Streams use for map computation from list with counter

Question

I have the following for loop which I would like to replace by a simple Java 8 stream statement:

List<String> words = new ArrayList<>("a", "b", "c");
Map<String, Long> wordToNumber = new LinkedHashMap<>();
Long index = 1L;

for (String word : words) {
  wordToNumber.put(word, index++);
}

I basically want a sorted map (by insertion order) of each word to its number (which is incremented at each for loop by 1), but done simpler, if possible with Java 8 streams.

Answer 1

The following should work (though it's not clear why Long is needed because the size of List is int)

Map<String, Long> map = IntStream.range(0, words.size())
    .boxed().collect(Collectors.toMap(words::get, Long::valueOf));

The code above works if there's no duplicate in the words list.

If duplicate words are possible, a merge function needs to be provided to select which index should be stored in the map (first or last)

Map<String, Long> map = IntStream.range(0, words.size())
    .boxed().collect(
        Collectors.toMap(words::get, Long::valueOf, 
        (w1, w2) -> w2, // keep the index of the last word as in the initial code
        LinkedHashMap::new // keep insertion order
    ));

---

Similarly, the map can be built by streaming words and using external variable to increment the index (AtomicLong and getAndIncrement() may be used instead of long[]):

long[] index = {1L};
Map<String, Long> map = words.stream()
    .collect(
        Collectors.toMap(word -> word, word -> index[0]++, 
        (w1, w2) -> w2, // keep the index of the last word
        LinkedHashMap::new // keep insertion order
    ));