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Importance of Collectors.flatMapping() method in Java 9?
In Java 9, a new method added to the Collectors class: flatMapping(). It is similar to the Collectors.mapping() method in which the flatMapping() method allows us to handle nested collections. The Collectors.flatMapping() method takes a function to be applied to input elements and a collector to accumulate the elements passed through the function. Unlike the Collectors.mapping() method, the Collectors.flatMapping() method deals with a stream of elements that allows us to get rid of unnecessary intermediary collections.
Syntax
public static <T,U,A,R> Collector<T,?,R> flatMapping(Function<? super T,? extends Stream<? extends U>> mapper, Collector<? super U,A,R> downstream)
Example
import java.util.stream.Collectors;
import java.util.Stream;
import java.util.Collection;
import java.util.List;
import java.util.Map;
public class FlatMappingMethodTest {
public static void main(String args[]) {
Map<Integer, List<Integer>> map = Stream.of(List.of(1, 2, 3, 4, 5, 6), List.of(7, 8, 9, 10))
.collect(Collectors.groupingBy(
Collection::size,
Collectors.flatMapping(
l -> l.stream()
.filter(i -> i % 2 == 0),
Collectors.toList())
)
);
System.out.println(map);
}
}Output
{4=[8, 10], 6=[2, 4, 6]}
Updated on: 05-Mar-2020
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