Map<K, V>.fromIterable constructor Null safety

Creates a Map instance in which the keys and values are computed from the iterable.

For each element of the iterable, a key/value pair is computed by applying key and value respectively to the element of the iterable.

Equivalent to the map literal:

<K, V>{for (var v in iterable) key(v): value(v)}

The literal is generally preferable because it allows for a more precise typing.

The example below creates a new map from a list of integers. The keys of map are the list values converted to strings, and the values of the map are the squares of the list values:

List<int> list = [1, 2, 3];
var map = Map<String, int>.fromIterable(list,
    key: (item) => item.toString(),
    value: (item) => item * item);
// map is {"1": 1, "2": 4, "3": 9}

If no values are specified for key and value, the default is the identity function. In that case, the iterable element must be assignable to the key or value type of the created map.

In the following example, the keys and corresponding values of map are the list values directly:

var map = Map<int, int>.fromIterable(list);
// map is {1: 1, 2: 2, 3: 3}

The keys computed by the source iterable do not need to be unique. The last occurrence of a key will overwrite the value of any previous occurrence.

The created map is a LinkedHashMap. A LinkedHashMap requires the keys to implement compatible operator== and hashCode. It iterates in key insertion order.