The below example shows how we can use flatMap() to get the unique combination of two integers for all integers up to n. For n=3, this would be
(1, 1) (1, 2) (1, 3) (2, 2) (2, 3) (3, 3).
To obtain a stream of 3 integers (1, 2, 3) use IntStream.rangeClosed(1, 3)
Map works with a stream of types, so convert this to a stream of 3 Integers by using .boxed().
Now starting from 1, we want to make a combination of that number with each integer following it.
So for each i we can obtain another stream by IntStream.rangeClosed(i, n).
If we use map(), as below, we'll end up with a stream of streams.
map(a -> IntStream.rangeClosed(a, 3) .mapToObj( b -> new double[]{a, b})) This would be a stream of 3 streams. The first stream will have [1, 1] [1, 2] [1, 3], the second will have [2, 2] and [2, 3]. The third will have [3, 3].
If instead, we use flatMap() as below, we'll end up with one stream.
flatMap(a -> IntStream.rangeClosed(a, 3) .mapToObj( b -> new double[]{a, b})) This would be the stream
[1, 1] [1, 2] [1, 3] [2, 2] [2, 3] [3, 3].
The complete program.
import java.util.stream.IntStream; public class FlatMapDemo { public static void main(String[] args) { System.out.println("\n\nStep 1 one primitive integer stream \n------"); IntStream.rangeClosed(1, 3) // one stream .forEach(t -> System.out.print(t + " - primitive " + ", ")); System.out.println("\n\nStep 2 boxed to Integer stream \n------"); IntStream.rangeClosed(1, 3).boxed() // one stream .forEach(t -> System.out.print(t + " - " + t.getClass().getTypeName() + ", ")); System.out.println("\n\nStep 3 Print stream Regular map \n------"); IntStream.rangeClosed(1, 3) .boxed() .map(a -> IntStream.rangeClosed(a, 3) // regular map, stream of streams .mapToObj( b -> new int[]{a, b})) .forEach(t -> System.out.println(t + " - contains " + t.count())); System.out.println("\n\nStep 4 Print stream Flat map \n------"); IntStream.rangeClosed(1, 3) .boxed() .flatMap(a -> IntStream.rangeClosed(a, 3) // flat map, one stream .mapToObj( b -> new int[]{a, b})) .forEach(t -> System.out.println(t + " - " + t.getClass().getTypeName() + " Contents: " + t[0] + ", " + t[1])); } } The output will be
Step 1 one primitive integer stream ------ 1 - primitive , 2 - primitive , 3 - primitive , Step 2 boxed to Integer stream ------ 1 - java.lang.Integer, 2 - java.lang.Integer, 3 - java.lang.Integer, Step 3 Print stream Regular map ------ java.util.stream.IntPipeline$1@6aceb1a5 - contains 3 java.util.stream.IntPipeline$1@4c75cab9 - contains 2 java.util.stream.IntPipeline$1@1ef7fe8e - contains 1 Step 4 Print stream Flat map ------ [I@6979e8cb - int[] Contents: 1, 1 [I@763d9750 - int[] Contents: 1, 2 [I@5c0369c4 - int[] Contents: 1, 3 [I@2be94b0f - int[] Contents: 2, 2 [I@d70c109 - int[] Contents: 2, 3 [I@17ed40e0 - int[] Contents: 3, 3 Link to blogpost
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