Dart LinkedHashMap

last modified April 4, 2025

In Dart, LinkedHashMap is an ordered collection of key-value pairs. It maintains insertion order while providing fast lookup operations.

LinkedHashMap implements the Map interface using a hash table and linked list. This combination preserves order while maintaining O(1) access time for keys.

Creating a LinkedHashMap

LinkedHashMap can be created using its constructor or from another Map.

main.dart

import ‘dart:collection’;

void main() { var orderedCapitals = LinkedHashMap<String, String>(); orderedCapitals[‘USA’] = ‘Washington’; orderedCapitals[‘Japan’] = ‘Tokyo’; orderedCapitals[‘France’] = ‘Paris’;

print(orderedCapitals); }

We create a LinkedHashMap of country-capital pairs. The entries will be printed in the same order they were inserted, unlike regular HashMap.

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

LinkedHashMap.from

We can create a LinkedHashMap from an existing Map while preserving order.

main.dart

import ‘dart:collection’;

void main() { var unordered = {‘z’: 1, ‘a’: 2, ’m’: 3}; var ordered = LinkedHashMap.from(unordered);

print(‘Original: $unordered’); print(‘LinkedHashMap: $ordered’); }

The LinkedHashMap.from constructor creates a new map with the same entries. The insertion order is preserved from the original map’s iteration order.

$ dart main.dart Original: {z: 1, a: 2, m: 3} LinkedHashMap: {z: 1, a: 2, m: 3}

Maintaining Insertion Order

LinkedHashMap preserves insertion order even when entries are modified.

main.dart

import ‘dart:collection’;

void main() { var tasks = LinkedHashMap<int, String>(); tasks[1] = ‘Buy groceries’; tasks[2] = ‘Clean house’; tasks[3] = ‘Pay bills’;

// Modify existing entry tasks[2] = ‘Deep clean house’;

// Add new entry tasks[4] = ‘Walk dog’;

print(tasks); }

Modifying an existing value doesn’t change its position in the iteration order. New entries are always added to the end of the iteration sequence.

$ dart main.dart {1: Buy groceries, 2: Deep clean house, 3: Pay bills, 4: Walk dog}

Accessing First and Last Elements

LinkedHashMap provides easy access to its first and last entries.

main.dart

import ‘dart:collection’;

void main() { var colors = LinkedHashMap<String, String>(); colors[‘red’] = ‘#FF0000’; colors[‘green’] = ‘#00FF00’; colors[‘blue’] = ‘#0000FF’;

print(‘First key: ${colors.keys.first}’); print(‘Last value: ${colors.values.last}’); print(‘First entry: ${colors.entries.first}’); }

We can access the first and last elements through keys, values, or entries. This is particularly useful for ordered collections like LinkedHashMap.

$ dart main.dart First key: red Last value: #0000FF First entry: MapEntry(red: #FF0000)

Removing Elements While Preserving Order

Removing elements from LinkedHashMap maintains the order of remaining entries.

main.dart

import ‘dart:collection’;

void main() { var inventory = LinkedHashMap<String, int>(); inventory[‘apples’] = 10; inventory[‘bananas’] = 5; inventory[‘oranges’] = 8; inventory[‘pears’] = 12;

print(‘Original: $inventory’);

inventory.remove(‘bananas’); print(‘After remove: $inventory’);

inventory.removeWhere((key, value) => value < 9); print(‘After removeWhere: $inventory’); }

When removing entries, the remaining elements keep their original order. The removeWhere method filters based on a condition while preserving order.

$ dart main.dart Original: {apples: 10, bananas: 5, oranges: 8, pears: 12} After remove: {apples: 10, oranges: 8, pears: 12} After removeWhere: {apples: 10, pears: 12}

Best Practices

• Order Matters: Use LinkedHashMap when insertion order is important.

• Memory Consideration: LinkedHashMap uses more memory than HashMap.

• Performance: Both have O(1) access time but LinkedHashMap has overhead.

• Key Consistency: Ensure keys have proper hashCode and == implementations.

Source

Dart LinkedHashMap Documentation

This tutorial covered Dart’s LinkedHashMap with examples demonstrating its ordered nature and key features compared to regular HashMap.

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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