Dart Map Interface

last modified April 4, 2025

In Dart, Map is a collection of key-value pairs where each key is unique. It provides efficient lookup, insertion, and deletion operations.

The Map interface is implemented by classes like HashMap and LinkedHashMap. Keys must have consistent Object.== and Object.hashCode implementations for proper functioning.

Creating a Map

The simplest way to create a Map is using the literal syntax with curly braces.

void main() {
 var capitals = {
 'USA': 'Washington',
 'Japan': 'Tokyo',
 'France': 'Paris'
 };
 print(capitals);
 print(capitals['Japan']); // Access by key
}

We create a Map of country-capital pairs using literal syntax. The type is inferred as Map<String, String>. We access values using the [] operator.

$ dart main.dart
{USA: Washington, Japan: Tokyo, France: Paris}
Tokyo

Map Operations

Basic Map operations include adding, updating, and removing key-value pairs.

void main() {
 var scores = {'Alice': 90, 'Bob': 85};
 
 // Add new entry
 scores['Charlie'] = 95;
 
 // Update existing entry
 scores['Alice'] = 92;
 
 // Remove entry
 scores.remove('Bob');
 
 print(scores);
 print('Length: ${scores.length}');
}

We demonstrate basic Map operations. The length property returns the number of key-value pairs. The remove method deletes an entry by its key.

$ dart main.dart
{Alice: 92, Charlie: 95}
Length: 2

Map Utility Methods

Map provides several utility methods for common operations like containsKey.

void main() {
 var inventory = {
 'apples': 10,
 'oranges': 5,
 'bananas': 20
 };
 print(inventory.containsKey('apples')); // true
 print(inventory.containsValue(15)); // false
 
 inventory.update('apples', (value) => value + 5);
 inventory.putIfAbsent('pears', () => 8);
 
 print(inventory);
}

containsKey checks for key existence, while containsValue checks for values. update modifies existing values, and putIfAbsent adds new entries safely.

$ dart main.dart
true
false
{apples: 15, oranges: 5, bananas: 20, pears: 8}

Iterating Over a Map

There are several ways to iterate through a Map's entries, keys, and values.

void main() {
 var colors = {
 'red': '#FF0000',
 'green': '#00FF00',
 'blue': '#0000FF'
 };
 // Iterate keys
 print('Keys:');
 for (var key in colors.keys) {
 print(key);
 }
 // Iterate values
 print('\nValues:');
 colors.values.forEach(print);
 // Iterate entries
 print('\nEntries:');
 colors.forEach((key, value) => print('$key: $value'));
}

We demonstrate three iteration approaches. The keys and values properties return iterables. The forEach method provides both key and value in the callback.

$ dart main.dart
Keys:
red
green
blue
Values:
#FF0000
#00FF00
#0000FF
Entries:
red: #FF0000
green: #00FF00
blue: #0000FF

Map Transformation

Maps can be transformed using methods like map, where, and addAll.

void main() {
 var original = {'a': 1, 'b': 2, 'c': 3};
 
 // Transform values
 var squared = original.map((key, value) => 
 MapEntry(key, value * value));
 
 // Filter entries
 var filtered = Map.fromEntries(original.entries
 .where((entry) => entry.value > 1));
 
 // Merge maps
 var merged = {...original, 'd': 4, 'e': 5};
 
 print('Squared: $squared');
 print('Filtered: $filtered');
 print('Merged: $merged');
}

The map method creates a new Map by transforming each entry. We filter using where and spread operator (...) for merging. These operations return new Maps.

$ dart main.dart
Squared: {a: 1, b: 4, c: 9}
Filtered: {b: 2, c: 3}
Merged: {a: 1, b: 2, c: 3, d: 4, e: 5}

Best Practices

• Immutable Keys: Prefer immutable objects for Map keys.
• Null Safety: Use nullable types if values can be null.
• Efficient Lookups: Use containsKey for key existence checks.
• Proper Hashing: Override hashCode when using custom keys.

Source

Dart Map Documentation

This tutorial covered Dart's Map interface with practical examples demonstrating its core functionality and common usage patterns.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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