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LinkedHashMap<K, V> class

An insertion-ordered Map with expected constant-time lookup.

A non-constant map literal, like {"a": 42, "b": 7}, is a LinkedHashMap.

The keys, values and entries are iterated in key insertion order.

The map uses a hash-table to look up entries, so keys must have suitable implementations of Object.operator== and Object.hashCode. If the hash codes are not well-distributed, the performance of map operations may suffer.

The insertion order of keys is remembered, and keys are iterated in the order they were inserted into the map. Values and entries are iterated in their corresponding key's order. Changing a key's value, when the key is already in the map, does not change the iteration order, but removing the key and adding it again will make it be last in the iteration order.

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).

The keys of a LinkedHashMap 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 ==.

Example:

final planetsByDiameter = {0.949: 'Venus'}; // A new LinkedHashMap

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

planetsByDiameter[1] = 'Earth';
planetsByDiameter.addAll({0.532: 'Mars', 11.209: 'Jupiter'});

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

print(planetsByDiameter.isEmpty); // false
print(planetsByDiameter.length); // 4
print(planetsByDiameter);
// {0.949: Venus, 1.0: Earth, 0.532: Mars, 11.209: Jupiter}

The forEach method calls a function for each key/value entry of the map.

planetsByDiameter.forEach((key, value) {
  print('$key \t $value');
  // 0.949    Venus
  // 1.0      Earth
  // 0.532    Mars
  // 11.209   Jupiter
});

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

final keyOneExists = planetsByDiameter.containsKey(1); // true
final keyFiveExists = planetsByDiameter.containsKey(5); // false

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

final earthExists = planetsByDiameter.containsValue('Earth'); // true
final saturnExists =  planetsByDiameter.containsValue('Saturn'); // false

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

final removedValue = planetsByDiameter.remove(1);
print(removedValue); // Earth
print(planetsByDiameter); // {0.949: Venus, 0.532: Mars, 11.209: Jupiter}

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

planetsByDiameter.removeWhere((key, value) => key == 0.949);
print(planetsByDiameter); // {0.532: Mars, 11.209: Jupiter}

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.

planetsByDiameter.update(0.949, (v) => 'Venus', ifAbsent: () => 'Venus');
planetsByDiameter.putIfAbsent(0.532, () => "Another Mars if needed");
print(planetsByDiameter); // {0.532: Mars, 11.209: Jupiter, 0.949: Venus}

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

planetsByDiameter.updateAll((key, value) => 'X');
print(planetsByDiameter); // {0.532: X, 11.209: X, 0.949: X}

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

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

See also:

Map, the general interface of key/value pair collections.
HashMap is unordered (the order of iteration is not guaranteed).
SplayTreeMap iterates the keys in sorted order.

Implemented types

Map<K, V>

Constructors

LinkedHashMap({bool equals(K, K)?, int hashCode(K)?, bool isValidKey(dynamic)?}) factory: Creates an insertion-ordered hash-table based Map.
LinkedHashMap.from(Map other) factory: Creates a LinkedHashMap that contains all key value pairs of other.
LinkedHashMap.fromEntries(Iterable<MapEntry<K, V>> entries) factory: Creates a LinkedHashMap containing the entries of entries.
LinkedHashMap.fromIterable(Iterable iterable, {K key(dynamic element)?, V value(dynamic element)?}) factory: Creates a LinkedHashMap where the keys and values are computed from the iterable.
LinkedHashMap.fromIterables(Iterable<K> keys, Iterable<V> values) factory: Creates a LinkedHashMap associating the given keys to values.
LinkedHashMap.identity() factory: Creates an insertion-ordered identity-based map.
LinkedHashMap.of(Map<K, V> other) factory: Creates a LinkedHashMap that contains all key value pairs of other. Example:

Properties

entries → Iterable<MapEntry<K, V>> read-only, inherited: The map entries of this.
hashCode → int read-only, inherited: The hash code for this object.
isEmpty → bool read-only, inherited: Whether there is no key/value pair in the map.
isNotEmpty → bool read-only, inherited: Whether there is at least one key/value pair in the map.
keys → Iterable<K> read-only, inherited: The keys of this.
length → int read-only, inherited: The number of key/value pairs in the map.
runtimeType → Type read-only, inherited: A representation of the runtime type of the object.
values → Iterable<V> read-only, inherited: The values of this.

Methods

addAll(Map<K, V> other) → void inherited: Adds all key/value pairs of other to this map.
addEntries(Iterable<MapEntry<K, V>> newEntries) → void inherited: Adds all key/value pairs of newEntries to this map.
cast<RK, RV>() → Map<RK, RV> inherited: Provides a view of this map as having RK keys and RV instances, if necessary.
clear() → void inherited: Removes all entries from the map.
containsKey(Object? key) → bool inherited: Whether this map contains the given key.
containsValue(Object? value) → bool inherited: Whether this map contains the given value.
forEach(void action(K key, V value)) → void inherited: Applies action to each key/value pair of the map.
map<K2, V2>(MapEntry<K2, V2> convert(K key, V value)) → Map<K2, V2> inherited: Returns a new map where all entries of this map are transformed by the given convert function.
noSuchMethod(Invocation invocation) → dynamic inherited: Invoked when a non-existent method or property is accessed.
putIfAbsent(K key, V ifAbsent()) → V inherited: Look up the value of key, or add a new entry if it isn't there.
remove(Object? key) → V? inherited: Removes key and its associated value, if present, from the map.
removeWhere(bool test(K key, V value)) → void inherited: Removes all entries of this map that satisfy the given test.
toString() → String inherited: A string representation of this object.
update(K key, V update(V value), {V ifAbsent()?}) → V inherited: Updates the value for the provided key.
updateAll(V update(K key, V value)) → void inherited: Updates all values.

Operators

operator ==(Object other) → bool inherited: The equality operator.
operator [](Object? key) → V? inherited: The value for the given key, or null if key is not in the map.
operator []=(K key, V value) → void inherited: Associates the key with the given value.

© 2012 the Dart project authors
Licensed under the BSD 3-Clause "New" or "Revised" License.
https://api.dart.dev/stable/2.18.5/dart-collection/LinkedHashMap-class.html