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Java 8 Streams - Stream.flatMapToDouble 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:

DoubleStream flatMapToDouble(Function<? super T,? extends DoubleStream> mapper)

This intermediate operation returns an DoubleStream 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.OptionalDouble;
import java.util.stream.DoubleStream;

public class FlatMapToDoubleExample {

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

        DoubleStream dStream =
                listOfLists.stream().flatMapToDouble(childList ->
                        childList.stream()
                                 .mapToDouble(Double::new));
        //let's peek and find average of the elements
        OptionalDouble opt = dStream.peek(System.out::println)
                                    .average();
        if (opt.isPresent()) {
            System.out.println("average: " + opt.getAsDouble());
        }
    }
}

Output

1.0
2.0
5.0
6.0
3.0
4.0
average: 3.5




package com.logicbig.example.stream;

import java.util.Arrays;
import java.util.OptionalDouble;
import java.util.stream.DoubleStream;

public class FlatMapToDoubleExample2 {

    public static void main(String... args) {

        String[][] arrayOfArrays = {{"1", "2"}, {"5", "6"}, {"3", "4"}};


        DoubleStream dStream = Arrays.stream(arrayOfArrays)
                                     .flatMapToDouble(childArray -> Arrays.stream(childArray)
                                                                          .mapToDouble(Double::new));
        OptionalDouble opt = dStream.peek(System.out::println)
                                    .average();
        if (opt.isPresent()) {
            System.out.println("average: " + opt.getAsDouble());
        }
    }
}

Output

1.0
2.0
5.0
6.0
3.0
4.0
average: 3.5

See Also