Toward understanding of map and flatmap in Stream (1)

question

From the list of two ʻInteger Generate a list ofPair`s that are a combination of all the elements of each other.

ex.) [1,2,3], [5,6] -> [(1,5),(1,6),(2,5),(2,6),(3,5),(3.6)]

trial1 Maybe I can go with a map.

List<Integer> ints1 = Arrays.asList(1,2,3);
List<Integer> ints2 = Arrays.asList(5,6);
ints1.stream()
     .map( i -> ints2.stream().map( j -> Pair.of(i,j)))
     .collect(toList()); 

No good. List <Stream <Pair <Integer, Integer >> is returned.

In the first place ... The Stream.map function takes the type Function <? Super T ,? extends R> as an argument and returns <R> Stream <R>.

Stream map(Function<? super T,? extends R> mapper) ref. Stream.map

So, the nested map is Returns Stream <Pair <Integer, Integer >> and The original map that received the return value It will return Stream <Stream <Pair <Integer, Integer >>>.

And by the termination operation List <Stream <Pair <Integer, Integer >> is finally returned. (The future will be about what the termination operation is doing ...)

So flatMap

By using flatMap Replace each value of Stream with another Stream, All generated Streams will be aggregated into a single Stream. ref. Java8 in Action

By definition Passed as an argument to flatMap The second argument of Function <? Super T ,? extends Stream <? extends R >> mapper is It is ? extends Stream <? extends R> The return value is It is <R> Stream <R> of type R that constitutes Stream of the second argument of Function.

Stream flatMap(Function<? super T,? extends Stream<? extends R>> mapper) ref. flatMap

Based on the above, the code is as follows.

ints1.stream()
     .flatMap( i -> ints2.stream().map( j -> Pair.of(i,j)))
     .collect(toList());

flatMap( i -> ints2.stream().map( j -> Pair.of(i,j))) The type returned by With Stream <Pair <Integer, Intege >>, The termination operation returns List <Pair <Integer, Integer >>.

By using flatMap It feels like it makes Stream that was nested in trial1 good ^^; (Explanation miscellaneous)