How to convert List to Map with indexes using stream - Java 8?

It is better to use Function.identity() in place of i->i because as per answer for this question:

As of the current JRE implementation, Function.identity() will always return the same instance while each occurrence of identifier -> identifier will not only create its own instance but even have a distinct implementation class.

IntStream.range(0, alphabet.size())
         .boxed()
         .collect(toMap(alphabet::get, Function.identity()));

Avoid stateful index counters like the AtomicInteger-based solutions presented in other answers. They will fail if the stream were parallel. Instead, stream over indexes:

IntStream.range(0, alphabet.size())
         .boxed()
         .collect(toMap(alphabet::get, i -> i));

Above assumes that the incoming list is not supposed to have duplicate characters since it's an alphabet. If you have possibility of duplicate elements then multiple elements will map to same key and then you need to specify merge function. For example you can use (a,b) -> b or (a,b) ->a as the third parameter to toMap method.

You can collect the stream to a map and use the map size as index.

alphabet.stream()
    .collect(HashMap::new, (map, ch) -> map.put(ch, map.size()), Map::putAll);

Using streams with AtomicInteger in Java 8:

private Map<Character, Integer> numerateAlphabet(List<Character> alphabet) {
    AtomicInteger index = new AtomicInteger();
    return alphabet.stream().collect(
            Collectors.toMap(s -> s, s -> index.getAndIncrement(), (oldV, newV)->newV));
}