how internally map and flatmap works in java 8 [duplicate]

Map is used when you want to transform a stream.

For example you have a shopping cart and for each item in your cart you would like to get the item detail or something else.

FlatMap combines the above map operation with a flat operation

Example could be something like you have bunch of orders and would like to know the total count of items sold for all orders. You could use a flatmap operation to get the number of items

Documentation helps btw

Map is used to transform the object from one stream to another.

For example:

    class User {
        private Long id;

        public Long getId(){
           return id;
       }
}

Consider you have a list of users and your usecase is to get the user id's alone. Here comes advantage of Map.

List<User> users = new ArrayList<>();

List<Long> ids = users.stream().map(user -> 
  user.getId() ).collect(Collectors.toList());

FlatMap is also same as Map but it has another advantage of merging the multiple list into single list.

Example:

List<List<String>> stringlist = Arrays.asList( Arrays.asList("a"),
Arrays.asList("b") );

List<String> strings = stringlist.stream().flatMap(Collection::stream)
  .collect(Collectors.toList()));