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

You can do something like

    Map invoiceMap = invoices.stream()
            .collect(Collectors.groupingBy(                   // group invoices by month
                    invoice -> invoice.month
            ))
            .entrySet().stream()                              // once you have them grouped stream then again so...
            .collect(Collectors.toMap(
                    entry -> entry.getKey(),                  // we can mantain the key (month)
                    entry -> entry.getValue().stream()        // and streaming all month's invoices
                        .reduce((invoice, invoice2) ->        // add all the ammounts
                                new Invoice(invoice.month, invoice.amount.add(invoice2.amount)))
                            .orElse(new Invoice(entry.getKey(), new BigDecimal(0)))          // In case we don't have any invoice (unlikeable)
            ));