Create a list of objects from another list using Java 8 streams

Question

I have these three dto classes :

public class OrganismeDTO {
    private Long organismeId;
    private String libelle;
    private List<RoleDTO> rolesDTO;
}

public class RoleDTO {
    private String lib;
    private Long roleId;
    private List<ProfilDTO> profilsDTO;
}

public class UtilisateurRoleDto {
    private String organismeLib;
    private String roleLib;
}

And in my DAO I have a function that will return a list of UtilisateurRoleDto.

What I want, is to create a list of OrganismeDTOfrom UtilisateurRoleDto list (Which I can get from my DAO), so for example, if I have a list of UtilisateurRoleDto as following:

"Organisme 1", "Role 1"
"Organisme 1", "Role 2"
"Organisme 2", "Role 1"
"Organisme 2", "Role 3"
"Organisme 3", "Role 3"

I want a list of OrganismeDTO in result as following (where each OrganismeDTO has a list of RoleDTO) :

"Organisme 1" : ["Role 1", "Role 2"]
"Organisme 2" : ["Role 1", "Role 3"]
"Organisme 3" : ["Role 3"]

How can I implement this using Java 8 streams?

Update:

When I construct the RoleDTO I need to inject RoleDTO.profilsDTO by a list I get from a function that takes OrganismeDTO.roleId and returns List<ProfilDTO>.

I think if you create a map from UtilisateurRoleDto list then will help you to find the solution. — Vishnu KR
– Vishnu KR, Commented Oct 3, 2017 at 11:52
@nullpointer I didnt understand your question, in fact, RoleDTO.lib is the same as UtilisateurRoleDto.roleLib — Renaud is Not Bill Gates
– Renaud is Not Bill Gates, Commented Oct 3, 2017 at 13:21

Oleg · Accepted Answer · 2017-10-03 11:53:21Z

1

You can do it with a custom collect(I added getters and some obvious constructors):

Map<String, OrganismeDTO> map = dt.stream().collect(HashMap::new, (m, t) -> {
    m.computeIfAbsent(t.getOrganismeLib(), x -> new OrganismeDTO(t.getOrganismeLib())).getRolesDTO().add(new RoleDTO(t.getRoleLib()));
}, (m1, m2) -> {
    m2.forEach((k,v) -> {
        OrganismeDTO organismeDTO = m1.get(k);
        if (organismeDTO != null ) {
            organismeDTO.getRolesDTO().addAll(v.getRolesDTO());
        } else {
            m1.put(k, v);
        }
    });
});

And then all that's left if to create List from the values.

List<OrganismeDTO> list = map.values().stream().collect(Collectors.toList());

answered Oct 3, 2017 at 11:53

Oleg

6,3542 gold badges27 silver badges40 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Renaud is Not Bill Gates Over a year ago

When I tried this I got a NullPointerException in this line : .add(new RoleDTO(t.getRoleLib()), is it because getRolesDTO() is not initialized as an ArrayList ?

Oleg Over a year ago

@AimadMAJDOU Yes, in my "obvious constructor" I initialize it with an ArrayList.

Renaud is Not Bill Gates Over a year ago

What I mean is the getRolesDTO() will be null since it has to be initialized as a new ArrayList, so what I did is: private List<RoleDTO> rolesDTO = new ArrayList<>(); in the OrganismeDTO class and the error is gone.

Renaud is Not Bill Gates Over a year ago

This solution is working for me, thanks for your help, it would be great if you could explain the code you wrote.

Schidu Luca · Accepted Answer · 2017-10-03 11:32:25Z

1

The only way I can think of at the moment is this:

Map<String, List<String>> collect = list.stream()
            .collect(Collectors.groupingBy(UtilisateurRoleDto::getOrganismeLib,
                    Collectors.mapping(UtilisateurRoleDto::getRoleLib, Collectors.toList())));

List<OrganismeDTO> result = collect.entrySet().stream()
        .map(entry -> new OrganismeDTO(entry.getKey(),
                entry.getValue().stream()
                        .map(RoleDTO::new)
                        .collect(toList())))
        .collect(Collectors.toList());

Firstly you can group your UtilisateurRoleDto by it's organismeLib field. After that you process the obtained result and map it to your wanted entities.

edited Oct 3, 2017 at 11:32

answered Oct 3, 2017 at 11:11

Schidu Luca

3,9471 gold badge14 silver badges28 bronze badges

3 Comments

Renaud is Not Bill Gates Over a year ago

Thank you, but I forgot to add the id field for OrganismeDTO and RoleDTO, could you please recheck my update.

Schidu Luca Over a year ago

From where should I get those ids ? or do I have to generate them randomly ? ...

Renaud is Not Bill Gates Over a year ago

They are already defined : OrganismeDTO.organismeId and RoleDTO.roleId

Collectives™ on Stack Overflow

Create a list of objects from another list using Java 8 streams

Update:

2 Answers 2

4 Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Update:

2 Answers 2

4 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related