CODEWITHSUNDEEP
dartcollections
Valentin Vignal
Valentin Vignal

Reputation: 8248

Dart - Map with Set as key

I'm trying to construct a Map object with Set as keys:

final myObject = <Set<MyType1>, MyType2>{};

I can successfully insert new values in my object

myObject[{myType1}] = myType2;

However, I cannot get back anything from it as 2 sets with the same values won't be consider equal:

{0} == {0}; // <- false

myObject[{myType1}]; // <- null

Is there something built-in that could allow me to use a Map<Set<T>, U> object?

Upvotes: 1

Views: 627

Answers (2)

paldepind
paldepind

Reputation: 4870

Unfortunately, it is not possible to extend any of the Set classes as they only have factory constructors. This would otherwise have been a clean way to implement a set with a custom notion of equality and hashing.

As an alternative, the HashMap constructor accepts a custom hash and equality function which can be used:

Map<K, V> hashMapDeep<K extends Set, V>() => HashMap(
      equals: setEquals,
      hashCode: Object.hashAllUnordered,
    );

This uses Object.hashAllUnordered to create an order-independent has of the set and setEquals from Flutter for the equality comparison.

Upvotes: 0

ch271828n
ch271828n

Reputation: 17617

You may create your own class that wraps a Set with your own equals and hashCode. Then the problem is solved.

A very rough example to demonstrate my idea:

class MySet<T> {
  final Set<T> inner;
  ...

  @override
  bool equals(other) => setEquals(this, other); // https://api.flutter.dev/flutter/foundation/setEquals.html

  @override
  int hashCode => ...
}

P.S. The equals/hashCode of a Set is not implemented as whether the contents are the same, because that would be very costly.

Upvotes: 1

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