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Java 8 Streams - Stream.flatMapToLong Examples

[Last Updated: Nov 8, 2025]

Java 8 Streams Java Java API 

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Interface:

java.util.stream.Stream

java.lang.AutoCloseableAutoCloseablejava.util.stream.BaseStreamBaseStreamjava.util.stream.StreamStreamLogicBig

Method:

LongStream flatMapToLong(Function<? super T,? extends LongStream> mapper)

Tlhis intermediate operation returns an LongStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.


Examples


package com.logicbig.example.stream;

import java.util.Arrays;
import java.util.List;
import java.util.stream.LongStream;

public class FlatMapToLongExample {

    public static void main(String... args) {
        List<List<String>> listOfLists = Arrays.asList(
                Arrays.asList("1", "2"),
                Arrays.asList("5", "6"),
                Arrays.asList("3", "4")
        );


        LongStream longStream = listOfLists.stream()
                                           .flatMapToLong(childList ->
                                                   childList.stream()
                                                            .mapToLong(Long::new));
        long sum = longStream.peek(System.out::println)
                             .sum();
        System.out.println("sum: " + sum);
    }
}

Output

1
2
5
6
3
4
sum: 21




Using an array of arrays. 

package com.logicbig.example.stream;

import java.util.Arrays;
import java.util.stream.LongStream;

public class FlatMapToLongExample2 {

    public static void main(String... args) {
        String[][] arrayOfArrays = {{"1", "2"}, {"5", "6"}, {"3", "4"}};


        LongStream longStream = Arrays.stream(arrayOfArrays)
                                      .flatMapToLong(childArray -> Arrays.stream(childArray)
                                                                         .mapToLong(Integer::new));
        long sum = longStream.peek(System.out::println)
                             .sum();
        System.out.println("sum: " + sum);
    }
}

Output

1
2
5
6
3
4
sum: 21

See Also