CODEWITHSUNDEEP
dart
iDecode
iDecode

Reputation: 28996

How to compare two objects for equality which have a List?

I have a class

class Foo {
  final List<int> a;
  final List<int> b;
  Foo(this.a, this.b);

  @override
  bool operator ==(Object other) => identical(this, other) || other is Foo && listEquals(a, other.a) && listEquals(b, other.b);

  @override
  int get hashCode => Object.hash(a, b);
}

I want to check if two instances of Foo are equal with hashCodes.

final list1 = Foo([0], [0]);
final list2 = Foo([0], [0]);

print(list1 == list2); // Prints true, all good
print(list1.hashCode == list2.hashCode); // Prints false.

How to properly override hashCode so that the above print statement also prints true without using a third party package?

Upvotes: 1

Views: 492

Answers (2)

lrn
lrn

Reputation: 71828

Wherever you got the listEquals method from, should also provide you with a compatible listHash function. If it doesn't, you can use ListEquality from package:collection, it also has a hashCode method.

Then you can do:

 int get hashCode => Object.hash(
     const ListEquality().hash(a), 
     const ListEquality().hash(b));

Upvotes: 2

h8moss
h8moss

Reputation: 5030

You could use the hashAll method to hash a list of all of your objects:

int get hashCode => Object.hashAll([...a, ...b]);

This should get you the result you want.

void main() {
  List<int> a1 = [0];
  List<int> a2 = [1];
  List<int> b1 = [0];
  List<int> b2 = [1];
  
  final simpleHashA = Object.hash(a1, a2);
  final simpleHashB = Object.hash(b1, b2);
  
  final correctHashA = Object.hashAll([...a1, ...a2]);
  final correctHashB = Object.hashAll([...b1, ...b2]);
  
  print(simpleHashA == simpleHashB); // prints false
  print(correctHashA == correctHashB); // prints true
  
}

Upvotes: 2

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