description

# HashSet<E> class abstract final

An unordered hash-table based Set implementation.

The elements of a `HashSet` must have consistent equality and hashCode implementations. This means that the equals operation must define a stable equivalence relation on the elements (reflexive, symmetric, transitive, and consistent over time), and that the hashCode must be consistent with equality, so that it's the same for objects that are considered equal.

Most simple operations on `HashSet` are done in (potentially amortized) constant time: add, contains, remove, and length, provided the hash codes of objects are well distributed.

The iteration order of the set is not specified and depends on the hashcodes of the provided elements. However, the order is stable: multiple iterations over the same set produce the same order, as long as the set is not modified.

Note: Do not modify a set (add or remove elements) while an operation is being performed on that set, for example in functions called during a forEach or containsAll call, or while iterating the set.

Do not modify elements in a way which changes their equality (and thus their hash code) while they are in the set. Some specialized kinds of sets may be more permissive with regards to equality, in which case they should document their different behavior and restrictions.

Example:

    final letters = HashSet<String>();

To add data to a set, use add or addAll.

    letters.add('A');
    letters.addAll({'B', 'C', 'D'});

To check if the set is empty, use isEmpty or isNotEmpty. To find the number of elements in the set, use length.

    print(letters.isEmpty); // false
    print(letters.length); // 4
    print(letters); // fx {A, D, C, B}

To check whether the set has an element with a specific value, use contains.

    final bExists = letters.contains('B'); // true

The forEach method calls a function with each element of the set.

    letters.forEach(print);
    // A
    // D
    // C
    // B

To make a copy of the set, use toSet.

    final anotherSet = letters.toSet();
    print(anotherSet); // fx {A, C, D, B}

To remove an element, use remove.

    final removedValue = letters.remove('A'); // true
    print(letters); // fx {B, C, D}

To remove multiple elements at the same time, use removeWhere or removeAll.

    letters.removeWhere((element) => element.startsWith('B'));
    print(letters); // fx {D, C}

To removes all elements in this set that do not meet a condition, use retainWhere.

    letters.retainWhere((element) => element.contains('C'));
    print(letters); // {C}

To remove all elements and empty the set, use clear.

    letters.clear();
    print(letters.isEmpty); // true
    print(letters); // {}

See also:

- Set is the general interface of collection where each object can occur only once.
- LinkedHashSet objects stored based on insertion order.
- SplayTreeSet iterates the objects in sorted order.

Implemented types
  - Set<E>
Available extensions
  - EnumByName
  - FutureIterable
  - IterableExtensions
  - IterableToJSIterable
  - NullableIterableExtensions

## Constructors

HashSet({bool equals(E, E)?, int hashCode(E)?, bool isValidKey(dynamic)?})
  Create a hash set using the provided `equals` as equality.
  factory
HashSet.from(Iterable elements)
  Create a hash set containing all `elements`.
  factory
HashSet.identity()
  Creates an unordered identity-based set.
  factory
HashSet.of(Iterable<E> elements)
  Create a hash set containing all `elements`.
  factory

## Properties

first → E
  The first element.
  no setter inherited
firstOrNull → T?

  Available on Iterable<T>, provided by the IterableExtensions extension

  The first element of this iterator, or `null` if the iterable is empty.
  no setter
hashCode → int
  The hash code for this object.
  no setter inherited
indexed → Iterable<(int, T)>

  Available on Iterable<T>, provided by the IterableExtensions extension

  Pairs of elements of the indices and elements of this iterable.
  no setter
isEmpty → bool
  Whether this collection has no elements.
  no setter inherited
isNotEmpty → bool
  Whether this collection has at least one element.
  no setter inherited
iterator → Iterator<E>
  Provides an iterator that iterates over the elements of this set.
  no setter override
last → E
  The last element.
  no setter inherited
lastOrNull → T?

  Available on Iterable<T>, provided by the IterableExtensions extension

  The last element of this iterable, or `null` if the iterable is empty.
  no setter
length → int
  The number of elements in this Iterable.
  no setter inherited
nonNulls → Iterable<T>

  Available on Iterable<T?>, provided by the NullableIterableExtensions extension

  The non-`null` elements of this iterable.
  no setter
runtimeType → Type
  A representation of the runtime type of the object.
  no setter inherited
single → E
  Checks that this iterable has only one element, and returns that element.
  no setter inherited
singleOrNull → T?

  Available on Iterable<T>, provided by the IterableExtensions extension

  The single element of this iterator, or `null`.
  no setter
toJSIterable → JSIterable<T>

  Available on Iterable<T>, provided by the IterableToJSIterable extension

  A JSIterable wrapper that proxies to the Dart iterable API.
  no setter
wait → Future<List<T>>

  Available on Iterable<Future<T>>, provided by the FutureIterable extension

  Waits for futures in parallel.
  no setter

## Methods

add(E value) → bool
  Adds `value` to the set.
  inherited
addAll(Iterable<E> elements) → void
  Adds all `elements` to this set.
  inherited
any(bool test(E value)) → bool
  Checks whether any element of this iterable satisfies `test`.
  inherited
asNameMap() → Map<String, T>

  Available on Iterable<T>, provided by the EnumByName extension

  Creates a map from the names of enum values to the values.
byName(String name) → T

  Available on Iterable<T>, provided by the EnumByName extension

  Finds the enum value in this list with name `name`.
cast<R>() → Set<R>
  Provides a view of this set as a set of `R` instances.
  inherited
clear() → void
  Removes all elements from the set.
  inherited
contains(Object? value) → bool
  Whether `value` is in the set.
  inherited
containsAll(Iterable<Object?> other) → bool
  Whether this set contains all the elements of `other`.
  inherited
difference(Set<Object?> other) → Set<E>
  Creates a new set with the elements of this that are not in `other`.
  inherited
elementAt(int index) → E
  Returns the `index`th element.
  inherited
elementAtOrNull(int index) → T?

  Available on Iterable<T>, provided by the IterableExtensions extension

  The element at position `index` of this iterable, or `null`.
every(bool test(E value)) → bool
  Checks whether every element of this iterable satisfies `test`.
  inherited
expand<T>(Iterable<T> toElements(E element)) → Iterable<T>
  Expands each element of this Iterable into zero or more elements.
  inherited
firstWhere(bool test(E value), {E orElse()?}) → E
  The first element that satisfies the given predicate `test`.
  inherited
fold<T>(T initialValue, T combine(T previousValue, E element)) → T
  Reduces a collection to a single value by iteratively combining each element of the collection with an existing value
  inherited
followedBy(Iterable<E> other) → Iterable<E>
  Creates the lazy concatenation of this iterable and `other`.
  inherited
forEach(void action(E element)) → void
  Invokes `action` on each element of this iterable in iteration order.
  inherited
intersection(Set<Object?> other) → Set<E>
  Creates a new set which is the intersection between this set and `other`.
  inherited
join([String separator = ""]) → String
  Converts each element to a String and concatenates the strings.
  inherited
lastWhere(bool test(E value), {E orElse()?}) → E
  The last element that satisfies the given predicate `test`.
  inherited
lookup(Object? object) → E?
  If an object equal to `object` is in the set, return it.
  inherited
map<T>(T toElement(E e)) → Iterable<T>
  The current elements of this iterable modified by `toElement`.
  inherited
noSuchMethod(Invocation invocation) → dynamic
  Invoked when a nonexistent method or property is accessed.
  inherited
reduce(E combine(E value, E element)) → E
  Reduces a collection to a single value by iteratively combining elements of the collection using the provided function.
  inherited
remove(Object? value) → bool
  Removes `value` from the set.
  inherited
removeAll(Iterable<Object?> elements) → void
  Removes each element of `elements` from this set.
  inherited
removeWhere(bool test(E value)) → void
  Removes all elements of this set that satisfy `test`.
  inherited
retainAll(Iterable<Object?> elements) → void
  Removes all elements of this set that are not elements in `elements`.
  inherited
retainWhere(bool test(E value)) → void
  Removes all elements of this set that fail to satisfy `test`.
  inherited
singleWhere(bool test(E value), {E orElse()?}) → E
  The single element that satisfies `test`.
  inherited
skip(int count) → Iterable<E>
  Creates an Iterable that provides all but the first `count` elements.
  inherited
skipWhile(bool test(E value)) → Iterable<E>
  Creates an `Iterable` that skips leading elements while `test` is satisfied.
  inherited
take(int count) → Iterable<E>
  Creates a lazy iterable of the `count` first elements of this iterable.
  inherited
takeWhile(bool test(E value)) → Iterable<E>
  Creates a lazy iterable of the leading elements satisfying `test`.
  inherited
toList({bool growable = true}) → List<E>
  Creates a List containing the elements of this Iterable.
  inherited
toSet() → Set<E>
  Creates a Set with the same elements and behavior as this `Set`.
  inherited
toString() → String
  A string representation of this object.
  inherited
union(Set<E> other) → Set<E>
  Creates a new set which contains all the elements of this set and `other`.
  inherited
where(bool test(E value)) → Iterable<E>
  Creates a new lazy Iterable with all elements that satisfy the predicate `test`.
  inherited
whereType<T>() → Iterable<T>
  Creates a new lazy Iterable with all elements that have type `T`.
  inherited

## Operators

operator ==(Object other) → bool
  The equality operator.
  inherited