Map<K, V> abstract interface

abstract interface class Map<K, V>

A collection of key/value pairs, from which you retrieve a value using its associated key.

There is a finite number of keys in the map, and each key has exactly one value associated with it.

Maps, and their keys and values, can be iterated. The order of iteration is defined by the individual type of map. Examples:

• The plain HashMap is unordered (no order is guaranteed),
• the LinkedHashMap iterates in key insertion order,
• and a sorted map like SplayTreeMap iterates the keys in sorted order.

It is generally not allowed to modify the map (add or remove keys) while an operation is being performed on the map, for example in functions called during a forEach call. Modifying the map while iterating the keys or values may also break the iteration.

It is generally not allowed to modify the equality of keys (and thus not their hashcode) while they are in the map. Some specialized subtypes may be more permissive, in which case they should document this behavior.

Key equality must be an equality relation. If the key stored in a map and the key used for lookup do not agree on whether the two are equal, so equality is not symmetric, then lookup behavior is unspecified.

Implementers

• HashMap<K, V>
• HttpSession
• LinkedHashMap<K, V>
• MapBase<K, V>
• MapView<K, V>
• UnmodifiableMapBase<K, V>
• UnmodifiableMapView<K, V>

Constructors

Map() factory

factory Map()

Creates an empty LinkedHashMap.

This constructor is equivalent to the non-const map literal <K, V>{}.

A LinkedHashMap requires the keys to implement compatible operator== and hashCode. It iterates in key insertion order.

Implementation

&#47;&#47; TODO: @Deprecated("Use literal <K, V>{} instead")
external factory Map();

Map.from() factory

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

Creates a LinkedHashMap with the same keys and values as other.

The keys must all be instances of K and the values of V. The other map itself can have any type, unlike for Map.of, and the key and value types are checked (and can fail) at run-time.

Prefer using Map.of when possible, and only use Map.from to create a new map with more precise types than the original, and when it's known that all the keys and values have those more precise types.

A LinkedHashMap requires the keys to implement compatible operator== and hashCode. It iterates in key insertion order.

final planets = <num, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth', 4: 'Mars'};
final mapFrom = Map<int, String>.from(planets);
print(mapFrom); // {1: Mercury, 2: Venus, 3: Earth, 4: Mars}

Implementation

factory Map.from(Map other) = LinkedHashMap<K, V>.from;

Map.fromEntries() factory

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

Creates a new map and adds all entries.

Returns a new Map<K, V> where all entries of entries have been added in iteration order.

If multiple entries have the same key, later occurrences overwrite the value of the earlier ones.

Equivalent to the map literal:

<K, V>{for (var e in entries) e.key: e.value}

Example:

final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
  'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14};
final map = Map.fromEntries(moonCount.entries);

Implementation

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

Map.fromIterable() factory

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

Creates a Map instance in which the keys and values are computed from the iterable.

For each element of the iterable, a key/value pair is computed by applying key and value respectively to the element of the iterable.

Equivalent to the map literal:

<K, V>{for (var v in iterable) key(v): value(v)}

The literal is generally preferable because it allows for a more precise typing.

The example below creates a new map from a list of integers. The keys of map are the list values converted to strings, and the values of the map are the squares of the list values:

final numbers = <int>[1, 2, 3];
final map = Map<String, int>.fromIterable(numbers,
    key: (item) => item.toString(),
    value: (item) => item * item);
print(map); // {1: 1, 2: 4, 3: 9}

If no values are specified for key and value, the default is the identity function. In that case, the iterable element must be assignable to the key or value type of the created map.

In the following example, the keys and corresponding values of map are the list values directly:

final numbers = <int>[1, 2, 3];
final map = Map.fromIterable(numbers);
print(map); // {1: 1, 2: 2, 3: 3}

The keys computed by the source iterable do not need to be unique. The last occurrence of a key will overwrite the value of any previous occurrence.

The created map is a LinkedHashMap. A LinkedHashMap requires the keys to implement compatible operator== and hashCode. It iterates in key insertion order.

Implementation

factory Map.fromIterable(
  Iterable iterable, {
  K key(dynamic element)?,
  V value(dynamic element)?,
}) = LinkedHashMap<K, V>.fromIterable;

Map.fromIterables() factory

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

Creates a map associating the given keys to the given values.

The map construction iterates over keys and values simultaneously, and adds an entry to the map for each pair of key and value.

final rings = <bool>[false, false, true, true];
final planets = <String>{'Earth', 'Mars', 'Jupiter', 'Saturn'};
final map = Map<String, bool>.fromIterables(planets, rings);
print(map); // {Earth: false, Mars: false, Jupiter: true, Saturn: true}

If keys contains the same object multiple times, the value of the last occurrence overwrites any previous value.

The two Iterables must have the same length.

The created map is a LinkedHashMap. A LinkedHashMap requires the keys to implement compatible operator== and hashCode. It iterates in key insertion order.

Implementation

factory Map.fromIterables(Iterable<K> keys, Iterable<V> values) =
    LinkedHashMap<K, V>.fromIterables;

Map.identity() factory

factory Map.identity()

Creates an identity map with the default implementation, LinkedHashMap.

An identity map uses identical for equality and identityHashCode for hash codes of keys instead of the intrinsic Object.== and Object.hashCode of the keys.

The map iterates in key insertion order.

Implementation

factory Map.identity() = LinkedHashMap<K, V>.identity;

Map.of() factory

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

Creates a LinkedHashMap with the same keys and values as other.

A LinkedHashMap requires the keys to implement compatible operator== and hashCode, and it allows null as a key. It iterates in key insertion order.

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final mapOf = Map<num, String>.of(planets);
print(mapOf); // {1: Mercury, 2: Venus, 3: Earth}

Implementation

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

Map.unmodifiable() factory

factory Map.unmodifiable(Map<dynamic, dynamic> other)

Creates an unmodifiable hash-based map containing the entries of other.

The keys must all be instances of K and the values of V. The other map itself can have any type.

The map requires the keys to implement compatible operator== and hashCode. The created map iterates keys in a fixed order, preserving the order provided by other.

The resulting map behaves like the result of Map.from, except that the map returned by this constructor is not modifiable.

final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final unmodifiableMap = Map.unmodifiable(planets);
unmodifiableMap[4] = 'Mars'; // Throws

Implementation

external factory Map.unmodifiable(Map<dynamic, dynamic> other);

Properties

entries no setter

Iterable<MapEntry<K, V>> get entries

The map entries of this 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

bool get isEmpty

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

Implementation

bool get isEmpty;

isNotEmpty no setter

bool get isNotEmpty

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

Implementation

bool get isNotEmpty;

keys no setter

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.

Implementation

Iterable<K> get keys;

length no setter

int get length

The number of key/value pairs in the 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

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.

Implementation

Iterable<V> get values;

Methods

addAll()

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}

Implementation

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

addEntries()

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}

Implementation

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

cast()

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.

Implementation

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

clear()

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(); // {}

Implementation

void clear();

containsKey()

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

Implementation

bool containsKey(Object? key);

containsValue()

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

Implementation

bool containsValue(Object? value);

forEach()

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
});

Implementation

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

map()

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.

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()

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().

Implementation

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

remove()

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}

Implementation

V? remove(Object? key);

removeWhere()

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}

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()

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}

Implementation

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

updateAll()

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}

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

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.

Implementation

V? operator [](Object? key);

operator []=()

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.

Implementation

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

Static Methods

castFrom()

Map<K2, V2> castFrom<K, V, K2, V2>(Map<K, V> source)

Adapts source to be a Map<K2, V2>.

Any time the set would produce a key or value that is not a K2 or V2, the access will throw.

Any time K2 key or V2 value is attempted added into the adapted map, the store will throw unless the key is also an instance of K and the value is also an instance of V.

If all accessed entries of source have K2 keys and V2 values, and if all entries added to the returned map have K keys and V values, then the returned map can be used as a Map<K2, V2>.

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.

Implementation

static Map<K2, V2> castFrom<K, V, K2, V2>(Map<K, V> source) =>
    CastMap<K, V, K2, V2>(source);