Dart .cast()

In Dart, the .cast() method returns a new Map containing the same key–value pairs as the original map, but with the specified key and value types. The returned map performs runtime type checks when reading or writing entries. If a key or value does not match the specified types, a TypeError is thrown.

This method is commonly used when working with dynamically typed maps that need to be treated as a map with specific types.

Syntax

mapVariable.cast<NewKeyType, NewValueType>()

mapVariable is the Map object whose entries are cast to new types.

Parameters:

• NewKeyType: The target key type to cast the map’s keys to.
• NewValueType: The target value type to cast the map’s values to.

Return value:

Returns a new Map<K, V> view of the original map, where keys and values are cast to the specified types.

Example

In this example, a dynamically typed map is cast to a Map<String, int> so its entries can be accessed with strong typing:

void main() {
  Map<dynamic, dynamic> rawData = {
    'Alice': 90,
    'Bob': 85,
  };

  Map<String, int> scores = rawData.cast<String, int>();

  print(scores);
}

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The output of this code is:

{Alice: 90, Bob: 85}

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