The flatMap function is very similar to the map function, but it is used when the operation might return more than one value and we want to keep a stream of single elements. In the case of map, a stream of collections would be returned instead. Let's see flatMap in use:

@Test
public void gettingLettersUsedInNames() {
  List<String> names = Arrays.asList("Alex", "Paul", "Viktor");
  List<String> lettersUsed = Collections.emptyList();

  assertThat(lettersUsed)
    .hasSize(12)
    .containsExactly("a","l","e","x","p","u","v","i","k","t","o","r");
}

One possible solution could be:

List<String> lettersUsed = names.stream()
  .map(String::toLowerCase)
  .flatMap(name -> Stream.of(name.split("")))
  .distinct()
  .collect(Collectors.toList());

This time we have used Stream.of(), a convenient method for creating Streams. Another really nice feature is the method distinct(), which returns a collection of unique elements, comparing them using the method equals().