What is the (kind of) inverse operation to Java's Stream.flatMap()?

Question

The Stream.flatMap() operation transforms a stream of

a, b, c

into a stream that contains zero or more elements for each input el

Answer 1

Take a look at collapse in StreamEx

StreamEx.of("a1", "a2", "c1", "c2", "c3").collapse((a, b) -> a.charAt(0) == b.charAt(0))
    .map(e -> e.substring(0, 1)).forEach(System.out::println);

Or my fork with more function: groupBy, split, sliding...

StreamEx.of("a1", "a2", "c1", "c2", "c3").collapse((a, b) -> a.charAt(0) == b.charAt(0))
.map(e -> e.substring(0, 1)).forEach(System.out::println);
// a
// c

StreamEx.of("a1", "a2", "c1", "c2", "c3").splitToList(2).forEach(System.out::println);
// [a1, a2]
// [c1, c2]
// [c3]

StreamEx.of("a1", "a2", "c1", "c2", "c3").groupBy(e -> e.charAt(0))
.forEach(System.out::println);
// a=[a1, a2]
// c=[c1, c2, c3]