Apply reduction only if certain condition is met

Question

Is there a way to let the 'reduction' of the reduce() method of Stream be optional?

I want to iterate over a list of Periods and join the periods that overlap and maintain both periods if they don't overlap:

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;
    }
});

Period is just an Interface I've created to demonstrate the code I will put it int the question. — alexpfx, Dec 04 '18 at 18:42
`reduce()` is meant to combine all elements of the stream into one. You can't return both. You can create a custom collector though. — daniu, Dec 04 '18 at 18:44
but how I could create a custom collector that is possible to evaluate the list[x] and list[x+1]? — alexpfx, Dec 04 '18 at 18:55

score 6 · Accepted Answer · answered Dec 04 '18 at 18:54

I don't think you can easily do it with streams alone. With Guava ranges, you can do something like this:

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());

This assumes a class structure like the following, but you can adjust as necessary:

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

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

user_3380739 · Answer 2 · 2018-12-04T21:42:11.850

6

Try collapse provided in StreamEx

// 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]

edited Dec 04 '18 at 21:42

answered Dec 04 '18 at 20:36

user_3380739

1
14
14

this look interesting, but I cannot use an external library – alexpfx Dec 04 '18 at 20:38

Apply reduction only if certain condition is met

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