Transform a flat list to domain objects with child objects using java streams

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I have incoming objects with a flat de-normalized structure which I instantiated from a JDBC resultset. The incoming objects mirror the resultset, there's loads of repeated data so I want to convert the data into a list of parent objects with nested child collections, i.e. an object graph, or normalized list.

The incoming object's class looks like this:

class IncomingFlatItem {
    String clientCode;
    String clientName;
    String emailAddress;
    boolean emailHtml;
    String reportCode;
    String reportLanguage;
}

So the incoming data contains multiple objects for each client, which I'd like to aggregate into one client object, which contains a list of email address objects for the client, and a list of report objects.

So the Client object would look like this:

class Client {
    String clientCode;
    String clientName;
    Set<EmailAddress> emailAddresses;
    Set<Report> reports;
}

Strangely I can't find an existing answer for this. I am looking at nesting streams or chaining streams but I'd like to find the most elegant approach and I definitely want to avoid a for-loop.

java-stream java
5 Answers

One thing you can do is use constructor parameters and a fluent API to your advantage. Thinking "nested" flows and the stream API (with dynamic data) can get complex very quickly.

This just uses a fluent API to simplify things (you can use a proper builder pattern instead)

class Client {
    String clientCode;
    String clientName;
    Set<EmailAddress> emailAddresses = new HashSet<>();
    Set<Report> reports = new HashSet<>();

    public Client(String clientCode, String clientName) {
        super();
        this.clientCode = clientCode;
        this.clientName = clientName;
    }

    public Client emailAddresses(String address, boolean html) {
        this.emailAddresses = 
             Collections.singleton(new EmailAddress(address, html));
        return this;
    }

    public Client reports(String... reports) {
        this.reports = Arrays.stream(reports)
                        .map(Report::new)
                        .collect(Collectors.toSet());
        return this;
    }

    public Client merge(Client other) {
        this.emailAddresses.addAll(other.emailAddresses);
        this.reports.addAll(other.reports);

        if (null == this.clientName)
            this.clientName = other.clientName;
        if (null == this.clientCode)
            this.clientCode = other.clientCode;

        return this;
    }
}

class EmailAddress {
    public EmailAddress(String e, boolean html) {

    }
}

class Report {
    public Report(String r) {

    }
}

And...

Collection<Client> clients = incomingFlatItemsCollection.stream()
        .map(flatItem -> new Client(flatItem.clientCode, flatItem.clientName)
                          .emailAddresses(flatItem.emailAddress, flatItem.emailHtml)
                          .reports(flatItem.reportCode, flatItem.reportLanguage))
        .collect(Collectors.groupingBy(Client::getClientCode,
                Collectors.reducing(new Client(null, null), Client::merge)))
        .values();

Or you can also just use mapping functions that convert IncomingFlatItem objects to Client.

user_57615580

You can do something on the lines of using mapping function to convert List<IncomingFlatItem> to Set<Reports/EmailAddress> as:

Function<List<IncomingFlatItem>, Set<EmailAddress>> inferEmailAddress =
        incomingFlatItems -> incomingFlatItems.stream()
                .map(obj -> new EmailAddress(obj.getEmailAddress(), 
                                             obj.isEmailHtml()))
                .collect(Collectors.toSet());

Function<List<IncomingFlatItem>, Set<Report>> inferReports =
        incomingFlatItems -> incomingFlatItems.stream()
                .map(obj -> new Report(obj.getReportCode(), 
                                       obj.getReportLanguage()))
                .collect(Collectors.toSet());

and further using groupingBy and mapping the entries to List<Client> as:

List<Client> transformIntoGroupedNormalisedContent(
                  List<IncomingFlatItem> incomingFlatItemList) {
    return incomingFlatItemList.stream()
            .collect(Collectors.groupingBy(inc ->
                    Arrays.asList(inc.getClientCode(), inc.getClientName())))
            .entrySet()
            .stream()
            .map(e -> new Client(e.getKey().get(0), 
                                 e.getKey().get(1),
                                 inferEmailAddress.apply(e.getValue()), 
                                 inferReports.apply(e.getValue())))
            .collect(Collectors.toList());
}
user_17461180

You can use this:

List<Client> clients = items.stream()
        .collect(Collectors.groupingBy(i -> Arrays.asList(i.getClientCode(), i.getClientName())))
        .entrySet().stream()
        .map(e -> new Client(e.getKey().get(0), e.getKey().get(1),
                e.getValue().stream().map(i -> new EmailAddress(i.getEmailAddress(), i.isEmailHtml())).collect(Collectors.toSet()),
                e.getValue().stream().map(i -> new Report(i.getReportCode(), i.getReportLanguage())).collect(Collectors.toSet())))
        .collect(Collectors.toList());

At the beginning you group your items by clientCode and clientName. After that you map the results to your Client object.

Make sure the .equals() and hashCode() methods are implemented for EmailAddress and Report to ensure they are distinct in the set.

user_96626010

Thanks to all the answerers who mentioned Collectors.groupingBy(). This was key to setting up a stream where I could use reduce(). I had erroneously believed I should be able to use reduce on its own to solve the problem, without groupingBy.

Thanks also to the suggestion to create a fluent API. I added IncomingFlatItem.getEmailAddress() and IncomingFlatItem.getReport() to fluently grab the domain objects from IncomingFlatItem - and also a method to convert the whole flat item to a proper domain object with its email and report nested already:

public Client getClient() {
    Client client = new Client();
    client.setClientCode(clientCode);
    client.setClientName(clientName);
    client.setEmailAddresses(new ArrayList());
    client.getEmailAddresses().add(this.getEmailAddress());
    client.setReports(new ArrayList<>());
    client.getReports().add(this.getReport());
    return client;
}

I also created business ID-based .equals() and .hashCode() methods on Client, EmailAddress and Report as recommended by @SamuelPhilip

Lastly for the domain objects, I created .addReport(Report r) and .addEmail(EmailAddress e) on my Client class, which would add the child object to Client if not already present. I ditched the Set collection type for List because the domain model standard is List and Sets would have meant lots of conversions to Lists.

So with that, the stream code and lambdas look succinct.

There are 3 steps:

  1. map IncomingFlatItems to Clients
  2. group the Clients into a map by client (relying heavily on Client.equals())
  3. reduce each group to one Client

So this is the functional algorithm:

List<Client> unflatten(List<IncomingFlatItem> flatItems) {
    return flatItems.parallelStream()
            .map(IncomingFlatItem::getClient)
            .collect(Collectors.groupingByConcurrent(client -> client))
            .entrySet().parallelStream()
            .map(kvp -> kvp.getValue()
                    .stream()
                    .reduce(new Client(), 
                            (client1, client2) -> {
                                    client1.getReports()
                                            .forEach(client2::addReport);
                                    client1.getEmailAddresses()
                                            .forEach(client2::addEmail);
                                    return client2;
                    }))
            .collect(Collectors.toList());
}

I took a long time due to going off on a tangent before I really understood reduce - I found a solution which passed my tests while using .stream() but totally failed with .parallelStream() hence its usage here. I had to use CopyOnWriteArrayList as well otherwise it would fall over randomly with ConcurrentModificationExceptions

user_13851740

If you don't like to iterate over entry sets (don't want to handle Map.Entry) or prefer a different solution without groupingBy, you can also use toMap with a merge function to aggregate your values. This approach works nicely because Client can hold the initial single item and the accumulated collection of all EmailAddress (Note: I used a utility function com.google.common.collectSets.union for conciseness, but you can just work with e.g. HashSet).

The following code demonstrates how to do it (add Reports in the same manner as EmailAddress, and add the other fields you want). I left the merge function inline and did not add an AllArgsConstructor, but feel free to refactor.

static Client mapFlatItemToClient(final IncomingFlatItem item) {
    final Client client = new Client();
    client.clientCode = item.clientCode;
    client.emailAddresses = Collections.singleton(mapFlatItemToEmail(item));
    return client;
}

static EmailAddress mapFlatItemToEmail(final IncomingFlatItem item) {
    final EmailAddress address = new EmailAddress();
    address.emailAddress = item.emailAddress;
    return address;
}

public static void example() {
    final List<IncomingFlatItem> items = new ArrayList<>();

    // Aggregated Client Info by Client Code
    final Map<String, Client> intermediateResult = items.stream()
            .collect(
                    Collectors.<IncomingFlatItem, String, Client> toMap(
                            flat -> flat.clientCode,
                            flat -> mapFlatItemToClient(flat),
                            (lhs, rhs) -> {
                                final Client client = new Client();
                                client.clientCode = lhs.clientCode;
                                client.emailAddresses = Sets.union(lhs.emailAddresses, rhs.emailAddresses);
                                return client;
                            }));

    final Collection<Client> aggregatedValues = intermediateResult.values();
}
user_64733980
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