Idiomatically enumerating a Stream of objects in Java 8

Question

How can one idiomatically enumerate a Stream<T> which maps each T instance to a unique integer using Java 8 stream methods (e.g. for an array T[] values, creating a Map<T,Integer> where Map.get(values[i]) == i evaluates to true)?

Currently, I'm defining an anonymous class which increments an int field for use with the Collectors.toMap(..) method:

private static <T> Map<T, Integer> createIdMap(final Stream<T> values) {
    return values.collect(Collectors.toMap(Function.identity(), new Function<T, Integer>() {

        private int nextId = 0;

        @Override
        public Integer apply(final T t) {
            return nextId++;
        }

    }));
}

However, is there not a more concise/elegant way of doing this using the Java 8 stream API? — bonus points if it can be safely parallelized.

Answer 1

Your approach will fail, if there is a duplicate element.

Besides that, your task requires mutable state, hence, can be solved with Mutable reduction. When we populate a map, we can simple use the map’s size to get an unused id.

The trickier part is the merge operation. The following operation simply repeats the assignments for the right map, which will handle potential duplicates.

private static <T> Map<T, Integer> createIdMap(Stream<T> values) {
    return values.collect(HashMap::new, (m,t) -> m.putIfAbsent(t,m.size()),
        (m1,m2) -> {
            if(m1.isEmpty()) m1.putAll(m2);
            else m2.keySet().forEach(t -> m1.putIfAbsent(t, m1.size()));
        });
}

If we rely on unique elements, or insert an explicit distinct(), we can use

private static <T> Map<T, Integer> createIdMap(Stream<T> values) {
    return values.distinct().collect(HashMap::new, (m,t) -> m.put(t,m.size()),
        (m1,m2) -> { int leftSize=m1.size();
            if(leftSize==0) m1.putAll(m2);
            else m2.forEach((t,id) -> m1.put(t, leftSize+id));
        });

}