Java streams, trying to turn a Map<String, List<T>> into a Map<String, Map<String, List<T>>> [duplicate]

Question

So I have the following object:

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class StakeHolderTypesDepartmentsNotifications {

    private StakeholderTypeEntity stakeholderTypeEntity;
    private StakeholderDepartmentEntity stakeholderDepartmentEntity;
    private StakeholderEntity stakeholderEntity;
    private StakeholderNotificationEntity stakeholderNotificationEntity;
}

So I initially had a list of this object, and I turned it into a Map<String, List<StakeHolderTypesDepartmentsNotifications> where the key is stakeholderDepartmentEntity.getId():

Map<String, List<StakeHolderTypesDepartmentsNotifications>> map =
    stakeholders.stream()
                .filter(entry -> entry.getStakeholderDepartmentEntity() != null)
                .collect(groupingBy(
                        entry -> entry.getStakeholderDepartmentEntity().getId()));

However now I would like the following:

Map<String, Map<String,List<StakeHolderTypesDepartmentsNotifications>>

Where the key of the top level map is stakeholderTypeEntity.getId(), where the value is the Map from above, where the key is the stakeholderDepartmentEntity.getId()

I don't understand the problem, you want to change the previous Map to new Map by java stream? Or do you want to create new Map by streaming the previous Map? — AP11
– AP11, Commented Jan 21, 2021 at 9:15

Nikolas · Accepted Answer · 2021-01-21 09:20:45Z

3

You can achieve it using another groupingBy as a downstream collector:

Map<String, Map<String, List<StakeHolderTypesDepartmentsNotifications>>> resultMap = 
    stakeholders
            .stream()
            .filter(e -> e.getStakeholderTypeEntity() != null)
            .filter(e -> e.getStakeholderDepartmentEntity() != null)
            .collect(Collectors.groupingBy(
                    e -> e.getStakeholderDepartmentEntity().getId(),
                    Collectors.groupingBy(
                        e -> e.getStakeholderTypeEntity().getId())));

Of course, an additional check whether getStakeholderTypeEntity() doesn't return null might make sense.

edited Jan 21, 2021 at 9:20

answered Jan 21, 2021 at 9:14

Nikolas

45.1k19 gold badges132 silver badges201 bronze badges

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

Comments

Lino · Accepted Answer · 2021-01-21 09:15:24Z

2

Map<String, Map<String, List<StakeHolderTypesDepartmentsNotifications>>> result = stakeholders
    .stream()
    .filter(e -> e.getStakeholderTypeEntity() != null && 
                 e.getStakeholderDepartmentEntity() != null)
    .collect(groupingBy(e -> e.getStakeholderTypeEntity().getId(), 
             groupingBy(e -> e.getStakeholderDepartmentEntity().getId())));

answered Jan 21, 2021 at 9:15

Lino

20k6 gold badges55 silver badges73 bronze badges

Comments

Nowhere Man · Accepted Answer · 2021-01-21 09:30:29Z

You should be able to do this by applying Collectors.groupingBy several times:

Map<String, Map<String,List<StakeHolderTypesDepartmentsNotifications>>> result = 
    stakeholders
        .stream()
        .filter(entry -> Objects.nonNull(entry.getStakeholderTypeEntity())
            && Objects.nonNull(entry.getStakeholderDepartmentEntity()))
        .collect(
            Collectors.groupingBy(
                entry -> entry.getStakeholderTypeEntity().getId(),
                Collectors.groupingBy(
                    entry -> entry.getStakeholderDepartmentEntity().getId()
                )
            )
        );

Aside comment: it's worth to refactor the names of the fields to remove redundant verbosity:

public class StakeHolderTypesDepartmentsNotifications {
    // just a model for an sql query that joins across all these tables
    private StakeholderTypeEntity type;
    private StakeholderDepartmentEntity department;
    private StakeholderEntity stakeholder;
    private StakeholderNotificationEntity notification;
}

Then building the map would look as follows:

Map<String, Map<String,List<StakeHolderTypesDepartmentsNotifications>>> result = 
    stakeholders
        .stream()
        .filter(entry -> Objects.nonNull(entry.getType()) && Objects.nonNull(entry.getDepartment()))
        .collect(
            Collectors.groupingBy(
                entry -> entry.getType().getId(),
                Collectors.groupingBy(-> entry.getDepartment().getId())
            )
        );

Collectives™ on Stack Overflow

Java streams, trying to turn a Map<String, List<T>> into a Map<String, Map<String, List<T>>> [duplicate]

3 Answers 3

Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Linked

Related