Java: removeIf sum of grouping of 2 fields equals 0

Question

I have a list of items. Each item has 3 properties: name, section and amount. The amount can be positive/negative. The same item can be several times in the same section.

I want to group the items in the list by section and name. If the sum of this grouping equals 0 - remove all the items with this section and name.

I got this far

items.stream().collect(Collectors.groupingBy(ItemData::getSection))....

Now I need the second grouping and compare it to 0 in order to add it as a predicate to removeIf

items.removeIf(item -> predicate(item));

Answer 1

You can first group the ItemData based on section and name with sum of amount as value

   Map<String,Long> result = details.stream()
            .collect(Collectors.groupingBy(det -> det.getSection()+"_"+det.getName(),
                    Collectors.summingLong(det->det.getAmount())));

And then remove the entries from Map having value !=0

   result.entrySet().removeIf(det->det.getValue() != 0);

And then remove the elements from items list having entry in Map with key comparison

items.removeIf(item -> result.containsKey(item.getSection()+"_"+item.getName()));

You can also eliminate the entries having value 0 by using stream

Map<String,Long> result = details.stream()
            .collect(Collectors.groupingBy(det -> det.getSection()+"_"+det.getName(),
                    Collectors.summingLong(det->det.getCount())))
           .entrySet()
           .stream()
           .filter(entry->entry.getValue()==0)
           .collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));