Everything You Must Know About FlatMap in Java

Java Streams operations are widely used in modern Java Programming. Map () and flatMap() are the important ones that we should understand to transform streams efficiently.

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What is flatMap()?

The flatMap() operation transforms each element of a stream to zero, one, or multiple elements.

It uses a function that returns a stream for each input stream element. flatMap() then flattens these multiple output streams into a single one.

List<List<Integer>> listOfLists = Arrays.asList(
    Arrays.asList(1, 2),
    Arrays.asList(3, 4, 5),
    Arrays.asList(6, 7, 8),
 Arrays.asList(9)
);

List<Integer> flattenedList = listOfLists.stream()
    .flatMap(List::stream) // Flatten the Stream<List<Integer>> into Stream<Integer>
    .collect(Collectors.toList());

System.out.println(flattenedList); // Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

In the above example, we can see that we have transformed multiple lists into a single list by flattening all those previous streams.

map() vs flatMap in Java