I have the following expression:
scheduleIntervalContainers.stream() .filter(sic -> ((ScheduleIntervalContainer) sic).getStartTime() != ((ScheduleIntervalContainer)sic).getEndTime()) .collect(Collectors.toList()); ...where scheduleIntervalContainers has element type ScheduleContainer:
final List<ScheduleContainer> scheduleIntervalContainers Is it possible to check the type before the filter?
You can apply another filter in order to keep only the ScheduleIntervalContainer instances, and adding a map will save you the later casts:
scheduleIntervalContainers.stream() .filter(sc -> sc instanceof ScheduleIntervalContainer) .map(sc -> (ScheduleIntervalContainer) sc) .filter(sic -> sic.getStartTime() != sic.getEndTime()) .collect(Collectors.toList()); Or, as Holger commented, you can replace the lambda expressions with method references if you prefer that style:
scheduleIntervalContainers.stream() .filter(ScheduleIntervalContainer.class::isInstance) .map(ScheduleIntervalContainer.class::cast) .filter(sic -> sic.getStartTime() != sic.getEndTime()) .collect(Collectors.toList()); 
.filter(ScheduleIntervalContainer.class::isInstance) .map(ScheduleIntervalContainer.class::cast), whatever style you prefer.
List<ScheduleContainer> or a List<ScheduleIntervalContainer>? It should be the latter.A pretty elegant option is to use method reference of class:
scheduleIntervalContainers .stream() .filter( ScheduleIntervalContainer.class::isInstance ) .map( ScheduleIntervalContainer.class::cast ) .filter( sic -> sic.getStartTime() != sic.getEndTime()) .collect(Collectors.toList() ); 
.class required? isn't isInstance part of Object? Is Class a class in Java?
ScheduleIntervalContainer wouldn't really be a an instance.There is a small problem with @Eran solution - typing class name in both filter and map is error-prone - it is easy to forget to change the name of the class in both places. An improved solution would be something like this:
private static <T, R> Function<T, Stream<R>> select(Class<R> clazz) { return e -> clazz.isInstance(e) ? Stream.of(clazz.cast(e)) : null; } scheduleIntervalContainers .stream() .flatMap(select(ScheduleIntervalContainer.class)) .filter( sic -> sic.getStartTime() != sic.getEndTime()) .collect(Collectors.toList()); However there might be a performance penalty in creating a Stream for every matching element. Be careful to use it on huge data sets. I've learned this solution from @Tagir Vailev
select(A.class) will return null for anything that is not an A. Adding .filter(Objects::nonNull) would help. BTW: @Eran's approach is null-safe.flatMap says "If a mapped stream is null an empty stream is used, instead.". So your solution was correct, even without the null-filter.
null when you could've just returned an empty streamInstead of a filter + map like other answers suggest, I would recommend this utility method:
public static <Super, Sub extends Super> Function<Super, Stream<Sub>> filterType(Class<Sub> clz) { return obj -> clz.isInstance(obj) ? Stream.of(clz.cast(obj)) : Stream.empty(); } Use it as:
Stream.of(dog, cat fish) .flatMap(filterType(Dog.class)); Compared to filter + map it has the following advantages:
Not exactly a stream, but Guava's FluentIterable does this very cleanly:
FluentIterable.from(scheduleIntervalContainers) .filter(ScheduleIntervalContainer.class) .filter(sic -> sic.getStartTime() != sic.getEndTime()) .toList(); 