Flutter Dart List - How to check if a list contains an object

Question

I wrote a function which returns true if an element is present in a list. I created an earlier version in which the list contained "ints" and it worked fine. Now I have a list of "complex" objects, and it seems it never detects if the object is present. Here is the code :

bool addNewWordToBufferCarnetList(int? wordId) {
    CarnetWords newWord =
        CarnetWords(iD: wordId!, wrong: 0, right: 0, mastery: 0);
         if (_bufferCarnetList.contains(newWord) == true) {
          return false;
    } else {
      _bufferCarnetList.add(newWord);
      notifyListeners();
      return true;
    }
  }

Is there another way to see if the list contains the object ?

read carefully List.contains method documentation - it tells what you need to do to use it correctly — pskink
– pskink, Commented May 29, 2021 at 13:13
Hmm, I have read it several times... but I don't see what I am doing wrong. Is it because it is now a complex object ? Should I just use a for loop and do it "manually" ? — SylvainJack
– SylvainJack, Commented May 29, 2021 at 13:19
they say: "The equality used to determine whether element is equal to an element of the iterable defaults to the Object.== of the element." - so == operator is wrong in your class, if in doubts check how other classes implement it — pskink
– pskink, Commented May 29, 2021 at 13:22
I guess I don't understand their sentence... am unfortunately a beginner and not a native English speaker... So can the List.contains method be used in my case ? — SylvainJack
– SylvainJack, Commented May 29, 2021 at 13:23
if you have a list of Rect objects for example, you can use List.contains beacause Rect class implements == operator, like this: api.flutter.dev/flutter/dart-ui/Rect/operator_equals.html - go to the bottom of this page and you will see the implementation, so what you need to do is to implement == operator in your CarnetWords class — pskink
– pskink, Commented May 29, 2021 at 13:28

SylvainJack · Accepted Answer · 2021-05-29 14:27:34Z

2

As suggested and explained by pskink, I had to override the == operator in my CarnetWords class for the "equality" to function. Here is the code :

class CarnetWords {
 @override
  bool operator ==(Object other) {
    if (identical(this, other)) return true;
    if (runtimeType != other.runtimeType) return false;
    return other is CarnetWords && other.iD == iD;
  }
 @override
  int get hashCode => iD.hashCode;

  int? iD;
  int? wrong;
  int? right;
  double? mastery;
  CarnetWords(
      {@required this.iD,
      @required this.wrong,
      @required this.right,
      @required this.mastery});
}

Thanks pskink for your help and patience !

answered May 29, 2021 at 14:27

SylvainJack

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4 Comments

SylvainJack Over a year ago

Hello again :) Are you online ?

SylvainJack Over a year ago

I had a crazy pb with Textfield. I don't know if you can see my post from here. I managed to find a way around it, but I still don't understand why it didn't work...

pskink Over a year ago

i dont see any Textfield in the code you posted

SylvainJack Over a year ago

In this post : stackoverflow.com/questions/67769519/…

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