How to not duplicate same item in a list dart? - arrays

Question

I have created a listView and button and when I click the button it adds an item to listView.
The problem is I don't want actually to repeat the same item in the list.
I've tried the .contains method but it didn't work.
I want a good solution please,

Answer 1

There are different ways to achieve this:
1) Iterate the list and check if every element doesn't have the
properties you consider equal:
items = [Item(id: 1), Item(id: 2)];
newItem = Item(id: 2);
if (items.every((item) => item.id != newItem.id)) {
items.add(newItem);
}
2) Use contains() and override == operator (and override hashCode too)
in the object class with the properties you consider equal.
items = [Item(id: 1), Item(id: 2)];
newItem = Item(id: 2);
if (!items.contains(newItem)) {
items.add(newItem);
}
// inside Item class
#override
bool operator ==(other) {
return this.id == other.id;
}
#override
int get hashCode => id.hashCode;
3) Instead of List use Set, where each element can occur only once. Its default implementation is LinkedHashSet that keeps track of the order.

Answer 2

Instead of List, Use Set.
void main() {
Set<String> currencies = {'EUR', 'USD', 'JPY'};
currencies.add('EUR');
currencies.add('USD');
currencies.add('INR');
print(currencies);
}
output: {EUR, USD, JPY, INR} // unique items only
Reference: Set<E> class

Answer 3

Check if the List already contains the element before you add it:
https://api.flutter.dev/flutter/dart-core/List-class.html
if(!List.contains(element) { add }
The contains method checks for equality, not for reference, so it must work as long as you compare a similar element. If your code isn't working, please provide it to us. Thanks.

Answer 4

If your list contains custom objects you may need to override the equality operator in the custom class.
You could also use a Set instead of a List.