Merge/aggregate objects within a list java

Question

I have a custom type which looks like this:

public class SampleObject {
    private UUID id;
    private List<ValueObject> values;
    
    // getters, all-args constructor, etc.
}

The ValueObject just contains some Strings.

And I have a List of SampleObjects payload. Within that list, there are several objects with the same id but different values in the List<ValueObject>.

What I want to archive is to merge all objects with the same id and add all the different lists of the ValueObjects in one merged object.

At the end, I need again a list of SampleObjects.

What I tried is the following:

List<SampleObject> payload = // initializing the List

payload.stream().collect(Collectors.groupingBy(SampleObject::getId));

But this returns a Map of List of SampleObjects. And I don't know how to merge then the objects under the value of this Map.

So you have a list of SampleObjects payload and you want to collect ValueObjects that are contained in the lists corresponding to the same id into a single List, and create a new SampleObject based on each aggregated List, right? And the result should be of type List<SampleObject> ? — Alexander Ivanchenko
– Alexander Ivanchenko, Commented Dec 16, 2022 at 15:52
You forget to provide the field name for List<ValueObject> in the SampleObject. — Alexander Ivanchenko
– Alexander Ivanchenko, Commented Dec 16, 2022 at 15:53
The ValueObject just contains some strings. Yes, within the List<SampleObject> are entities that have the same id but different ValueObject. As ValueObject is a list, I want to merge all enteries that have the same id to one SampleObject, this object should contain all ValueObjects of the duplicates. Thank you for your help! — noob123
– noob123, Commented Dec 16, 2022 at 16:22

Alexander Ivanchenko · Accepted Answer · 2022-12-16 16:31:04Z

1

`Collectors.flatMapping()`

In order to combine together and store into a single list all the ValueObjects that correspond to the same id you can use Collector flatMapping(), which expects a function that produces a Stream from the element (similarly to flatMap() operation) and a downstream Collector which tells how the new elements produced by the function need to be stored.

A combination of Collectors groupingBy() and flatMapping() would give you a map of type Map<UUID, List<ValueObject>>. To produce a List of SampleObjects out of it, you can create a stream map entries, transform each entry into a SampleObject and then collect the elements into a List.

That's how implementation might look like:

List<SampleObject> payload = // initializing the list
    
List<SampleObject> mergedData = payload.stream()
    .collect(Collectors.groupingBy( // intermediate Map<UUID, List<ValueObject>>
        SampleObject::getId,
        Collectors.flatMapping(
            sampleObject -> sampleObject.getValues().stream(),
            Collectors.toList()
        )
    ))
    .entrySet().stream()
    .map(entry -> new SampleObject(entry.getKey(), entry.getValue()))
    .toList(); // for Java 16+ or collect(Collectors.toList())

edited Dec 16, 2022 at 16:31

answered Dec 16, 2022 at 16:20

Alexander Ivanchenko

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1 Comment

noob123 Over a year ago

Thank you again for the answer but I have a question afterwards, does this change the order of the initial list?

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`Collectors.flatMapping()`

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