In a social media application, You have a collection of posts, where each post can have multiple comments.
To retrieve all the comments from all the posts
and represents them as a single stream,
Which java Stream operation would you use?
The correct
answer is flatmap().
So what is flatmap,
flatMap() = Flattening + map()
It is an intermediate operation that is always lazy as described in Java Stream API
flattening means, merging multiple collections/arrays into
one.
OR,
Flattening is the process of converting several lists of lists and merge all those lists to create a single list containing all the elements from all the lists.
Flattening Example
Consider the following lists of lists:
Before Flattening: [[1, 2, 3, 4], [7, 8, 9, 0], [5, 6], [12, 18, 19, 20, 17], [22],[23,24,25,26,27]]
After Flattening: [1, 2, 3, 4, 7, 8, 9, 0, 5, 6, 12, 18, 19, 20, 17, 22,23,24,25,26,27]
Syntax
<R>
Stream<R> flatMap(Function<? super T,? extends Stream<? extends
R>> mapper)
- R represents the element type of the new stream.
- mapper is a non-interfering, stateless function to apply to each element which produces a stream of new values.
- The method returns a new stream of objects of type R.
Similar flatMap() Methods
IntStream flatMapToInt(Function<? super T,? extends IntStream> mapper)
LongStream flatMapToLong(Function<? super T,? extends LongStream> mapper)
DoubleStream flatMapToDouble(Function<? super T,? extends DoubleStream> mapper)
Some examples where flatMap can be used are:
Count of words of a text file.
Convert a Nested List into a single List.
Convert a Nested Array into a single Lis
