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

问题

I've created method whih numerating each character of alphabet. I'm learning streams(functional programming) and try to use them as often as possible, but I don't know how to do it in this case:

private Map<Character, Integer> numerateAlphabet(List<Character> alphabet) {
    Map<Character, Integer> m = new HashMap<>();
    for (int i = 0; i < alphabet.size(); i++)
        m.put(alphabet.get(i), i);
    return m;
}

So, how to rewrite it using streams of Java 8?

回答1:

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.

回答2:

It is better to use Function.identity() in place of i->i :

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

回答3:

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));
}

回答4:

using AtomicInteger

    AtomicInteger counter = new AtomicInteger();
    Map<Character, Integer> map = characters.stream()
            .collect(Collectors.toMap((c) -> c, (c) -> counter.incrementAndGet()));
    System.out.println(map);

来源：https://stackoverflow.com/questions/33138577/how-to-convert-list-to-map-with-indexes-using-stream-java-8