How to get list of unique objects with their number from the List<Object>

Question

I nave list of objects , e.g.

List<Product> products = [product1, product2, product1, product2, product1, product1]

How I can to get list of unique objects with their number from the List?

import 'package:built_value/serializer.dart';
import 'package:built_value/built_value.dart';

part 'product.g.dart';

abstract class Product implements Built<Product, ProductBuilder>{
  int get id;
  String get title;
  String get image;
  double get price;
  int get volume;

  static Serializer<Product> get serializer => _$productSerializer;
  Product._();
  factory Product([updates(ProductBuilder b)]) = _$Product;
}

I would like to get other List with objects:

class OrderPosition {
int id;
String title;
int count; // number of unique elements from list 'products'
}

For example:

List<OrderPosition> = [
OrderPosition(1, title1, 4),
OrderPosition(2, title2, 2)
]

Answer 1

class Product {
  Product(this.id); // for operator == to work properly
  final int id;     // make at least the id immutable

  String title;
  String image;
  double price;
  int volume;

  bool operator ==(Object other) => identical(this, other) || (other as Product).id == id;

  int get hashCode => id.hashCode;
}

var uniqueProducts = products.toSet().toList();
var result = <OrderPosition>[];
for(var i = 0; i < uniqueProducts.length; i++) {
  result.add(
    OrderPosition(i, 
                  uniqueProducts[i].title, 
                  products.where((e) => e == uniqueProducts[i]).length)));
}

Answer 2

Another alternative, using Set.putIfAbsent, is to collect the information in one iteration:

var uniqueProductMap = Map<Product, OrderPosition>.identity();
for (var product in products) {
  var position = uniqueProductMap.putIfAbsent(product, 
     () => OrderPosition(product.id, product.title, 0);
  position.count++;
}
var uniqueProducts = uniqueProductMap.values.toList();

This uses identical for deciding which elements are unique. If you want to use equality instead, replace the map with just <Product, OrderPosition>{}. If you want to use another equivalence not supported by the object itself, you can use:

var uniqueProductMap = LinkedHashMap<Product, OrderPosition>(
    equals: (p1, p2) => p1.id == p2.id,
    hashCode: (p) => p.id.hashCode);

(I fully support the recommendation to make id final if you use it for equality comparisons).