HashSet<E> class Null safety

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.