LinkedHashMap class - dart:collection library (original) (raw)
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 hashCodemust 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 theiterable.
factory
LinkedHashMap.fromIterables(Iterable<K> keys, Iterable<V> values)
Creates a LinkedHashMap associating the given keys to values.
factory
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 setterinherited
The hash code for this object.
no setterinherited
Whether there is no key/value pair in the map.
no setterinherited
Whether there is at least one key/value pair in the map.
no setterinherited
The keys of this Map.
no setterinherited
The number of key/value pairs in the map.
no setterinherited
A representation of the runtime type of the object.
no setterinherited
The values of this Map.
no setterinherited
Methods
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
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
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
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