Java Stream.flatMap Method

September 25, 2023

Java 8 Stream.flatMap() method is used to flatten stream of collections to a stream of objects. In short, we can say that it allows us to combine the objects from multiple collections into a single collection.

Flattening Example

Before flattening: [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 
After flattening: [1, 2, 3, 4, 5, 6, 7, 8, 9]

Stream flatMap Examples

Codes for flattening nested lists

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

        List<Integer> results = listOfLists.stream()
                .flatMap(Collection::stream)
                .toList();

        log.info("results: {}", results);
    }

Codes for flattening nested arrays

    private static void flatteningNestedArrays() {
        Integer[][] nestedArrays = new Integer[][]{
                {1, 2, 3},
                {4, 5, 6},
                {7, 8, 9}
        };

        List<Integer> results = Arrays.stream(nestedArrays)
                .flatMap(Arrays::stream)
                .toList();

        log.info("results: {}", results);
    }