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

Here is the solution by My library: AbacusUtil

Stream.of(invoices)
      .groupBy2(Invoice::getMonth, Invoice::getAmount, BigDecimal::add)  
      .map(e -> new Invoice(e.getKey(), e.getValue())) // Probably we should not modify original invoices. create new instances.
      .toList();