, 2: Earth, 5: Jupiter, 6: Saturn}

Inherited from Map.

Implementation

void addAll(Map<K, V> other);

addEntries() inherited

void addEntries(Iterable<MapEntry<K, V>> newEntries)

Adds all key/value pairs of newEntries to this map.

If a key of newEntries is already in this map, the corresponding value is overwritten.

The operation is equivalent to doing this[entry.key] = entry.value for each MapEntry of the iterable.

final planets = <int, String>{1: 'Mercury', 2: 'Venus',
  3: 'Earth', 4: 'Mars'};
final gasGiants = <int, String>{5: 'Jupiter', 6: 'Saturn'};
final iceGiants = <int, String>{7: 'Uranus', 8: 'Neptune'};
planets.addEntries(gasGiants.entries);
planets.addEntries(iceGiants.entries);
print(planets);
// {1: Mercury, 2: Venus, 3: Earth, 4: Mars, 5: Jupiter, 6: Saturn,
//  7: Uranus, 8: Neptune}

Inherited from Map.

Implementation

void addEntries(Iterable<MapEntry<K, V>> newEntries);

cast() inherited

Map<RK, RV> cast<RK, RV>()

Provides a view of this map as having RK keys and RV instances, if necessary.

If this map is already a Map<RK, RV>, it is returned unchanged.

If this set contains only keys of type RK and values of type RV, all read operations will work correctly. If any operation exposes a non-RK key or non-RV value, the operation will throw instead.

Entries added to the map must be valid for both a Map<K, V> and a Map<RK, RV>.

Methods which accept Object? as argument, like containsKey, remove and operator [], will pass the argument directly to the this map's method without any checks. That means that you can do mapWithStringKeys.cast<int,int>().remove("a") successfully, even if it looks like it shouldn't have any effect.

Inherited from Map.

Implementation

Map<RK, RV> cast<RK, RV>();

clear() inherited

void clear()

Removes all entries from the map.

After this, the map is empty.

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
planets.clear(); // {}

Inherited from Map.

Implementation

void clear();

containsKey() inherited

bool containsKey(Object? key)

Whether this map contains the given key.

Returns true if any of the keys in the map are equal to key according to the equality used by the map.

final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
  'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14};
final containsUranus = moonCount.containsKey('Uranus'); // true
final containsPluto = moonCount.containsKey('Pluto'); // false

Inherited from Map.

Implementation

bool containsKey(Object? key);

containsValue() inherited

bool containsValue(Object? value)

Whether this map contains the given value.

Returns true if any of the values in the map are equal to value according to the == operator.

final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
  'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14};
final moons3 = moonCount.containsValue(3); // false
final moons82 = moonCount.containsValue(82); // true

Inherited from Map.

Implementation

bool containsValue(Object? value);

forEach() inherited

void forEach(void Function(K key, V value) action)

Applies action to each key/value pair of the map.

Calling action must not add or remove keys from the map.

final planetsByMass = <num, String>{0.81: 'Venus', 1: 'Earth',
  0.11: 'Mars', 17.15: 'Neptune'};

planetsByMass.forEach((key, value) {
  print('$key: $value');
  // 0.81: Venus
  // 1: Earth
  // 0.11: Mars
  // 17.15: Neptune
});

Inherited from Map.

Implementation

void forEach(void action(K key, V value));

map() inherited

Map<K2, V2> map<K2, V2>(MapEntry<K2, V2> Function(K key, V value) convert)

Returns a new map where all entries of this map are transformed by the given convert function.

Inherited from Map.

Implementation

Map<K2, V2> map<K2, V2>(MapEntry<K2, V2> convert(K key, V value));

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

putIfAbsent() inherited

V putIfAbsent(K key, V Function() ifAbsent)

Look up the value of key, or add a new entry if it isn't there.

Returns the value associated to key, if there is one. Otherwise calls ifAbsent to get a new value, associates key to that value, and then returns the new value.

That is, if the key is currently in the map, map.putIfAbsent(key, ifAbsent) is equivalent to map[key]. If the key is not currently in the map, it's instead equivalent to map[key] = ifAbsent() (but without any guarantee that the [] and []= operators are actually called to achieve that effect).

final diameters = <num, String>{1.0: 'Earth'};
final otherDiameters = <double, String>{0.383: 'Mercury', 0.949: 'Venus'};

for (final item in otherDiameters.entries) {
  diameters.putIfAbsent(item.key, () => item.value);
}
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}

// If the key already exists, the current value is returned.
final result = diameters.putIfAbsent(0.383, () => 'Random');
print(result); // Mercury
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}

The ifAbsent function is allowed to modify the map, and if so, it behaves the same as the equivalent map[key] = ifAbsent().

Inherited from Map.

Implementation

V putIfAbsent(K key, V ifAbsent());

remove() inherited

V? remove(Object? key)

Removes key and its associated value, if present, from the map.

Returns the value associated with key before it was removed. Returns null if key was not in the map.

Note that some maps allow null as a value, so a returned null value doesn't always mean that the key was absent.

final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final removedValue = terrestrial.remove(2); // Venus
print(terrestrial); // {1: Mercury, 3: Earth}

Inherited from Map.

Implementation

V? remove(Object? key);

removeWhere() inherited

void removeWhere(bool Function(K key, V value) test)

Removes all entries of this map that satisfy the given test.

final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
terrestrial.removeWhere((key, value) => value.startsWith('E'));
print(terrestrial); // {1: Mercury, 2: Venus}

Inherited from Map.

Implementation

void removeWhere(bool test(K key, V value));

toString() inherited

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Object.

Implementation

external String toString();

update() inherited

V update(K key, V Function(V value) update, {(V Function())? ifAbsent})

Updates the value for the provided key.

Returns the new value associated with the key.

If the key is present, invokes update with the current value and stores the new value in the map.

If the key is not present and ifAbsent is provided, calls ifAbsent and adds the key with the returned value to the map.

If the key is not present, ifAbsent must be provided.

final planetsFromSun = <int, String>{1: 'Mercury', 2: 'unknown',
  3: 'Earth'};
// Update value for known key value 2.
planetsFromSun.update(2, (value) => 'Venus');
print(planetsFromSun); // {1: Mercury, 2: Venus, 3: Earth}

final largestPlanets = <int, String>{1: 'Jupiter', 2: 'Saturn',
  3: 'Neptune'};
// Key value 8 is missing from list, add it using [ifAbsent].
largestPlanets.update(8, (value) => 'New', ifAbsent: () => 'Mercury');
print(largestPlanets); // {1: Jupiter, 2: Saturn, 3: Neptune, 8: Mercury}

Inherited from Map.

Implementation

V update(K key, V update(V value), {V ifAbsent()?});

updateAll() inherited

void updateAll(V Function(K key, V value) update)

Updates all values.

Iterates over all entries in the map and updates them with the result of invoking update.

final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
terrestrial.updateAll((key, value) => value.toUpperCase());
print(terrestrial); // {1: MERCURY, 2: VENUS, 3: EARTH}

Inherited from Map.

Implementation

void updateAll(V update(K key, V value));

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

external bool operator ==(Object other);

operator inherited

V? operator [](Object? key)

The value for the given key, or null if key is not in the map.

Some maps allow null as a value. For those maps, a lookup using this operator cannot distinguish between a key not being in the map, and the key being there with a null value. Methods like containsKey or putIfAbsent can be used if the distinction is important.

Inherited from Map.

Implementation

V? operator [](Object? key);

operator []=() inherited

void operator []=(K key, V value)

Associates the key with the given value.

If the key was already in the map, its associated value is changed. Otherwise the key/value pair is added to the map.

Inherited from Map.

Implementation

void operator []=(K key, V value);