Streams flatMap

Description

Streams map() operation creates a one-to-one mapping.

Streams flatMap() supports one-to-many mapping. It maps each element to a stream and then flaten the stream of streams to a stream.

Example

The following code maps a stream of three numbers: 1, 2, and 3 to produce a stream that contains the numbers and their next numbers. The output stream should be 1,2,2,3,3,4.

import java.util.stream.Stream;
/*from   w w w. j  a v  a 2 s.c o m*/
public class Main {
  public static void main(String[] args) {
    Stream.of(1, 2, 3)
    .flatMap(n -> Stream.of(n, n+1))
    .forEach(System.out::println);

  }
}

The code above generates the following result.

Example 2

The following code shows how to convert a stream of strings to a stream of characters.

import java.util.stream.Stream;
//  w  ww.j  av a2s. co  m
public class Main {
  public static void main(String[] args) {
    Stream.of("XML", "Java",  "CSS")
        .map(name  ->  name.chars())
        .flatMap(intStream ->  intStream.mapToObj(n ->  (char)n))
        .forEach(System.out::println); 

  }
}

The code maps the strings to IntStream returns from chars() method of the String class.

The output of the map() method is Stream<IntStream>.

The flatMap() method maps the Stream<IntStream> to Stream<Stream<Character>> and finally, flattens it to produce a Stream<Character>.

The code above generates the following result.

Example 3

The following code flatMaps the stream of string values to a IntStreams, then maps IntStream to Stream of characters.

import java.util.stream.IntStream;
import java.util.stream.Stream;
// www .  j  a va2 s. c  om
public class Main {
  public static void main(String[] args) {
    Stream.of("XML", "Java",  "CSS")
        .flatMap(name ->  IntStream.range(0, name.length())
        .mapToObj(name::charAt))
        .forEach(System.out::println);
  }
}

The code above generates the following result.