What are the differences between the different Map implementations in Dart?

Question

Dart has a Map type, with implementations like HashMap, LinkedHashMap, and SplayTreeMap. What\'s the difference between those different Map implementations?

Answer 1

Key points to note about Dart Map

A Dart Map is a collection of key-value pairs. It maps each key to exactly one value. We can iterate over a Map.

There are three types of map, depending in the order of iteration:

• HashMap is unordered. The key-value pair coming later could be ordered first.
• LinkedHashMap has predictable iteration order by the insertion order. The key-value pair coming later will be ordered later.
• SplayTreeMap iterates the keys in sorted order. It is a self-balancing binary tree, frequently accessed elements will be moved more closely to the root of the tree.

By default, creating an instance using Map constructor (Map(), Map.from(), Map.of()) will return a LinkedHashMap.

wherever we use Map, you can think about LinkedHashMap, and you can also replace Map with LinkedHashMap.

Create a new Map in Dart/Flutter

Using new keyword, we can create a new Map.

Don’t forget to import dart:collection library before using these syntax containing HashMap, LinkedHashMap, SplayTreeMap, also other code in the rest of this answer

import 'dart:collection';

main() {
  HashMap hashMap = new HashMap();
  LinkedHashMap linkedHashMap = new LinkedHashMap();
  SplayTreeMap treeMap = new SplayTreeMap();
}

With recent Dart version, we don’t need to use new keyword anymore. For example, HashMap map = HashMap() is accepted.

We can create a new LinkedHashMap using Map constructor like this.

Map map = Map();
if (map is LinkedHashMap) {
  print("This is a LinkedHashMap.");
}

// Result: This is a LinkedHashMap.

In the code above, we specify the type of key-value pairs: . You can also declare a Map, LinkedHashMap, SplayTreeMap without setting the type for their keys and values. What does it mean? It will not constrain us to add a pair into the Map.

These Maps now accept other types: , …

Map map = Map();
HashMap map1 = HashMap();
LinkedHashMap map2 = LinkedHashMap();
SplayTreeMap map3 = SplayTreeMap();

More : A complete reference