description

# putIfAbsent method

String putIfAbsent(

1. String key,
2. String ifAbsent()

)
override

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.

That is, if the key is currently in the map, `map.putIfAbsent(key, ifAbsent)` is equivalent to `map[key]`. If the key is not currently in the map, it's instead equivalent to `map[key] = ifAbsent()` (but without any guarantee that the `[]` and `[]=` operators are actually called to achieve that effect).

    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}

The `ifAbsent` function is allowed to modify the map, and if so, it behaves the same as the equivalent `map[key] = ifAbsent()`.

## Implementation

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