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Java Streams flatMap Demo

vikrampawar profile image vikrampawar ・2 min read

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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