description

# LinkedHashMap<K, V> class abstract final

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

## Properties

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

## Methods

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

## Operators

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