A SequenceInputStream represents the logical concatenation of other input streams. It starts out with an ordered collection of input streams and reads from the first one until end of file is reached, whereupon it reads from the second one, and so on, until end of file is reached on the last of the contained input streams.
Stream<String> a = Stream.of("one", "two");
Stream<String> b = Stream.of("three", "four");
Stream.concat(a, b);
or
Stream.of(a, b, c)
.reduce(Stream::concat)
.orElseGet(Stream::empty);
or
Stream.of(a, b, c).flatMap(s -> s);