HashSet<E> constructor Null safety

HashSet<E>(
  1. {bool equals(
    1. E,
    2. E
    ),
  2. int hashCode(
    1. E
    ),
  3. bool isValidKey(
    1. dynamic
    )}
)

Create a hash set using the provided equals as equality.

The provided equals must define a stable equivalence relation, and hashCode must be consistent with equals. If the equals or hashCode methods won't work on all objects, but only on some instances of E, the isValidKey predicate can be used to restrict the keys that the functions are applied to. Any key for which isValidKey returns false is automatically assumed to not be in the set when asking contains.

If equals or hashCode are omitted, the set uses the elements' intrinsic Object.== and Object.hashCode.

If you supply one of equals and hashCode, you should generally also to supply the other.

If the supplied equals or hashCode functions won't work on all E objects, and the map will be used in a setting where a non-E object is passed to, e.g., contains, then the isValidKey function should also be supplied.

If isValidKey is omitted, it defaults to testing if the object is an E instance. That means that:

HashSet<int>(equals: (int e1, int e2) => (e1 - e2) % 5 == 0,
             hashCode: (int e) => e % 5)

does not need an isValidKey argument, because it defaults to only accepting int values which are accepted by both equals and hashCode.

If neither equals, hashCode, nor isValidKey is provided, the default isValidKey instead accepts all values. The default equality and hashcode operations are assumed to work on all objects.

Likewise, if equals is identical, hashCode is identityHashCode and isValidKey is omitted, the resulting set is identity based, and the isValidKey defaults to accepting all keys. Such a map can be created directly using HashSet.identity.

Implementation

external factory HashSet(
    {bool Function(E, E)? equals,
    int Function(E)? hashCode,
    bool Function(dynamic)? isValidKey});