Java 11 Stream to sort a list of object and add sequence based on sorting

Question

I need add sequence number to a list of object based on created date field Code is written in Java 11. I have a list like the below one

public class UserInformation {
    private String userSeqNumber;
    private String userDepartment;
    private Date createdDate;
}

I can sort the list by created date but at the same time i need to add the userSequenceNumber based on the created date in ascending order. I tried a messed up code here, can someone please help me.

userInformations.stream()
        .filter(c-> c.getUserDepartment().equalsIgnoreCase(request.getUserDepartment()))
        .sorted(Comparator.comparing(UserInformation::getCreatedDate))
        .forEach(f -> f.setUserSeqNumber());

So output should be for each department, sequence number should be incremented based on the number of entries using the created date.

UserInformation:[{"1","IT",01-01-2022}, {"2","IT",01-02-2022},{"1","OPS",01-01-2022}, {"2,"OPS",01-02-2022}]

Unless setReferralSeqNumber() produces some output, the posted code is not capable of producing output. Unless your problem is precisely that this code doesn’t produce any output, you should add an explanation about your actual problem, i.e. in what regard does the code not do what you expect. — Holger
– Holger, Commented Feb 14, 2022 at 12:27
User Information seems to have duplicate sequence numbers. And what is SIUInformation? I thought you were simply sorting dates amongst themselves. Please provide more detail and include input and expected output. BTW, most of the Date class is mostly deprecated. LocalDate would be much better unless you must use legacy classes. — WJS
– WJS, Commented Feb 14, 2022 at 12:31
Another question. Some of your provided dates are the same but the departments are different. Does it matter which comes first? Is it dependent on the department? — WJS
– WJS, Commented Feb 14, 2022 at 13:39

Nico Van Belle · Accepted Answer · 2022-02-14 12:35:54Z

It could be I don't understand the issue because your code example refers to unknown fields (referringDepartment, referralSeqNumber) and unknown classes (SIUInformation) but I'm guessing this is likely what you more or less need?

userInformations.stream()
        .collect(Collectors.groupingBy(UserInformation::getUserDepartment))
        .values()
        .forEach(listForDepartment -> {
              AtomicInteger x = new AtomicInteger(1);
              listForDepartment.stream()
                  .sorted(Comparator.comparing(UserInformation::getCreatedDate))
                  .forEachOrdered(item -> item.setUserSeqNumber(String.valueOf(x.getAndIncrement())));
         });

First you group the UserInformation objects by department (resulting in Map<String List<UserInformation>) and then you loop the values in the Map created by this action. You then sort the values by createdDate using sorted() (Date comparator is by default ascending) and you then start filling in the userSeqNumber according the order of the elements, restarting from 1 for every department in the map.
Note I use forEachOrdered instead of forEach as I explicitly want to document that the order matters, even though in this case, it won't break the code if forEach is used instead.

MikeFHay · Accepted Answer · 2022-02-14 14:20:08Z

0

Streams don't track the number of items that they have processed. The simplest way to achieve what you want is to do it in 2 parts.

List<UserInformation> sorted = userInformations.stream()
        .filter(c-> c.getReferringDepartment().equalsIgnoreCase(referralReq.getReferringDepartment()))
        .sorted(Comparator.comparing(SIUInformation::getCreatedDate))
        .collect(Collectors.toList());

IntStream.range(0, sorted.size()).forEach(i -> sorted.get(i).setReferralSeqNumber(i));

answered Feb 14, 2022 at 14:20

MikeFHay

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WJS · Accepted Answer · 2022-02-14 15:57:23Z

0

You can't really set the sequence number until all is sorted. So imo, there is not much point in using streams. Based on existing information, I would just do the following:

userInformation.sort(Comparator
        .comparing(UserInformation::getCreatedDate));
// now assign the ids.
int id = 1;
for(UserInformation user : userInformation) {
    user.setUserSeqNumber(id++ +"");
}

answered Feb 14, 2022 at 15:57

WJS

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