Assign unique IDs to objects using Java 8 streams

Question

static <T> Map<T, Integer> assignIds(Collection<T> objects);

I want to write a function that takes a collection of unique objects and assigns a different ID number to each. The ID numbers should be assigned sequentially.

I could easily do this with an explicit loop like:

Map<T, Integer> ids = new HashMap<>();
int id = 0;

for (T object: objects) {
    ids.put(object, id++);
}

Is there an elegant way to do this with the new Java 8 Stream API?

Answer 1

Here's one way:

static <T> Map<T, Integer> assignIds(Collection<T> objects) {
    AtomicInteger ai = new AtomicInteger();
    return objects.stream()
                  .collect(Collectors.toMap(o -> o, o -> ai.getAndIncrement()));
}

The above solution could also make use of parallelStream() instead of stream().

Here's another that works sequentially:

static <T> Map<T, Integer> assignIds(Collection<T> objects) {
    Map<T, Integer> result = new HashMap<>();
    objects.stream().forEachOrdered(o -> result.put(o, result.size()));        
    return result;
}

Building upon ZouZou's answer...

static <T> Map<T, Integer> assignIds(Collection<T> objects) {
    OfInt ids = IntStream.range(0, objects.size()).iterator();
    return objects.stream().collect(Collectors.toMap(o -> o, o -> ids.next()));
}

The idiomatic way to do this in for instance Scala would be to use zipWithIndex. There's no such method in the Java 8 Streams API, not even a zip method which you could combine with an IntStream.