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
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