An insertion-orderedMap with expected constant-time lookup.

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

Thekeys,values andentries are iterated in key insertion order.

The map uses a hash-table to look up entries, so keys must havesuitable implementations ofObject.operator== andObject.hashCode.If the hash codes are not well-distributed, the performance of mapoperations 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 againwill make it be last in the iteration order.

Notice:Do not modify a map (add or remove keys) while an operationis being performed on that map, for example in functionscalled during aforEach orputIfAbsent call,or while iterating the map (keys,values orentries).

The keys of aLinkedHashMap must have consistentObject.==andObject.hashCode implementations. This means that the== operatormust define a stable equivalence relation on the keys (reflexive,symmetric, transitive, and consistent over time), and thathashCodemust 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, useoperator[]=,addAll oraddEntries.

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

To check if the map is empty, useisEmpty orisNotEmpty.To find the number of map entries, uselength.

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

TheforEach 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, usecontainsKey.

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

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

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

To remove an entry with a specific key, useremove.

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

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

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 onwhether there already is an entry with that key,useputIfAbsent orupdate.

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,useupdateAll.

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

To remove all entries and empty the map, useclear.

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({boolequals(K,K)?,inthashCode(K)?,boolisValidKey(dynamic)?})

Creates an insertion-ordered hash-table basedMap.

factory

LinkedHashMap.from(Mapother)

Creates aLinkedHashMap that contains all key value pairs ofother.

factory

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

Creates aLinkedHashMap containing the entries ofentries.

factory

LinkedHashMap.fromIterable(Iterableiterable, {Kkey(dynamicelement)?,Vvalue(dynamicelement)?})

Creates aLinkedHashMap where the keys and values are computed from theiterable.

factory

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

Creates aLinkedHashMap associating the givenkeys tovalues.

factory

LinkedHashMap.identity()

Creates an insertion-ordered identity-based map.

factory

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

Creates aLinkedHashMap that contains all key value pairs ofother.Example:

factory

Properties

entries→Iterable<MapEntry<K,V>>

The map entries of thisMap.

no setterinherited

hashCode→int

The hash code for this object.

no setterinherited

isEmpty→bool

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

no setterinherited

isNotEmpty→bool

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

no setterinherited

keys→Iterable<K>

The keys of thisMap.

no setterinherited

length→int

The number of key/value pairs in the map.

no setterinherited

runtimeType→Type

A representation of the runtime type of the object.

no setterinherited

values→Iterable<V>

The values of thisMap.

no setterinherited

Methods

addAll(Map<K,V>other)→ void

Adds all key/value pairs ofother to this map.

inherited

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

Adds all key/value pairs ofnewEntries to this map.

inherited

cast<RK,RV>()→Map<RK,RV>

Provides a view of this map as havingRK keys andRV instances,if necessary.

inherited

clear()→ void

Removes all entries from the map.

inherited

containsKey(Object?key)→bool

Whether this map contains the givenkey.

inherited

containsValue(Object?value)→bool

Whether this map contains the givenvalue.

inherited

forEach(voidaction(Kkey,Vvalue))→ void

Appliesaction to each key/value pair of the map.

inherited

map<K2,V2>(MapEntry<K2,V2>convert(Kkey,Vvalue))→Map<K2,V2>

Returns a new map where all entries of this map are transformed bythe givenconvert function.

inherited

noSuchMethod(Invocationinvocation)→ dynamic

Invoked when a nonexistent method or property is accessed.

inherited

putIfAbsent(Kkey,VifAbsent())→ V

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

inherited

remove(Object?key)→ V?

Removeskey and its associated value, if present, from the map.

inherited

removeWhere(booltest(Kkey,Vvalue))→ void

Removes all entries of this map that satisfy the giventest.

inherited

toString()→String

A string representation of this object.

inherited

update(Kkey,Vupdate(Vvalue), {VifAbsent()?})→ V

Updates the value for the providedkey.

inherited

updateAll(Vupdate(Kkey,Vvalue))→ void

Updates all values.

inherited

Operators

operator ==(Objectother)→bool

The equality operator.

inherited

operator [](Object?key)→ V?

The value for the givenkey, ornull ifkey is not in the map.

inherited

operator []=(Kkey,Vvalue)→ void

Associates thekey with the givenvalue.

inherited