Is there a way to "expand" a list of objects into a bigger list with stream API?

Question

Consider this example: I have a list of RangeSet that contains, for instance, timestamps. I want to get the total duration of ranges using java8 streams instead of the imperative way:

// "list" is List<RangeSet<Long>>

long totalTime = list.stream()
    .expand(rangeset -> rangeset.asRanges())
    .map(range -> range.upperEndpoint() - range.lowerEndpoint())
    .reduce(0, (total, time) -> total + time);

The "expand" of course doesn't exist ; the idea is it would convert each single object in the stream to a list of other objects, and add that list to the resulting stream.

Is there something similar, or another way to do it fluently?

Answer 1

It looks like you're just looking for Stream.flatMap, e.g.

long totalTime = list.stream()
    .flatMap(rangeset -> rangeset.asRanges().stream())
    .map(range -> range.upperEndpoint() - range.lowerEndpoint())
    .reduce(0, (total, time) -> total + time);