how to get from Stream using FlatMap List<> of items with List

Question

I have a Collection of a Dto (Pojo).

Collection<Dto> dtos = new ArrayList<>();
... <filling dto> now it has items.

Dto has a field List:

class Dto {
  private List<Tag> tags; 
} 

@RequiredArgsConstructor
class Tag {
  private String name;

}

Example of Data:

dtos.get(1) => new Tag('name1.1') -> new Tag('name1.2') 
dtos.get(2) => new Tag('name2.1') 
dtos.get(3) => new Tag('name3.1') > new Tag('name3.2')
dtos.get(4) => null

a QUESTIONL How to get using only stream Java8+ all List is it possible to use flatMap()

You ideas?

Answer 1

I think this should be straightforward, you filter the null tags then flatmap them and collect to a list

public static void main(String[] args) {
    Dto dto1 = new Dto(Arrays.asList(new Tag("name1.1"), new Tag("name1.2")));
    Dto dto2 = new Dto(Arrays.asList(new Tag("name2.1")));
    Dto dto3 = new Dto(Arrays.asList(new Tag("name3.1"), new Tag("name3.2")));
    Dto dto4 = new Dto(null);

    Collection<Dto> dtos = new ArrayList<>(Arrays.asList(dto1, dto2, dto3, dto4));

    List<Tag> tags = dtos.stream().filter(dto -> dto.getTags() != null).flatMap(dto ->
            dto.getTags().stream()).collect(Collectors.toList());

}

Answer 2

You have a few issues with initialization and size of your buffers.

• The size of your buffer is too short by 1 byte that is needed for 0-termination. Adding the last number causes buffer overrun and undefined behaviour.
• In addition the calculated size is only sufficient as long as number_of_ints<10 because it only allows for single digit numbers.
• That buffer is not initialized and us very likely not holding an empty string. Accessing it (via strcat etc.) invokes undefined behaviour.
• The size you provide to snprintf is not related to the size of the buffer.

You should apply these changes:

#include <stdio.h>
#include <string.h>

int main(void)
{
    int number_of_ints = 5;
    char s[number_of_ints*2+1];
    char suffix[4];
    s[0] = 0;
    for (int i = 1; i <= number_of_ints; i++)
    {
        snprintf(suffix, sizeof(suffix), "%d*", i);
        strncat(s, suffix, 2);
        printf("%s\n", s);
    }
}