List contains method in dart-CodePudding

Contain method can not detect the object inside of list What is the reason of that?

I expected to get the result of if block enter image description here Look at this picture as well.In the second picture it works

CodePudding user response：

it's because your Contact object has no == relation between its instances, so trying to compare two instances like this:

Contact(false, "") == Contact(false, "") // false

And the contains method follows the same comparison I did to find if a list contains that object.

It would help if you told Dart when two objects of that class should be considered equal to each other, by overriding the == operator like this:

class Country {
  final bool? brotherCountry;
  final String? name;
  Country(this.brotherCountry, this.name);
  
  @override
  bool operator ==(covariant Country other) {
    return other.name == name && other.brotherCountry ==brotherCountry;
    
  }
}

now trying this:

Country(false, "") == Country(false, "") // true

and so on now the contains method will work as you expect

[Country(false, ""), Country(false, "test")].contains(Country(false, "")); // true

CodePudding user response：

You can override the == operator on Country like so to get it to work:

class Country {
  bool? brotherCountry;
  String? name;
  Country(this.brotherCountry, this.name);

  @override
  bool operator ==(Object other) {
    if (other is Country) {
      return name == other.name && brotherCountry == other.brotherCountry;
    }
    return false;
  }

  @override
  int get hashCode => brotherCountry.hashCode ^ name.hashCode;
}

Alternatively, there's this package https://pub.dev/packages/equatable, and you can use it like so:

class Country extends Equtable {
  bool? brotherCountry;
  String? name;
  Country(this.brotherCountry, this.name);

  @override
  List<Object> props => [brotherCountry, name];
}

As pointed about by jamesdlin, it's not because of call by value

From Effective dart, it says:

Any two objects that are equal must have the same hash code. Otherwise, maps and other hash-based collections will fail to recognize that the two objects are equivalent.