仅在满足特定条件时应用减少量

Question

有没有办法让Stream的reduce()方法的"减少"成为可选项？

我想迭代一个Periods列表并加入重叠的句点,如果它们不重叠则保持两个句点:

interface Period {
    boolean overlaps(Period other);
}


List<Period> periods = new ArrayList<>();

periods.stream().reduce(new BinaryOperator<Period>() {
    @Override
    public Period apply(Period period, Period period2) {
        if (period.overlaps(period2)){
            // join period and period2 into period.
        }else{
            "return both"
            // don't reduce and maintain period and period2 in the list.
        }
        return null;
    }
});

Answer 1

我不认为你可以轻松单独使用流.使用Guava系列,您可以执行以下操作:

periods.stream()
        .map(p -> Range.closedOpen(p.getStart(), p.getEnd()))
        .collect(TreeRangeSet::<Integer>create, RangeSet::add, RangeSet::addAll)
        .asRanges()
        .stream()
        .map(r -> new PeriodImpl(r.lowerEndpoint(), r.upperEndpoint()))
        .collect(Collectors.toList());

假设类结构如下,但您可以根据需要进行调整:

interface Period {
    int getStart();
    int getEnd();
}

class PeriodImpl implements Period {
    PeriodImpl(int start, int end) {
        //...
    }
    //...
}

Answer 2

尝试collapse在StreamEx中提供

// Here I use Range type provided in Google Guava for test.
List<Range<Integer>> list = Arrays.asList(Range.openClosed(1, 3), Range.openClosed(2, 4), Range.closed(5, 5));

StreamEx.of(list)
    .collapse(Range::isConnected, Range::span)
    .forEach(System.out::println);
// (1..4]
// [5..5]