public class Dto {

    List<Long> longs = new ArrayList<>();

    public List<Long> getLongs() {
        return longs;
    }

    public void setLongs(List<Long> longs) {
        this.longs = longs;
    }
}

public class Entity {

    List<Long> longs = new ArrayList<>();

    public List<Long> getLongs() {
        return longs;
    }

    public void setLongs(List<Long> longs) {
        this.longs = longs;
    }
}

@Mapper(nullValueMappingStrategy = NullValueMappingStrategy.RETURN_DEFAULT)
public interface EntityMapper {

    Dto asTarget(Entity entity);

}

    public EntityMapperImpl() {
    }

    public Dto asTarget(Entity entity) {
        Dto dto = new Dto();
        if (entity != null) {
            List<Long> list = entity.getLongs();
            if (list != null) {
                dto.setLongs(new ArrayList(list));
            } else {
                dto.setLongs((List)null);
            }
        }

        return dto;
    }
}