Summarize data with Java streams

Question

Is it possible to summarize values with streams? For example, I have a class Dataset that divides a csv file for me into lines that contain values:

{customer=Max , articlenr=1234, paid=12}
{customer=Lisa , articlenr=21, paid=20}
{customer=Max , articlenr=19, paid=100}

Now I want to create a List with a Stream which gives me a list of my customers back(no other values), sorted by the sum what they have ever paid (in this example Max has paid 112 and Lisa 20). So I would expect getting Max first in the List and the Lisa.

Answer 1

Collect the sum of paid to a Map<String, Integer>, then stream the entries, sorting them in reverse order by paid, then collect the keys to a List.

List<MyClass> records; // your list

List<String> customers = records.stream()
        .collect(groupingBy(MyClass::getCustomer, Collectors.summingInt(MyClass::getPaid)))
        .entrySet().stream()
        .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
        .map(Map.Entry::getKey)
        .collect(toList());

Here is a complete working example in Java 23.

record Payment( String name , int amount ) { }

List < Payment > payments =
        List.of (
                new Payment ( "Max" , 12 ) ,
                new Payment ( "Lisa" , 20 ) ,
                new Payment ( "Max" , 100 )
        );
List < String > namesSortedByPaymentTotal =
        payments
                .stream ( )
                .collect ( groupingBy ( Payment :: name , Collectors.summingInt ( Payment :: amount ) ) )
                .entrySet ( ).stream ( )
                .sorted ( Map.Entry. < String, Integer > comparingByValue ( ).reversed ( ) )
                .map ( Map.Entry :: getKey )
                .toList ( );

namesSortedByPaymentTotal.forEach ( System.out :: println );

When run:

Max
Lisa

To verify our work, modify that code.

payments
        .stream ( )
        .collect ( groupingBy ( Payment :: name , Collectors.summingInt ( Payment :: amount ) ) )
        .entrySet ( ).stream ( )
        .sorted ( Map.Entry. < String, Integer > comparingByValue ( ).reversed ( ) )
        .forEach ( System.out :: println );

Max=112
Lisa=20