Dart HashSet

last modified April 4, 2025

In Dart, HashSet is an unordered collection of unique elements. It provides fast lookup, addition, and deletion operations using hash tables.

HashSet implements the Set interface and requires elements to have consistent Object.== and Object.hashCode implementations. Duplicate elements are not allowed.

Creating a HashSet

The simplest way to create a HashSet is using the constructor. We can specify the element type and optional equality comparer.

main.dart

import ‘dart:collection’;

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

print(colors); }

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

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

HashSet from Iterable

We can create a HashSet from an existing Iterable using HashSet.from constructor. This automatically removes duplicates.

main.dart

import ‘dart:collection’;

void main() { var numbers = [1, 2, 3, 2, 4, 1, 5]; var uniqueNumbers = HashSet.from(numbers);

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

This creates a HashSet from a list with duplicates. The resulting HashSet contains only unique values from the original list.

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

Checking Contents

HashSet provides methods to check for element presence and collection state. These operations are typically O(1) complexity.

main.dart

import ‘dart:collection’;

void main() { var primes = HashSet.from([2, 3, 5, 7, 11]);

print(primes.contains(5)); // true print(primes.contains(4)); // false print(primes.isEmpty); // false print(primes.length); // 5 print(primes.first); // 2 (arbitrary) }

We check various properties of the HashSet. The first element is arbitrary since HashSet doesn’t maintain insertion order.

$ dart main.dart true false false 5 2

Set Operations

HashSet supports standard set operations like union, intersection, and difference. These methods return new sets without modifying the original.

main.dart

import ‘dart:collection’;

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

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

We demonstrate three fundamental set operations. Each operation creates a new HashSet containing the result of the operation.

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

Iterating Over HashSet

We can iterate through HashSet elements using various methods. The iteration order is undefined but consistent for the same set.

main.dart

import ‘dart:collection’;

void main() { var languages = HashSet.from([‘Dart’, ‘Java’, ‘Python’, ‘Go’]);

print(‘For loop:’); for (var lang in languages) { print(lang); }

print(’\nForEach:’); languages.forEach((lang) => print(lang));

print(’\nIterator:’); var it = languages.iterator; while (it.moveNext()) { print(it.current); } }

This demonstrates three ways to iterate through a HashSet. All methods will process elements in the same (arbitrary) order for a given set.

$ dart main.dart For loop: Dart Java Python Go

ForEach: Dart Java Python Go

Iterator: Dart Java Python Go

Removing Elements

HashSet provides methods to remove elements individually or clear the entire set. These operations are typically O(1) complexity.

main.dart

import ‘dart:collection’;

void main() { var fruits = HashSet.from([‘apple’, ‘banana’, ‘orange’, ‘kiwi’]);

print(‘Original: $fruits’);

fruits.remove(‘banana’); print(‘After remove: $fruits’);

fruits.removeWhere((fruit) => fruit.length > 5); print(‘After removeWhere: $fruits’);

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

We demonstrate element removal methods. removeWhere allows filtering elements based on a condition, while clear empties the entire set.

$ dart main.dart Original: {apple, banana, orange, kiwi} After remove: {apple, orange, kiwi} After removeWhere: {apple, kiwi} After clear: {}

HashSet with Custom Objects

When using custom objects in HashSet, we must implement hashCode and == properly. This ensures correct behavior for uniqueness and lookups.

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; }

void main() { var library = HashSet<Book>(); var book1 = Book(‘Dart in Depth’, ‘John Doe’); var book2 = Book(‘Flutter Basics’, ‘Jane Smith’); var book3 = Book(‘Dart in Depth’, ‘John Doe’); // Same as book1

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

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

The Book class implements == and hashCode to ensure logical equality. book1 and book3 are considered the same, so only one is added to the set.

$ dart main.dart Library size: 2 book1 and book3 same: true

Best Practices

• Element Immutability: Use immutable objects for stable hashing.

• Proper Hashing: Always override hashCode when overriding ==.

• Null Safety: Consider non-nullable types for clarity.

• Performance: HashSet excels at contains checks (O(1)).

• Ordering: Don’t rely on iteration order being consistent.

Source

Dart HashSet Documentation

This tutorial covered Dart’s HashSet with practical examples demonstrating its key features and usage patterns for unordered unique collections.

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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