Java 8 Streams FlatMap method example

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It doesn’t make sense to flatMap a Stream that’s already flat, like the Stream<Integer> you’ve shown in your question.

However, if you had a Stream<List<Integer>> then it would make sense and you could do this:

Stream<List<Integer>> integerListStream = Stream.of(
    Arrays.asList(1, 2), 
    Arrays.asList(3, 4), 
    Arrays.asList(5)
);

Stream<Integer> integerStream = integerListStream .flatMap(Collection::stream);
integerStream.forEach(System.out::println);

Which would print:

1
2
3
4
5

To do this pre-Java 8 you just need a loops:

List<List<Integer>> integerLists = Arrays.asList(
    Arrays.asList(1, 2), 
    Arrays.asList(3, 4), 
    Arrays.asList(5)
)

List<Integer> flattened = new ArrayList<>();

for (List<Integer> integerList : integerLists) {
    flattened.addAll(integerList);
}

for (Integer i : flattened) {
    System.out.println(i);
}

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