JAVA EE: How to use flatMap method of Java 8 streams

Wednesday 6 September 2017

How to use flatMap method of Java 8 streams | Streams in Java 8

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How to use flatMap method of Java 8 streams | Streams in Java 8

FlatMapDemo.java

import java.util.Arrays;
import java.util.stream.Stream;

public class FlatMapDemo
{
    public static void main(String[] args)
    {

        String[][] strArray = new String[][] { { "a", "b" }, { "c", "d" }, { "e", "f" } };

        // Stream<String[]>
        Stream<String[]> strArrayStream = Arrays.stream(strArray);
        
        //Convert  Stream<String[]> to Stream<String>
        Stream<String> strStream = strArrayStream.flatMap((x -> Arrays.stream(x)));

        // filter a stream of string
        Stream<String> filterdStream = strStream
                .filter(x -> "c".equals(x.toString()));

        filterdStream.forEach(System.out::println); //Output is c
    }

}

Output

NonFlatMapDemo.java

import java.util.Arrays;
import java.util.stream.Stream;

/**
 * 
 * This example will print an empty result, because filter() has
 * no idea how to filter a stream of String[].
 *
 */
public class NonFlatMapDemo
{
    public static void main(String[] args)
    {

        String[][] strArray = new String[][] { { "a", "b" }, { "c", "d" },{ "e", "f" } };

        // Stream<String[]>
        Stream<String[]> strArrayStream = Arrays.stream(strArray);

        // filter a stream of string[], and return a string[]?
        Stream<String[]> filterdStream = strArrayStream
                .filter(x -> "c".equals(x.toString()));

        filterdStream.forEach(System.out::println); //Output is empty...
    }

}