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

Answer 1

A more concise way to build a Collector for this is as follows:

Collectors.reducing((a, b) -> null);

The reducing collector will store the first value, and then on successive passes, pass the current running value and the new value into the lambda expression. At this point, null can always be returned since this will not be called with the first value, which will simply be stored.

Plugging this into the code:

Optional universalCommonProperty = someList.stream()
    .map(obj -> obj.getSomeProperty())
    .distinct()
    .collect(Collectors.reducing((a, b) -> null));