user3663882 :
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 (e.g. by composing standard collectors provided in JDK) rather then implementing all Collector's method from scratch?
Tunaki :
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]"
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