description

# update abstract method

V update(

1. K key,
2. V update(
   1. V value
   ), {
3. V ifAbsent()?,

})

Updates the value for the provided `key`.

Returns the new value associated with the key.

If the key is present, invokes `update` with the current value and stores the new value in the map.

If the key is not present and `ifAbsent` is provided, calls `ifAbsent` and adds the key with the returned value to the map.

If the key is not present, `ifAbsent` must be provided.

    final planetsFromSun = <int, String>{1: 'Mercury', 2: 'unknown',
      3: 'Earth'};
    // Update value for known key value 2.
    planetsFromSun.update(2, (value) => 'Venus');
    print(planetsFromSun); // {1: Mercury, 2: Venus, 3: Earth}

    final largestPlanets = <int, String>{1: 'Jupiter', 2: 'Saturn',
      3: 'Neptune'};
    // Key value 8 is missing from list, add it using [ifAbsent].
    largestPlanets.update(8, (value) => 'New', ifAbsent: () => 'Mercury');
    print(largestPlanets); // {1: Jupiter, 2: Saturn, 3: Neptune, 8: Mercury}

## Implementation

    V update(K key, V update(V value), {V ifAbsent()?});