Java 8 Collector that returns a value if there's only a single value [duplicate]

Answer 1

If you don't mind using Guava, you can wrap your code with Iterables.getOnlyElement, so it would look something like that:

SomeProperty distinctProperty = Iterables.getOnlyElement(
        someList.stream()
                .map(obj -> obj.getSomeProperty())
                .distinct()
                .collect(Collectors.toList()));

IllegalArgumentException will be raised if there is more than one value or no value, there is also a version with default value.

Answer 2

It seems RxJava has similar functionality in its single() operator.

single( ) and singleOrDefault( )

if the Observable completes after emitting a single item, return that item, otherwise throw an exception (or return a default item)

I'd rather just have an Optional, and I'd rather it be a Collector.