What method truncates a list--for example to the first 100 elements--discarding the others (without iterating through individual elements)?
subList, as suggested in the other answers, is the first that comes to mind. I would also suggest a stream approach.
source.stream().limit(10).collect(Collectors.toList()); // truncate to first 10 elements
source.stream().skip(2).limit(5).collect(Collectors.toList()); // discards the first 2 elements and takes the next 5