如果过滤值为 null，如何跳过 stream 过滤器？

Question

I have a list of Java objects. In database some of objects has got field nextSyncDate and some not. What I want to do is to put filter on java stream but only if this field exists and is for example greater than today date. So simplify, I want to get objects which nextSyncDate is greater than today and objects which hasn't got this field (getting NullPointException after get() on this field).

I have tried something like this but it's not working like I want to..

List<MyObjects> objects;
objects.stream()
    .filter(obj -> Objects.nonNull(obj.getNextSyncDate()) && obj.getNextSyncDate().before(new Date()))

On the other hand, all objects has got field counter . What I want to do additionally is to set nextSyncDate (for example for tomorrow) for every object which counter is greater than 15. I tried to .map() objects before .filter() but it's also not working.

Answer 1

When creating a stream, you are not directly modifying its source. You need to collect the data. Your filter seems to work as intended. I would personally not recommend having a filtered stream with side effects (do your " counter > 15 then set nextSyncDate " logic somewhere else) but you could do it with another stream operation such as peek ( map is for transformation which isn't necessary in your case but could be used):

List<MyObjects> objects = /* omitted */;
List<MyObjects> filteredObjects = objects.stream()
    .peek(MyObjects::setNextSycDateWithSomeLogic())
    .filter(obj -> Objects.nonNull(obj.getNextSyncDate()) && obj.getNextSyncDate().before(new Date()))
    .collect(Collectors.toList());

Answer 2

The filter can include the counter requirement as well:

    final Date TODAY = Date.from(LocalDate.now().atStartOfDay(ZoneId.systemDefault()).toInstant());
    final int COUNTER_THRESHOLD = 15;
    objects.stream()
            .filter(obj -> Objects.nonNull(obj.getNextSyncDate())
                    && obj.getNextSyncDate().before(TODAY)
                    && obj.getCounter() > COUNTER_THRESHOLD)

The intermediate stream provided by the filter has the objects which need to be updated, so a forEach can be applied:

    final Date TODAY = Date.from(LocalDate.now().atStartOfDay(ZoneId.systemDefault()).toInstant());
    final Date TOMORROW = Date.from(LocalDate.now().plusDays(1).atStartOfDay(ZoneId.systemDefault()).toInstant());
    final int COUNTER_THRESHOLD = 15;
    objects.stream()
            .filter(obj -> Objects.nonNull(obj.getNextSyncDate())
                    && obj.getNextSyncDate().before(TODAY)
                    && obj.getCounter() > COUNTER_THRESHOLD)
            .forEach(obj -> obj.setNextSyncDate(TOMORROW));

Also, consider using LocalDate instead of deprecated Date.