使用 Java8 Stream 从地图中查找最高值

问题描述

我编写了以下方法来查找映射到最高值的键并尝试转换为 java Streams.你能帮忙吗?

I wrote following method to find the keys mapped to the highest values and trying to convert to java Streams. Can you please help?

private List<Integer> testStreamMap(Map<Integer, Long> mapGroup) 
{
    List<Integer> listMax = new ArrayList<Integer>();
    Long frequency = 0L;
    for (Integer key : mapGroup.keySet()) {
        Long occurrence = mapGroup.get(key);
        if (occurrence > frequency) {
            listMax.clear();
            listMax.add(key);
            frequency = occurrence;
        } else if (occurrence == frequency) {
            listMax.add(key);
        }
    }
    return listMax;
}

推荐答案

您可以通过

Integer max=mapGroup.entrySet().stream().max(Map.Entry.comparingByValue()).get().getKey();

但不幸的是，没有内置函数可以获取所有等效的最大值.

but unfortunately, there is no built-in function for getting all equivalent maximums.

最简单、直接的解决方案是先找到最大值，然后检索映射到该值的所有键:

The simplest, straight-forward solution is to find the maximum value first and retrieve all keys mapping to that value afterwards:

private List<Integer> testStreamMap(Map<Integer, Long> mapGroup) {
    if(mapGroup.isEmpty())
        return Collections.emptyList();
    long max = mapGroup.values().stream().max(Comparator.naturalOrder()).get();
    return mapGroup.entrySet().stream()
        .filter(e -> e.getValue() == max)
        .map(Map.Entry::getKey)
        .collect(Collectors.toList());
}

在如何强制 max() 返回 ALL 中讨论了在单次通过中获取流的所有最大值的解决方案Java Stream 中的最大值?".如果您的输入是一个普通的Map(例如HashMap)，可以廉价地迭代多次.

Solutions for getting all maximum values of a stream in a single pass, are discussed in "How to force max() to return ALL maximum values in a Java Stream?". You will see that single-pass solutions are much more complicated and not worth the effort if your input is an ordinary Map (e.g. HashMap), which can be iterated multiple times cheaply.

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