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Flutter – Comparing the same result of List<Map<String, Object>> is not true

I’m trying to compare values from a List which consists of Map<String, Object>, but the result always returns false despite the value I’m comparing it with is the same. Is there something I did wrong here?

List<Map<String, Object>> orderList = [{"Nasi Goreng": 1}];
...
ElevatedButton(onPressed: () {
  print(orderList);
  assert(orderList[0] == {"Nasi Goreng": 1});
}, child: Text("+"))

I tried using assert, contains and indexOf, but all those returned false, false and -1. I expected one of those should at least return true or returns the index of the item (which is 0), but I keep getting false. What I get from running assert is:

Failed assertion: line 178 pos 68: 'orderList[0] == {"Nasi Goreng": 1}': is not true.
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2

Answers


  1. 0

    When you use comparison in Dart, the ‘==’ condition means that the references of the objects are identical, not their value.
    There is, however, a class named ‘collection’. So, first, add it:

    dependencies:
  collection: ^1.15.0

    Then, the following code should work:

    import 'package:collection/collection.dart';  // Import the collection package

List<Map<String, Object>> orderList = [{"Nasi Goreng": 1}];

ElevatedButton(
  onPressed: () {
    print(orderList);

    // Now we will use MapEquality to compare the map contents (not reference)
    final mapEquality = MapEquality();
    bool isEqual = mapEquality.equals(orderList[0], {"Nasi Goreng": 1});
    
    // Print whether the maps are equal
    print(isEqual);  // Now it should print true now

    // Assert based on map content equality, to be on the safe side
    assert(isEqual);
  },
  child: Text("+"),
)
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  2. 0

    When you try to compare Maps in Dart by using ==, Dart will basically compare each hashCode of those Maps. Since these Maps are two different Object, the hashCode of each Map is distinct, and == comparison will always return false:

    print(orderList[0].hashCode) // 329301310
print({"Nasi Goreng": 1}.hashCode) // 743029473

    To compare equality of Maps, you can use mapEquals from ‘package:flutter/foundation.dart’
    (As one of solutions of similar question produced in how to check two maps are equal in dart)

    Example:

    import 'package:flutter/foundation.dart';

void main() {
  const orderedList = [{"Nasi Goreng": 1}];
  print( mapEquals(orderedList[0], {"Nasi Goreng": 1}) );
}
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