java8转换字符串数组来映射（奇数索引是键，偶数索引是值）

Question

Now I have a String array,

String[] a= {"from","a@a.com","to","b@b.com","subject","hello b"};

from command line arguments.

I want to convert it to Map,

{"from":"a@a.com","to":"b@b.com","subject":"hello b"}

Does exist convenient manner in java8 to achieve this? Now my way is

Map<String,String> map = new HashMap<>();
for (int i = 0; i < args.length; i+=2) {
    String key = args[i].replaceFirst("-+", ""); //-from --> from
    map.put(key, args[i+1]);
}

Answer 1

You can use an IntStream to iterate on the indices of the array (which is required in order to process two elements of the array each time) and use the Collectors.toMap collector.

The IntStream will contain a corresponding index for each pair of elements of the input array. If the length of the array is odd, the last element will be ignored.

Map<String,String> map = 
    IntStream.range(0,a.length/2)
             .boxed()
             .collect(Collectors.toMap(i->a[2*i].replaceFirst("-+", ""),
                                       i->a[2*i+1]));

Answer 2

In Java 8 you can use Stream.iterate to divide list to sublists of 2 elements

    String[] a= {"from","a@a.com","to","b@b.com","subject","hello b"};

    Map<String, String> map = Stream.iterate(
        Arrays.asList(a), l -> l.subList(2, l.size()))
            .limit(a.length / 2)
            .collect(Collectors.toMap(
                l -> l.get(0).replaceFirst("-+", ""),
                l -> l.get(1))
            );

Another recursive solution using simple Iterator

Map<String, String> map = buildMap(new HashMap<>(), Arrays.asList(a).iterator());

private static Map<String, String> buildMap(
        Map<String, String> map, Iterator<String> iterator) {
    if (iterator.hasNext()) {
        map.put(iterator.next().replaceFirst("-+", ""), iterator.next());
        createMap(map, iterator);
    }
    return map;
}

Answer 3

You can do this quite elegant with Javaslang :

String[] a= {"from","a@a.com","to","b@b.com","subject","hello b"};

Map<String, String> map = Stream.of(a).grouped(2) // use javaslang.collection.Stream here
  .map(group -> group.toJavaArray(String.class))
  .toJavaStream() // this is the plain old java.util.Stream
  .collect(toMap(tuple -> tuple[0], tuple -> tuple[1]));

The grouped function groups your stream in groups of 2 elements. These can be transformed to string arrays and those can be the base of a Map . Probably Javaslang allows you to do even more elegant.

Answer 4

The trick is to first join the string, then split it up into key/value pairs , then the task is simple:

Map<String,String> map = Arrays.stream(
    String.join(",", a)
          .split(",(?!(([^,]*,){2})*[^,]*$)"))
          .map(s -> s.split(","))
          .collect(Collectors.toMap(s -> s[0], s -> s[1]))
          ;