如何在Java 8中拼合地图列表 [英] How to flatten List of Maps in java 8

问题描述

我有地图列表的要求

[{Men=1},{Men=2, Women=3},{Women=2,Boys=4}]

现在我需要将其制作为一个flatMap，使其看起来像

Now I need make it a flatMap such that it looks like

Gender=countOfValues

在上面的示例中，输出为

In the above example the output would be

{Men=3,Women=5,Boys=4}

当前，我有以下代码:

private Map<String, Long> getGenderMap(
        List<Map<String, Long>> genderToCountList) {
    Map<String, Long> gendersMap = new HashMap<String, Long>();
    Iterator<Map<String, Long>> genderToCountListIterator = genderToCountList
            .iterator();
    while (genderToCountListIterator.hasNext()) {
        Map<String, Long> genderToCount = genderToCountListIterator.next();
        Iterator<String> genderToCountIterator = genderToCount.keySet()
                .iterator();
        while (genderToCountIterator.hasNext()) {
            String gender = genderToCountIterator.next();
            if (gendersMap.containsKey(gender)) {
                Long count = gendersMap.get(gender);
                gendersMap.put(gender, count + genderToCount.get(gender));
            } else {
                gendersMap.put(gender, genderToCount.get(gender));
            }
        }
    }
    return gendersMap;
}

我们如何使用Java8使用Lambda表达式编写这段代码?

How do we write this piece of code using Java8 using lambda expressions?

推荐答案

我不会为此使用任何lambda，但是我使用了Java 8中引入的Map.merge和方法引用.

I wouldn't use any lambdas for this, but I have used Map.merge and a method reference, both introduced in Java 8.

Map<String, Long> result = new HashMap<>();
for (Map<String, Long> map : genderToCountList)
    for (Map.Entry<String, Long> entry : map.entrySet())
        result.merge(entry.getKey(), entry.getValue(), Long::sum);

您也可以使用Stream s执行此操作:

You can also do this with Streams:

return genderToCountList.stream().flatMap(m -> m.entrySet().stream())
           .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Long::sum));

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