Group by, sum then sort a list of transaction objects java - list

Question

I have a list of transactions by day, the transaction contains the following attributes:
Transaction(int transactionID,
DateTime transactionDate,
String shopId,
int productReference,
int quantity,
float price);
Having a list List<Transaction>, I want to extract top 100 selled products by shop.
So I need to group transactions by shopId then by productReference, then summing quantities than sorting from most selled to least.
Thanks you for your Help

Answer 1

private static Collector<Transaction, ?, List<Transaction>> limit(int limit) {
return Collector.of(
ArrayList::new,
(list, transaction) -> { if (list.size() < limit) list.add(transaction); },
(list1, list2) -> {
list1.addAll(list2.subList(0, Math.min(list2.size(), Math.max(0, limit - list1.size()))));
return list1;
}
);
}
public static void main(String[] args) {
Map<String, List<Transaction>> groupedMap = listOfTransactions
.stream()
.sorted((t1, t2) -> Integer.compare(t2.getQuantity(), t1.getQuantity()))
.collect(
Collectors.groupingBy(
Transaction::getShopId,
limit(100)
)
);
}
As a result you'll get a map with shopId as a key, and lists of transactions sorted by quantity as a value.
Is it expected behavior?

Answer 2

I'd suggest using of additional Product type, with overridden equals() and hasCode() which will consist only of shopId and productReference . New type will serve as an output, which will make all the transformation job more obvious. Consider my version, with Lombok lib usage:
import lombok.*;
#Data
#RequiredArgsConstructor(staticName = "of")
#ToString
public class Product {
final String shopId;
final int productReference;
}
and the function code itself:
List<Product> products = transactions.stream()
// grouping transactions by the same product
.collect(Collectors.groupingBy(transaction -> Product.of(
transaction.getShopId(),
transaction.getProductReference())))
.entrySet().stream()
// summing all price * quantity occurrences to find top sellings
.collect(Collectors.toMap(
Map.Entry::getKey,
e -> e.getValue().stream()
.mapToDouble(p -> p.getQuantity() * p.getPrice())
.sum()))
.entrySet().stream()
// sorting by the most expensive ones at the top,
// limiting to 10 and collecting to the list
.sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))
.map(Map.Entry::getKey)
.limit(10)
.collect(Collectors.toList());