将列表转换为地图 - foreach或stream？

Question

I want to transform a List<String> into a Map<String, Integer> . The lists' values should serve as keys, and the map value will be statically initialized.

I want to keep the order of the list, thus using a LinkedHashMap , and ignore duplicates.

Question: is there any advantage using the java8 stream API? Because, comparing the following, simply regarding the syntax I'd never go for the verbose stream api, but always sticking to the old foreach loop:

    //foreach
    Map<String, Integer> map = new LinkedHashMap<>();
    for (String k : abk) {
        if (k != null) map.put(k, STATIC_VAL);
    }

    //streams
    Map<String, Integer> map2 = abk.stream()
            .filter(Objects::nonNull)
            .collect(Collectors.toMap(
                        Function.identity(),
                        k -> STATIC_VAL,
                        (v1, v2) -> v1,
                        LinkedHashMap::new));

Would you stick to the old approach or go with new lambda expressions?

Answer 1

For this simple case I would stick to the for loop. It is faster too by the way (besides being more readable), Stream s do have a cost of the infrastructure that has to be used. This new API is just a tool and as every tool is has pros/cons.

Generally I tent to used stream api only when I have to - let's say that the List needs filtered or mapped to something else, etc - then the Stream usage would make sense. If there are multiple operations that have to be done, it would make code way more readable then doing the same thing with a loop; but that's not always the case - as in your example.

Parallelization (in case you really need it is another aspect). Simple for loops are not easily parallelized, but with streams this is a breeze.

Answer 2

There's a variant on the traditional for loop that you can use as well:

Map<String, Integer> map = new LinkedHashMap<>();
abk.forEach(k -> if (k != null) map.put(k, STATIC_VAL));

But it's just a matter of style. Either this way or the traditional for loop looks good to me. Using streams, on the other hand, is more verbose and less readable, IMO.

Answer 3

You could write the for-each with stream. eg.

abk.stream()
    .filter(Object::notNull)
    .forEach(s->map.put(s, STATIC_VAL));