Elegant way to find out if a Java 8 stream filter has filtered something

Question

I have a List of Item fetched from a database.

I do something like:

List<Item> list;
list.stream()
    .filter( i -> i.condition() )
    .mapToLong( Item::asLong )
    .sum()
    ;

Now I want to know, if the filter has filtered out something so I could delete this from my database where it is no longer needed.

I could do:

List<Item> list2 = list.stream()
    .filter( i -> i.condition() )
    .collect( Collectors.toList() )
    ;

int theSizeIWant = list2.size();

list2.stream().mapToLong( Item::asLong )
     .sum()
     ;

but I wonder if there is a more elegant solution which doesn't need to create the intermediate list.

Answer 1

A possible solution would be to partition the Stream with the predicate and sum the values. Partitioning is done with partitioningBy(predicate, downstream). In this case, the predicate would be your filtering function and the downstream collector would be summingLong to sum the values.

public static void main(String[] args) {
    List<Item> list = new ArrayList<>();
    Map<Boolean, Long> map = 
        list.stream()
            .collect(Collectors.partitioningBy(
                Item::condition, 
                Collectors.summingLong(Item::asLong)
            ));
    long sumWithTrueCondition = map.get(true);
    long sumWithFalseCondition = map.get(false);
}

The map will contain the sum where the predicate is true (resp. false) with the true key (resp. false).

This way, if something has been filtered, sumWithTrueCondition will be strictly greater than 0. Furthermore, you can get the total sum by adding sumWithTrueCondition and sumWithFalseCondition.

Answer 2

Yes, you can do something like this.

Map<Boolean, List<Item>> partitions =
    list.stream.collect(Collectors.partitioningBy(i -> i.condition()));

Where partitions.get(true) will get you the good ones and partitions.get(false) will get you the ones to be deleted.