putIfAbsent method

V? putIfAbsent(
  1. K key,
  2. 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

V? putIfAbsent(K key, V ifAbsent()) {
  if (value == null) return null;
  late V _result;
  update((_) => _result = value!.putIfAbsent(key, ifAbsent));
  return _result;
}