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Collecting items from Multi#

There are cases where you want to accumulate the items from a Multi and produce a Uni as a final result. This is also called a reduction in functional programming.

The Uni fires its item when the Multi completes. Mutiny provides multiple operators to deal with that scenario. They are available from the collect() group. For example, you can store the items in a list, emit the list on completion, or use a Java Collector to customize the aggregation.

Danger

Don’t collect items from infinite streams or you will likely end with an out-of-memory failure!

Collecting items into a list#

One of the most common approaches to collect items is to store them in a list (Uni<List<T>>) It emits the final list when the Multi completes.

sequenceDiagram
    autonumber
    participant M as Multi
    participant O as Collect operator
    participant D as Subscriber

    M->>O: onItem(1)
    M->>O: onItem(2)
    M->>O: onItem(3)

    O->>D: onItem([1, 2, 3])

How to achieve this with Mutiny?

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Multi<String> multi = getMulti();
Uni<List<String>> uni = multi.collect().asList();

It’s important to note that the returned type is a Uni. It emits the list when the multi completes.

Collecting items into a map#

You can also collect the items into a Map. In this case, you need to provide a function to compute the key for each item:

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Multi<String> multi = getMulti();
Uni<Map<String, String>> uni =
        multi.collect()
                .asMap(item -> getUniqueKeyForItem(item));

If the key mapper function returns the same key for multiple items, the last one with that key is stored in the final Map. You can collect items in a multimap to handle items with the same keys.

Collecting items into a multimap#

A multimap is a Map<K, List<T>>. In the case of a conflicting key, it stores all the items in a list associated with that key.

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Multi<String> multi = getMulti();
Uni<Map<String, Collection<String>>> uni =
        multi.collect()
                .asMultiMap(item -> getKeyForItem(item));

Using a custom accumulator#

You can also use a custom accumulator function:

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Multi<String> multi = getMulti();
Uni<MyCollection> uni = multi.collect()
        .in(MyCollection::new, (col, item) -> col.add(item));

The in method receives two arguments:

  1. a supplier providing the new instance of your collection/container
  2. the accumulator function

You can also use a Java Collector. For example, in the next example, count the number of items, and produce the final count as item:

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Uni<Long> count = multi.collect()
        .with(Collectors.counting());

Getting the first and last items#

While they are not strictly speaking collecting items, collect().first() and collect().last() allow retrieving the first and last item from a Multi:

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Uni<String> first = multi.collect().first();
Uni<String> last = multi.collect().last();