putIfAbsent method

@override V putIfAbsent(K key, V ifAbsent())

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

Returns the value associated to key, if there is one. Otherwise calls ifAbsent to get a new value, associates key to that value, and then returns the new value.

final diameters = <num, String>{1.0: 'Earth'};
final otherDiameters = <double, String>{0.383: 'Mercury', 0.949: 'Venus'};

for (final item in otherDiameters.entries) {
  diameters.putIfAbsent(item.key, () => item.value);
}
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}

// If the key already exists, the current value is returned.
final result = diameters.putIfAbsent(0.383, () => 'Random');
print(result); // Mercury
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}

Calling ifAbsent must not add or remove keys from the map.

Implementation

@override
V putIfAbsent(K key, V Function() ifAbsent) {
  var len = _map.length;
  var result = _map.putIfAbsent(key, ifAbsent);
  if (hasObservers && len != _map.length) {
    notifyPropertyChange(#length, len, _map.length);
    notifyChange(MapChangeRecord<K, V>.insert(key, result));
    _notifyKeysValuesChanged();
  }
  return result;
}