Java 按键缩小地图

Question

我有一个配对列表如下：

// pairs = [(k1, obj1), (k2, obj1)]
List<Pair<String, Object> pairs;

然后我想把那些对象展开成很多对象，然后key缩减

pairs.stream().map(pair -> {
                // every object is expanded into many objects, obj11, obj12, ...
                List<Object> objects = expand(pair.getRight());
                // the return is [(k1, obj11), (k1, obj12), (k2, obj21), (k2, obj22)]
                return objects.stream().map(object -> new MutablePair<String, Object>(pair.getLeft(), object)).collect(Collectors.toList());
            }).reduce(...
                // how to reduce it by key and get a list of pairs 
            ...);

我的问题是，如何对展开的对象进行key reduce，得到pairs列表？

我的意思是预期结果是：

pairs = [(k1, obj11), (k1, obj12), (k2, obj21), (k2, obj22)]

Answer 1

看起来你需要 flatMap 而不是 map，collect 而不是 reduce:

List<Pair<String, Object>> expandedPairs =
    pairs.stream()
         .flatMap(pair -> expand(pair.getRight()).stream()
                                                 .map(object -> new MutablePair<String, Object>(pair.getLeft(), object)))
         .collect(Collectors.toList());

Java 按键缩小地图

Java Map Reduce By Key

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