Stream<Number> numbers = ... ;
     List<Integer> integers = numbers.<Integer>mapMulti((number, consumer) -> {
             if (number instanceof Integer)
                 consumer.accept((Integer) number);
         })
         .collect(Collectors.toList());

Implementation Requirements:
The default implementation invokes flatMap on this stream, passing a function that behaves as follows. First, it calls the mapper function with a Consumer that accumulates replacement elements into a newly created internal buffer. When the mapper function returns, it creates a stream from the internal buffer. Finally, it returns this stream to flatMap.