检查流中的 instanceof

小开

最佳答案

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());

小开

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() );

小开

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

小开

Instead 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:

If the class does not extend your class you will get a compile error
Single place, you can never forget to change a class in either filter or map

小开

Filter by class type with StreamEx

StreamEx.of(myCollection).select(TheThing.class).toList();