Check whether a list contain an attribute of an object in dart

Question

I need to check whether myItemsList contains myitem.itemId or not, If it exists need to add itemQuantity, if it not exists need to add myitem object to myItemsList.

List<MyItem> myItemsList = new List();

MyItem myitem = new MyItem (
  itemId: id,
  itemName: name,
  itemQuantity: qty,
);

if (myItemsList.contains(myitem.itemId)) {
  print('Already exists!');
} else {
  print('Added!');
  setState(() {
    myItemsList.add(myitem);
  });
}

MyItem class

class MyItem {
  final String itemId;
  final String itemName;
  int itemQuantity;

  MyItem ({
    this.itemId,
    this.itemName,
    this.itemQuantity,
  });
}

above code is not working as expected, please help me to figure out the issue.

You're calling myItemsList.contains(String), but myItemsList doesn't contain Strings; it contains MyItems. — jamesdlin
– jamesdlin, Commented Apr 19, 2019 at 7:08

Thilo · Accepted Answer · 2021-06-12 11:56:26Z

44

As already said before, contains compares two Objects with the == operator. So you currently compare MyItem with String itemId, which will never be the same.

To check whether myItemsList contains myitem.itemId you can use one of the following:

myItemsList.map((item) => item.itemId).contains(myitem.itemId);

or

myItemsList.any((item) => item.itemId == myitem.itemId);

answered Jun 12, 2021 at 11:56

Thilo

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Comments

Radek · Accepted Answer · 2020-12-08 07:55:44Z

31

Contains() compares the whole objects.

Besides overriding == operator or looping over, you can use list's singleWhere method:

  if ((myItemsList.singleWhere((it) => it.itemId == myitem.itemId,
          orElse: () => null)) != null) {

Edit: As Dharaneshvar experienced and YoApps mentioned in the comments .singleWhere raises StateError when more elements are found.

This is desired when you expect unique elements such as in the case of comparing IDs.

Raised error is the friend here as it shows that there is something wrong with the data.

For other cases .firstWhere() is the right tool:

  if ((myItemsList.firstWhere((it) => it.itemName == myitem.itemName,
          orElse: () => null)) != null) {

// EO Edit

Whole example:

List<MyItem> myItemsList = new List();

class MyItem {
  final String itemId;
  final String itemName;
  int itemQuantity;

  MyItem({
    this.itemId,
    this.itemName,
    this.itemQuantity,
  });
}

void main() {
  MyItem myitem = new MyItem(
    itemId: "id00",
    itemName: "name",
    itemQuantity: 50,
  );

  myItemsList.add(myitem);

  String idToCheck = "id00";

  if ((myItemsList.singleWhere((it) => it.itemId == idToCheck,
          orElse: () => null)) != null) {
    
    print('Already exists!');
  } else {
    print('Added!');
  }
}

edited Dec 8, 2020 at 7:55

answered May 6, 2020 at 21:22

Radek

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4 Comments

Dharaneshvar Over a year ago

Recently getting "Bad State: too many elements error" in Flutter after upgrading. Is there any changes to do or is there any alternative way to do this check?

Radek Over a year ago

@Dharaneshvar I wonder what is the code throwing that.

Yo Apps Over a year ago

EXACTLY! Thats why I would NOT recommend .singleWhere() .. but instead firstWhere(). Because if more than 1 element is found it will throw StateError. But firstWhere() will just return you that element or execute the orElse: block of code!

Radek Over a year ago

@YoApps I updated the answer to reflect when to use singleWhere vs firstWhere

ishaan · Accepted Answer · 2019-04-19 07:00:53Z

12

You're using contains slightly wrong.

From: https://api.dartlang.org/stable/2.2.0/dart-core/Iterable/contains.html

bool contains(Object element) {
  for (E e in this) {
    if (e == element) return true;
  }
  return false;
}

You can either override the == operator, see: https://dart-lang.github.io/linter/lints/hash_and_equals.html

@override    
bool operator ==(Object other) => other is Better && other.value == value;

Or you can loop over your list and search the normal way one by one, which seems slightly easier.

answered Apr 19, 2019 at 7:00

ishaan

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Comments

awaik · Accepted Answer · 2022-08-30 12:23:13Z

4

One more way to check does list contain object with property or not

if (myList.firstWhereOrNull((val) => val.id == someItem.id) != null) {}

answered Aug 30, 2022 at 12:23

awaik

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1 Comment

Rens Over a year ago

Make sure to add this import statement when using firstWhereOrNull: import 'package:collection/collection.dart';

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