putIfAbsent method Null safety

V putIfAbsent (
  1. K key,
  2. V ifAbsent(
      )
    )
    override

    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.

    Implementation

    V putIfAbsent(K key, V ifAbsent()) {
  if (containsKey(key)) {
    return this[key] as V;
  }
  return this[key] = ifAbsent();
}