[mypalmike]

I'm generally a proponent of OO, and I actually think this kind of filtering chain is one of a number of places where fp code can be more readable than imperative loops.

[Jtsummers]

It's also, more or less, the "fluent interface" style if laid out differently, other than the lambdas (but even those are in both C# and Java):

  timeslotRepo.findAll()
    .stream()
    .filter(timeslot->timeslot.getEndDate()==null)
    .first()
    .ifPresentOrElse(()->{}, ()->{new Timeslot(DateTime.now())})

https://en.wikipedia.org/wiki/Fluent_interface#Java