putIfAbsent method Null safety

String putIfAbsent (
  1. String key,
  2. String 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.


    String putIfAbsent(String key, String ifAbsent()) {
      if (!containsKey(key)) this[key] = ifAbsent();
      return this[key] as String;