flatMap() example

Example:

import java.util.Arrays;
import java.util.List;

public class FlatMapFlight {
    public static void main(String[] args) {
        System.out.println("main() [Start]");
        List<String> words = Arrays.asList("Hello","Good","Morning");
       
        words
        .stream()
        .map(word->word.split(""))
        .flatMap(Arrays::stream)
        .forEach(System.out::println);
       
        System.out.println("main() [End]");
    }
}

Ouput:
main() [Start]
H
e
l
l
o
G
o
o
d
M
o
r
n
i
n
g
main() [End]

Explanation:

 .map(word->word.split("")) :

• filter takes the lamda expression as an argument. 
• It takes a lamda expression having functional interface having abstract method which take one argument and return another type object in result. 
• It returns the Stream of result type parameters
• Here it returns each word in a separate stream of String  containing each character

    .flatMap(Arrays::stream):

• This method works on the stream returned from the previous method
• It returns the Stream merging all the words streams into a single stream
• If you will not use flatMap here then it will give you three streams objects for the give three words each contains each character in a string
• So, using flatMap will give you a single stream contains all words all character in a single stream.

     .forEach(System.out::println):

• This method works on the stream returned from the flatpMap method
• This is a terminal operation method because it doesnt return the stream.
• All the previous streams maintained as pipeline and processed when this method get called
• This method takes a consumer functional interface lamda expression or method reference as argument and it consumes the data of previous streams. Here it consumes and display each element.