Dart LinkedHashSet

last modified April 4, 2025

In Dart, LinkedHashSet is an ordered set implementation that maintains insertion order. It combines Set and List properties by ensuring uniqueness while preserving order.

LinkedHashSet implements the Set interface and uses a hash table with a linked list. Elements must have consistent Object.== and Object.hashCode implementations.

Creating a LinkedHashSet

The simplest way to create a LinkedHashSet is using the constructor.

main.dart

import ‘dart:collection’;

void main() { var colors = LinkedHashSet<String>(); colors.add(‘red’); colors.add(‘green’); colors.add(‘blue’); colors.add(‘red’); // Duplicate ignored

print(colors); }

We create a LinkedHashSet of colors. The generic type specifies String elements. We add elements with add(), and duplicates are automatically ignored.

$ dart main.dart {red, green, blue}

LinkedHashSet from Iterable

We can create a LinkedHashSet from an existing Iterable using LinkedHashSet.from.

main.dart

import ‘dart:collection’;

void main() { var numbers = [3, 1, 4, 1, 5, 9, 2, 6]; var uniqueNumbers = LinkedHashSet<int>.from(numbers);

print(‘Original: $numbers’); print(‘Unique ordered: $uniqueNumbers’); }

This creates a LinkedHashSet from a list with duplicates. The resulting set contains unique elements in their first occurrence order.

$ dart main.dart Original: [3, 1, 4, 1, 5, 9, 2, 6] Unique ordered: {3, 1, 4, 5, 9, 2, 6}

Checking Contents

LinkedHashSet provides methods to check for elements and set properties.

main.dart

import ‘dart:collection’;

void main() { var vowels = LinkedHashSet<String>.from([‘a’, ’e’, ‘i’, ‘o’, ‘u’]);

print(vowels.contains(’e’)); // true print(vowels.contains(‘x’)); // false print(vowels.first); // a print(vowels.last); // u print(vowels.length); // 5 }

We check various properties of the LinkedHashSet. The first and last properties reflect insertion order, not alphabetical order.

$ dart main.dart true false a u 5

Set Operations

LinkedHashSet supports standard set operations like union, intersection, and difference.

main.dart

import ‘dart:collection’;

void main() { var setA = LinkedHashSet<int>.from([1, 2, 3, 4]); var setB = LinkedHashSet<int>.from([3, 4, 5, 6]);

print(‘Union: ${setA.union(setB)}’); print(‘Intersection: ${setA.intersection(setB)}’); print(‘Difference: ${setA.difference(setB)}’); }

These operations return new sets. The union combines elements from both sets, intersection finds common elements, and difference shows elements in setA not in setB.

$ dart main.dart Union: {1, 2, 3, 4, 5, 6} Intersection: {3, 4} Difference: {1, 2}

Iterating Over LinkedHashSet

We can iterate through LinkedHashSet elements while maintaining insertion order.

main.dart

import ‘dart:collection’;

void main() { var planets = LinkedHashSet<String>.from([ ‘Mercury’, ‘Venus’, ‘Earth’, ‘Mars’, ‘Jupiter’, ‘Saturn’, ‘Uranus’, ‘Neptune’ ]);

print(‘Forward order:’); for (var planet in planets) { print(planet); }

print(’\nReversed order:’); for (var planet in planets.toList().reversed) { print(planet); } }

This demonstrates iteration in both forward and reversed order. The reversed iteration requires converting to a list first since sets don’t have reverse.

$ dart main.dart Forward order: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Reversed order: Neptune Uranus Saturn Jupiter Mars Earth Venus Mercury

Removing Elements

LinkedHashSet provides methods to remove elements while maintaining order.

main.dart

import ‘dart:collection’;

void main() { var letters = LinkedHashSet<String>.from([ ‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’ ]);

print(‘Original: $letters’);

letters.remove(‘C’); print(‘After remove: $letters’);

letters.removeWhere((letter) => letter.codeUnitAt(0) % 2 == 0); print(‘After removeWhere: $letters’);

letters.clear(); print(‘After clear: $letters’); }

We demonstrate three removal methods. removeWhere filters based on character codes, removing letters with even ASCII values.

$ dart main.dart Original: {A, B, C, D, E, F, G} After remove: {A, B, D, E, F, G} After removeWhere: {A, D, F} After clear: {}

LinkedHashSet with Custom Objects

When using custom objects, we must implement hashCode and == properly.

main.dart

import ‘dart:collection’;

class Book { final String title; final String author;

Book(this.title, this.author);

@override bool operator ==(Object other) => identical(this, other) || other is Book && title == other.title && author == other.author;

@override int get hashCode => title.hashCode ^ author.hashCode;

@override String toString() => ‘$title by $author’; }

void main() { var library = LinkedHashSet<Book>(); var book1 = Book(‘1984’, ‘George Orwell’); var book2 = Book(‘Brave New World’, ‘Aldous Huxley’); var book3 = Book(‘1984’, ‘George Orwell’); // Same as book1

library.add(book1); library.add(book2); library.add(book3);

print(library); print(‘book1 and book3 same: ${library.contains(book3)}’); }

The Book class implements == and hashCode to ensure logical equality. book1 and book3 are considered the same despite being different object instances.

$ dart main.dart {1984 by George Orwell, Brave New World by Aldous Huxley} book1 and book3 same: true

Best Practices

• Order Preservation: Use when insertion order matters.

• Element Uniqueness: Ensure elements implement proper == and hashCode.

• Performance: O(1) for add/contains/remove operations.

• Memory: Uses more memory than HashSet due to order tracking.

Source

Dart LinkedHashSet Documentation

This tutorial covered Dart’s LinkedHashSet with practical examples demonstrating its key features and 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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