V putIfAbsent(
K key,
V ifAbsent()
)

Look up the value of key, or add a new value 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.

Map<String, int> scores = {'Bob': 36}; for (var key in 'Bob', 'Rohan', 'Sophena') { scores.putIfAbsent(key, () => key.length); } scores'Bob'; // 36 scores'Rohan'; // 5 scores'Sophena'; // 7

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

Source

V putIfAbsent(K key, V ifAbsent()) {
  if (key == null) throw new ArgumentError(key);
  int comp = _splay(key);
  if (comp == 0) {
    _SplayTreeMapNode mapRoot = _root;
    return mapRoot.value;
  }
  int modificationCount = _modificationCount;
  int splayCount = _splayCount;
  V value = ifAbsent();
  if (modificationCount != _modificationCount) {
    throw new ConcurrentModificationError(this);
  }
  if (splayCount != _splayCount) {
    comp = _splay(key);
    // Key is still not there, otherwise _modificationCount would be changed.
    assert(comp != 0);
  }
  _addNewRoot(new _SplayTreeMapNode(key, value), comp);
  return value;
}