使用Java 8 Stream API合并映射列表

Question

我正在尝试学习Java 8 Stream,当我尝试将一些函数转换为java8来练习时.我遇到了一个问题.

我很好奇如何将跟随代码转换为java流格式.

/*
 * input example:
 * [
    {
        "k1": { "kk1": 1, "kk2": 2},
        "k2": {"kk1": 3, "kk2": 4}
    }
    {
        "k1": { "kk1": 10, "kk2": 20},
        "k2": {"kk1": 30, "kk2": 40}
    }
  ]
 * output:
 * {
        "k1": { "kk1": 11, "kk2": 22},
        "k2": {"kk1": 33, "kk2": 44}
   }
 *
 *
 */
private static Map<String, Map<String, Long>> mergeMapsValue(List<Map<String, Map<String, Long>>> valueList) {
    Set<String> keys_1 = valueList.get(0).keySet();
    Set<String> keys_2 = valueList.get(0).entrySet().iterator().next().getValue().keySet();
    Map<String, Map<String, Long>> result = new HashMap<>();
    for (String k1: keys_1) {
        result.put(k1, new HashMap<>());
        for (String k2: keys_2) {
            long total = 0;
            for (Map<String, Map<String, Long>> mmap: valueList) {
                Map<String, Long> m = mmap.get(k1);
                if (m != null && m.get(k2) != null) {
                    total += m.get(k2);
                }
            }
            result.get(k1).put(k2, total);
        }
    }
    return result;
}

Answer 1

这里的技巧是正确收集内部地图.工作流程将是:

• 将地图列表平面映射List<Map<String, Map<String, Long>>>为地图条目流Stream<Map.Entry<String, Map<String, Long>>>.
• 按每个条目的键分组,并将映射到相同键的值合并,将两个映射合并在一起.

通过合并它们来收集地图将理想地保证flatMapping收集器,遗憾的是,它不存在于Java 8中,尽管它将存在于Java 9中(参见JDK-8071600).对于Java 8,可以使用StreamEx库提供的(并MoreCollectors.flatMapping在以下代码中使用).

private static Map<String, Map<String, Long>> mergeMapsValue(List<Map<String, Map<String, Long>>> valueList) {
    return valueList.stream()
                    .flatMap(e -> e.entrySet().stream())
                    .collect(Collectors.groupingBy(
                        Map.Entry::getKey,
                        Collectors.flatMapping(
                            e -> e.getValue().entrySet().stream(),
                            Collectors.<Map.Entry<String,Long>,String,Long>toMap(Map.Entry::getKey, Map.Entry::getValue, Long::sum)
                        )
                    ));
}

如果不使用这个方便的收集器,我们仍然可以使用等效的语义构建自己的收集器:

private static Map<String, Map<String, Long>> mergeMapsValue2(List<Map<String, Map<String, Long>>> valueList) {
    return valueList.stream()
                    .flatMap(e -> e.entrySet().stream())
                    .collect(Collectors.groupingBy(
                        Map.Entry::getKey,
                        Collector.of(
                            HashMap::new,
                            (r, t) -> t.getValue().forEach((k, v) -> r.merge(k, v, Long::sum)),
                            (r1, r2) -> { r2.forEach((k, v) -> r1.merge(k, v, Long::sum)); return r1; }
                        )
                    ));
}