如果toList返回空列表，则收集器返回singletonList

Question

I have quite a large stream pipeline and therefore would like to keep it clean. I have the following part of larger pipeline

Integer defaultInt;
//...
Stream<Integer> ints;
ints.filter(/* predicate_goes_here */).collect(toSingletonIfEmptyCollector);

Where toSingletonIfEmptyCollector is supposed to act the same as Collectors.toList() does if it returns non-emtpy list and Collections.singletonList(defaultInt) if Collectors.toList() returned empty.

Is there a shorter way to implement it (eg by composing standard collectors provided in JDK) rather then implementing all Collector 's method from scratch?

Answer 1

You can use collectingAndThen and perform an additional finisher operation on the built-in toList() collector that will return a singleton list in case there was no elements.

static <T> Collector<T, ?, List<T>> toList(T defaultValue) {
    return Collectors.collectingAndThen(
              Collectors.toList(), 
              l -> l.isEmpty() ? Collections.singletonList(defaultValue) : l
           );
}

It would be used like this:

System.out.println(Stream.of(1, 2, 3).collect(toList(5))); // prints "[1, 2, 3]"
System.out.println(Stream.empty().collect(toList(5))); // prints "[5]"