Java 8 stream - merge collections of objects sharing the same Id

Question

I have a collection of invoices :

class Invoice {
  int month;
  BigDecimal amount
}

I\'d like to merge these invoices, so I get one invoice pe

Answer 1

Collection result = invoices.stream().collect(groupingBy(i -> i.month,
                collectingAndThen(
                    reducing((Invoice i1, Invoice i2) -> new Invoice(i1.month, i1.amount + i2.amount)),
                        Optional::get))).values();