Question 78812 in Flutter Question posted in
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List contains method in dart – Flutter

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 hereLook at this picture as well.In the second picture it works

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2

Answers


  1. 0

    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.

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  2. 0

    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
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