Back/Map — Key-Value Pairs & JSON
Lesson 18 min

Map — Key-Value Pairs & JSON

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Map: Key-Value Pairs & JSON

A Map is an unordered collection of key-value pairs. Keys must be unique, but values can be duplicated. Maps are highly versatile and are the primary data structure used to represent JSON payloads in web APIs.

Declaring Maps

You can declare a Map using curly braces {} or the Map class constructor.

dart
void main() {
  // Using Map Literal syntax
  var ages = {
    'Alice': 25,
    'Bob': 30,
    'Charlie': 35,
  }; // Inferred as Map<String, int>

  // Explicit type declaration
  Map<String, String> capitals = {
    'India': 'New Delhi',
    'USA': 'Washington D.C.',
  };

  // Empty Map declaration
  var emptyMap = <String, int>{};
  Map<int, String> statusCodes = Map();
}

Map Operations (CRUD)

1. Create & Update

To add a new key-value pair or update an existing value, use the square bracket index operator [].

dart
void main() {
  var scores = {'Math': 90};
  scores['Science'] = 95; // Create new key
  scores['Math'] = 92;    // Update existing key
  print(scores); // {Math: 92, Science: 95}
}

2. Read

Access a value by passing its key to the bracket operator. If the key doesn't exist, it returns null.

dart
void main() {
  var capitals = {'India': 'New Delhi'};
  print(capitals['India']);  // New Delhi
  print(capitals['France']); // null
  
  // Checking key existence
  if (capitals.containsKey('France')) {
    print("Found France!");
  }
}

3. Delete

Use the remove() method to delete a key-value pair.

dart
void main() {
  var user = {'id': 1, 'name': 'John', 'role': 'Admin'};
  user.remove('role');
  print(user); // {id: 1, name: John}
}

Iterating Over Maps

You can iterate over a map using forEach(), a for-in loop with keys, values, or entries.

dart
void main() {
  var currency = {'USD': 'Dollar', 'EUR': 'Euro', 'INR': 'Rupee'};

  // Method 1: Using forEach
  currency.forEach((code, name) {
    print("$code stands for $name");
  });

  // Method 2: Using for-in over entries
  for (var entry in currency.entries) {
    print("${entry.key} is the code for ${entry.value}");
  }
}

Maps and JSON

JSON (JavaScript Object Notation) maps directly to Dart's Map<String, dynamic> type. When you fetch data from an API, you parse the raw JSON string into a Dart Map to access properties.

dart
import 'dart:convert';

void main() {
  // A raw JSON string from a server
  String jsonString = '{"name": "Alice", "id": 101, "isActive": true}';

  // Decode JSON string to Map
  Map<String, dynamic> userMap = jsonDecode(jsonString);

  print("User Name: ${userMap['name']}"); // Alice
  print("User ID: ${userMap['id']}");     // 101
}

Summary

  • A Map stores data as key-value associations.
  • Keys must be unique; lookup by key is extremely fast (O(1)).
  • JSON payloads are represented as Map<String, dynamic> in Dart.
  • Use jsonDecode() from dart:convert to convert JSON strings into Dart Maps.
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