将列表转换为从列表中的唯一值的地图，用键作为单个元素

发布于 2025-02-05 20:13:23 字数 8 浏览 2 评论 0原文

continue

I have a list as below -

List<List<String>> originalList = 
    List.of(List.of("C1", "C2", "C3"), List.of("C2", "C3", "C1"));

I am collecting unique elements across lists and arrived at -

Set<String> uniqueValues = originalList.stream()
    .flatMap(Collection::stream)
    .collect(Collectors.toSet());

Now I'm trying to create a map which will look like below -

{C3=[C1, C2], C1=[C3, C2], C2=[C3, C1]}

I have the below snippet -

Map<String, Set<String>> mymap = uniqueValues
            .stream()
            .collect(Collectors.toMap(Function.identity(),
                    value -> uniqueValues, (left, right) -> {
                        left.removeAll(right);
                        return left;
                    }));

which is giving me -

{C3=[C3, C1, C2], C1=[C3, C1, C2], C2=[C3, C1, C2]}

What am I doing wrong ?

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层林尽染 2025-02-12 20:13:23

基本上，您的目标是创建一个地图，每个条目代表基于Set uniqueValues生成的组合。

显然，每种组合都是不同的，这意味着它需要自己的set。 IE而不是为每个 key 提供unique> uniquevalues并从中删除当前键。

另一个错误是，您不需要tomap（）收集器的味道，它期望三个参数，因为MergeFunction（第三个参数）旨在解决重复项，但会在其中解决。由于其源为set，因此在流中没有重复。

为此，我们需要一个版本的 tomap 只期望keymapper（（ 一个从流元素）和valueMapper（ 函数负责从流元素生成值的函数）的函数。

That's how it can be done:

Map<String, Set<String>> myMap = uniqueValues
    .stream()
    .collect(Collectors.toMap(
        Function.identity(),
        str -> {
            Set<String> value = new HashSet<>(uniqueValues);
            value.remove(str);
            return value;
        }
    ));

System.out.println(myMap);

Output:

{C3=[C1, C2], C1=[C3, C2], C2=[C3, C1]}

Basically, your goal is to create a map where each entry represents a combination generated based on a set uniqueValues.

And obviously each combination is distinct, which means it requires its own Set. I.e. instead of providing the uniqueValues as a value for every key, you need to generate a new HashSet based on the uniqueValues and remove the current key from it.

Another mistake is that you don't need a flavor of toMap() collector that expects three arguments because mergeFunction (the third argument) is meant to resolve duplicates, but there would be no duplicates in the stream since its source is a Set.

To achieve this we need a version of toMap that expects only a keyMapper (a function which produces a key from a stream element) and valueMapper (a function responsible for generating a value from a stream element).

That's how it can be done:

Map<String, Set<String>> myMap = uniqueValues
    .stream()
    .collect(Collectors.toMap(
        Function.identity(),
        str -> {
            Set<String> value = new HashSet<>(uniqueValues);
            value.remove(str);
            return value;
        }
    ));

System.out.println(myMap);

Output: