I am trying to combine following records by “key” however after this I also want to conditionally merge/reduce items with the same vat rate

    var item1 = new Item2();
    item1.setKey("ABC");
    item1.setCost(100.0);
    item1.setVatRate(18.0);

    var item2 = new Item2();
    item2.setKey("ABC");
    item2.setCost(100.0);
    item2.setVatRate(18.0);

    var item3 = new Item2();
    item3.setKey("ABC");
    item3.setCost(100.0);
    item3.setVatRate(17.0);

    var item4 = new Item2();
    item4.setKey("DEF");
    item4.setCost(100.0);
    item4.setVatRate(18.0);
    List<Item2> list = Arrays.asList(item1, item2, item3, item4);

    Map<String, List<Item2>> resulst = list.stream()
        .collect(Collectors.groupingBy(
            item -> item.getKey() , 
            LinkedHashMap::new,  
            Collectors.toList()  
        ));