How to flatten and group this HashMap, using streams? [closed]

问题

Given mapping of letters to numbers, I would like to return a list of Strings, where each String is a comma delimited list of the letters grouped by their associated number.

For this map

    Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("A", 1);
    map.put("B", 2);
    map.put("C", 4);
    map.put("D", 1);
    map.put("E", 1);
    map.put("F", 2);

I would like to return a List containing:

"A,D,E" "B,F", "C"

Any suggestions how this can be accomplished using the 1.8 streaming functions?

回答1:

This way doesn't reference map after it's initially streamed, and makes maximal use of the streaming facilities:

return map.entrySet().stream()
    .collect(Collectors.groupingBy(
        Map.Entry::getValue,
        Collectors.mapping(Map.Entry::getKey, Collectors.joining(","))))
    .values().stream().collect(Collectors.toList());

Or more concisely, but with less usage of streams (thanks @Kartik):

return new ArrayList<>(map.entrySet().stream()
    .collect(Collectors.groupingBy(
        Map.Entry::getValue,
        Collectors.mapping(Map.Entry::getKey, Collectors.joining(","))))
    .values());

In either of those, if you add TreeMap::new as an argument between the two existing arguments to Collectors.groupingBy, the "inside" pieces will be sorted.

回答2:

You can first group entries by value, and then use Collectors.joining(","):

List<String> result = map.entrySet().stream()
        .collect(Collectors.groupingBy(Map.Entry::getValue))
        .values().stream()
        .map(e -> e.stream()
                .map(Map.Entry::getKey)
                .collect(Collectors.joining(",")))
        .collect(Collectors.toList());

回答3:

Here's my attempt

List<String> results = map
    .entrySet()
    .stream()
    .collect(
        Collectors.groupingBy(
            Map.Entry::getValue,
            Collectors.mapping(
                Map.Entry::getKey,
                Collectors.toList())))
    .values()
    .stream()
    .map(letters -> String.join(",", letters))
    .collect(Collectors.toList());

来源：https://stackoverflow.com/questions/55055555/how-to-flatten-and-group-this-hashmap-using-streams

标签

java

java-8

java-stream