Stream flatMap vs map

Question

To filter a List we can use

a1.stream().filter(x->x>6).forEach(System.out::println);

and to filter two lists I used FlatMap

Stream<List<Integer>> s2=Stream.of(a1,a2);
s2.flatMap(x->x.stream()).sorted().filter(x->x>6).forEach(System.out::println);

but I tried filtering in this way for a single List

Stream<List<Integer>> s1=Stream.of(a1);
s1.map(x->x.stream()).filter(x->x>2).forEach(System.out::print);

I got an error--- The operator > is undefined for the argument type(s) Stream<Integer>, int

but when I use the flatMap in the map no error why

Stream<List<Integer>> s1=Stream.of(a1); 
s1.flatMap(x->x.stream()).filter(x->x>2).forEach(System.out::print);

Both `s2` and `s1` are streams of `List`, so why did you expect that `s1.map()` would do the same as `s2.flatMap()`? — Andreas, Jan 10 '21 at 10:50
Map is for one result only. Flatmap are for zero or more results. — Thorbjørn Ravn Andersen, Jan 10 '21 at 11:01
To stream over a single list, use `a1.stream()` instead of `Stream.of(a1)`. Then, you need neither, `map` nor `flatMap`. — Holger, Jan 10 '21 at 12:13

score 5 · Answer 1 · answered Jan 10 '21 at 10:45

5

If you are calling map(x->x.stream()) on a Stream<List<Integer>>, you'll get a Stream<Stream<Integer>>. You cannot apply .filter(x->x>2) on the elements of that Stream, since those elements are Stream<Integer>s, and the > operator requires two numeric operands.

If you use flatMap instead of map, you are converting your Stream<List<Integer>> to a Stream<Integer> whose elements are all the elements of all the Lists of the original Stream. Therefore, you can apply the .filter(x->x>2) filter on them.

answered Jan 10 '21 at 10:45

Eran

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Stream> s1=Stream.of(a1); s1.filter(x->x>2).forEach(System.out::print); error--The operator > is undefined for the argument type(s) List, int – Krissag Jan 10 '21 at 11:01
@Krissag well, in this case the elements of your `Stream` are `List`. You cannot ask if a `List` > 2. – Eran Jan 10 '21 at 11:03

score 2 · Answer 2 · edited Jan 28 '21 at 16:57

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Stream<List<Integer>> s1 = Stream.of(a1);
        s1.map(x -> x.stream()).filter(x -> x > 2).forEach(System.out::print);

This will give error as you are not using flat map so filtering not possible whole (Streams >2). You can try like this:

    Stream<List<Integer>> s1 = Stream.of(a1);
            s1.map(x -> x.stream()).filter(x -> x.findFirst().get() > 2).forEach(System.out::print);

As we cannot apply.filter(x->x>2) on we will requires two numeric operands. s1.map(x -> x.stream()).filter(x -> x.findFirst().get() > 2).forEach(System.out::print); if we will take this then filter will find find matching element value which is greater than 2. In short we have to use flatmap to find greater than 2 in those streams or any operation to fetch numeric operands from stream then only it work.

edited Jan 28 '21 at 16:57

dreamcrash

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answered Jan 10 '21 at 14:56

murari99732

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Explain your code or at least write a few words about it – Aalexander Jan 10 '21 at 22:12
As we cannot apply .filter(x->x>2) on we will requires two numeric operands. s1.map(x -> x.stream()).filter(x -> x.findFirst().get() > 2).forEach(System.out::print); if we will take this then filter will find find matching element value which is greater than 2. In short we have to use flatmap to find greater than 2 in those streams or any operation to fetch numeric operands from stream then only it work. – murari99732 Jan 12 '21 at 08:05
@murari99732 I just formatted you answer, if you do like fell free to rollback :) – dreamcrash Jan 28 '21 at 17:05

Stream flatMap vs map

2 Answers2