DefaultMap<K, V> class

The DefaultMap is a Map-like class that calls a factory function to supply missing values while storing values in an internal map.

It overrides the [] operator: when a value for a given key is present in the internal map, its value is returned. When the value is not set, the factory function is called to supply the default value.

Using DefaultMap can be useful, for example, for creating a letter counter (for example the ransom note coding exercise). See how it simplifies code, making your more concise, more explicit, and easier to read.

final string = 'mississippi'.split('');
// Without DefaultMap
final counterMap = <String, int>{};
for (final char in string) {
  if (!counterMap.containsKey(char)) counterMap[char] = 0;
  counterMap[char] = counterMap[char]! + 1;
}

// With DefaultMap
final counterMap = DefaultMap<String, int>(0);
for (final char in string) counterMap[char] += 1;

The class is inspired by Python's defaultdict.

Inheritance

Constructors

DefaultMap(V defaultValue)
Creates a DefaultMap with defaultValue as fallback.
DefaultMap.filled(V defaultValue, [Map<K, V>? inner])
Creates a DefaultMap with defaultValue as fallback, and inner as the map that will serve as the container map for the values.
DefaultMap.generate(V generator(K key), [Map<K, V>? inner])
Creates a DefaultMap where the fallback value is generated by the generator function.

Properties

entries → Iterable<MapEntry<K, V>>
The map entries of this.
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 this.
no setteroverride
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 this.
no setterinherited

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.
override
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> transform(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(covariant K key) → V
Removes value for key.
override
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 [](covariant K key) → V
Gets value for key.
override
operator []=(K key, V value) → void
Associates the key with the given value.
override