Remove duplicate from List<model> after merge - flutter

Question

I want to add a list to my main List and remove duplicate, like this:
class item {
int id;
String title;
item({this.id, this.title});
}
void main() {
// this is working for List<int>
List<int> c = [1, 2, 3];
List<int> d = [3, 4, 5];
c.addAll(d..removeWhere((e) => c.contains(e)));
print(c);
// but this is not working for List<item>
List<item> a = new List<item>();
a.add(new item(id: 1, title: 'item1'));
a.add(new item(id: 2, title: 'item2'));
List<item> b = new List<item>();
b.add(new item(id: 2, title: 'item2'));
b.add(new item(id: 3, title: 'item3'));
a.addAll(b..removeWhere((e) => a.contains(e)));
a.forEach((f) => print('${f.id} ${f.title}'));
}
and output is like this:
[1, 2, 3, 4, 5]
1 item1
2 item2
2 item2
3 item3
As you test this code on https://dartpad.dev/ output is ok for List<int> but there is duplicate in output for List<item>.

Answer 1

The first list have integer values and when you call contains it will check the values and will work correctly.
In second case you have item objects. Both lists have objects that may have same property values but both are two different object. For example, the below code will work correctly in your case, because the item2 object is same in both lists.
Item item2 = Item(id: 2, title: 'item2');
List<Item> a = new List<Item>();
a.add(new Item(id: 1, title: 'item1'));
a.add(item2);
List<Item> b = new List<Item>();
b.add(item2);
b.add(new Item(id: 3, title: 'item3'));
When you call contains it will use the Object.== method, so to handle this issue you have to override that method and specify your own equality logic.
class Item {
int id;
String title;
Item({this.id, this.title});
#override
bool operator == (Object other) {
return
identical(this, other) ||
other is Item &&
runtimeType == other.runtimeType &&
id == other.id;
}
}
Or you can use the equatable package to handle it better.
References:
contains method
operator == method

Answer 2

I think you need to iterate on your list a if you want to compare a property (e.g title)
a.addAll(
b
..removeWhere((e) {
bool flag = false;
a.forEach((x) {
if (x.title.contains(e.title)) {
flag = true;
}
});
return flag;
}),
);
As suggested below, those two list items are different

Answer 3

Thats because these 2 item's:
a.add(new item(id: 2, title: 'item2'));
b.add(new item(id: 2, title: 'item2'));
are different. They are 2 instances of 2 different objects that just have the same values for id and title. As the documentation tells:
The default behavior for all Objects is to return true if and only if this and other are the same object.
If you want to compare if the 2 fields are the same you can override the equality operator inside your item class:
operator ==(item other) {
return (this.id == other.id && this.title == other.title);
}
which gives the expected output:
[1, 2, 3, 4, 5]
1 item1
2 item2
3 item3