Stream flatMap() in Java with examples
Last Updated :
12 Mar, 2018
Stream flatMap(Function mapper) returns a stream 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. Stream flatMap(Function mapper) is an intermediate operation. These operations are always lazy. Intermediate operations are invoked on a Stream instance and after they finish their processing, they give a Stream instance as output.
Note : Each mapped stream is closed after its contents have been placed into this stream. If a mapped stream is null, an empty stream is used, instead.
flatMap() V/s map() :
1) map() takes a Stream and transform it to another Stream. It applies a function on each element of Stream and store return value into new Stream. It does not flatten the stream. But flatMap() is the combination of a map and a flat operation i.e, it applies a function to elements as well as flatten them.
2) map() is used for transformation only, but flatMap() is used for both transformation and flattening.
Syntax :
<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
where, R is the element type of the new stream.
Stream is an interface and T is the type
of stream elements. mapper is a stateless function
which is applied to each element and the function
returns the new stream.
Example 1 : flatMap() function with provided mapping function.
import java.util.*;
import java.util.stream.Stream;
class GFG {
public static void main(String[] args)
{
List<String> list = Arrays.asList( "5.6" , "7.4" , "4" ,
"1" , "2.3" );
list.stream().flatMap(num -> Stream.of(num)).
forEach(System.out::println);
}
}
|
Output :
5.6
7.4
4
1
2.3
Example 2 : flatMap() function with provided operation of mapping string with character at position 2.
import java.util.*;
import java.util.stream.Stream;
class GFG {
public static void main(String[] args)
{
List<String> list = Arrays.asList( "Geeks" , "GFG" ,
"GeeksforGeeks" , "gfg" );
list.stream().flatMap(str ->
Stream.of(str.charAt( 2 ))).
forEach(System.out::println);
}
}
|
Output :
e
G
e
g
How does flatMap() work ?
As already discussed in the post that flatMap() is the combination of a map and a flat operation i.e, it first applies map function and than flattens the result. Let us consider some examples to understand what exactly flattening a stream is.
Example 1 :
The list before flattening :
[ [2, 3, 5], [7, 11, 13], [17, 19, 23] ]
The list has 2 levels and consists of 3 small lists. After Flattening, it gets transformed into “one level” structure as shown :
[ 2, 3, 5, 7, 11, 13, 17, 19, 23 ]
Example 2 :
The list before flattening :
[ ["G", "E", "E"], ["K", "S", "F"], ["O", "R", "G"], ["E", "E", "K", "S"] ]
The list has 3 levels and consists of 4 small lists. After Flattening, it gets transformed into “one level” structure as shown :
["G", "E", "E", "K", "S", "F", "O", "R", "G", "E", "E", "K", "S"]
In short, we can say that if there is a Stream of List of <<Data Type>> before flattening, then on applying flatMap(), Stream of <<Data Type>> is returned after flattening.
Application :
import java.util.*;
import java.util.stream.Collectors;
class GFG
{
public static void main(String[] args)
{
List<Integer> PrimeNumbers = Arrays.asList( 5 , 7 , 11 , 13 );
List<Integer> OddNumbers = Arrays.asList( 1 , 3 , 5 );
List<Integer> EvenNumbers = Arrays.asList( 2 , 4 , 6 , 8 );
List<List<Integer>> listOfListofInts =
Arrays.asList(PrimeNumbers, OddNumbers, EvenNumbers);
System.out.println( "The Structure before flattening is : " +
listOfListofInts);
List<Integer> listofInts = listOfListofInts.stream()
.flatMap(list -> list.stream())
.collect(Collectors.toList());
System.out.println( "The Structure after flattening is : " +
listofInts);
}
}
|
Output :
The Structure before flattening is : [[5, 7, 11, 13], [1, 3, 5], [2, 4, 6, 8]]
The Structure after flattening is : [5, 7, 11, 13, 1, 3, 5, 2, 4, 6, 8]
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