如何创建嵌套地图映射＆lt; string，map＆lt; string，string＆gt;＆gt;与流API

发布于 2025-01-24 14:04:49 字数 771 浏览 0 评论 0原文

如何编写逻辑以获取map＆lt; string，map＆lt; string，string＆gt;＆gt;使用流？

这是我尝试过的代码：

public Map<String, Map<String, String>> getData(Set<String> Ids) {
    List<Events> events = repository.findAllByIdIn(Ids);

    int i = 0;
    return events.stream()
            .sorted(Comparator.comparing(Events::getMajor).thenComparing(Events::getMinor))
            .collect(Collectors.groupingBy(Events::getId, 
    //Error-Cannot resolve method 'getMajor()'
    Collectors.mapping(cae -> cae.getMajor() + "." + cae.getMinor(), 
        cae.getDates().get(i).getEndDate() != null ? "Predicted" : "Not Predicted"), 
             Collectors.toMap())));//Error-Cannot resolve method 'toMap()'
}

那么如何在另一个地图内循环循环？

原文

How can I write the logic to obtain a Map<String, Map<String, String>> using streams?

Here is the code I have tried :

public Map<String, Map<String, String>> getData(Set<String> Ids) {
    List<Events> events = repository.findAllByIdIn(Ids);

    int i = 0;
    return events.stream()
            .sorted(Comparator.comparing(Events::getMajor).thenComparing(Events::getMinor))
            .collect(Collectors.groupingBy(Events::getId, 
    //Error-Cannot resolve method 'getMajor()'
    Collectors.mapping(cae -> cae.getMajor() + "." + cae.getMinor(), 
        cae.getDates().get(i).getEndDate() != null ? "Predicted" : "Not Predicted"), 
             Collectors.toMap())));//Error-Cannot resolve method 'toMap()'
}

So how do I loop through a map inside another map?

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自控 2025-01-31 14:04:49

collector tomap（） 与收集者tolist（tolist> tolist（）和（） 不是参数，您需要提供信息，如何从单个流元素中提取键和值。

collectors.tomap（）期望至少两个 functions ：keymapper和valueemapper。如果可以使用tomap，则需要使用MergeFunction作为第三个参数来解决在同一密钥上碰撞的值。

因此，您需要将tomap（）作为第二个参数（ downstream collector ）放入groupingby中。这将是 collectors 和 functions 的句法正确的结构：

Collectors.groupingBy(classifier, Collectors.toMap(keyMapper, valueMapper))

以及代码的外观：

return events.stream()
        .sorted(Comparator.comparing(Events::getMajor)
            .thenComparing(Events::getMinor))
        .collect(Collectors.groupingBy(Events::getId,
            Collectors.toMap(cae -> cae.getMajor() + "." + cae.getMinor(),
                cae -> cae.getDates().get(i).getEndDate() != null ? 
                    "Predicted" : "Not Predicted")));

Note note ，collectors。映射（）并不是要产生地图，而是在变换流中的元素，您可以将其视为MAP（）操作未完成的操作是流管线的中间，而是在收集数据的过程中。

而且，如果我正确理解您的逻辑，则无需在此处应用映射（）。

Collector toMap() contrary to collectors toList() and toSet() isn't parameterless, you need to provide information how a key and a value should be extracted from a single stream element.

Collectors.toMap() expects at least two functions: keyMapper and valueMapper. In case if there could be duplicates, you need to use the flavor of toMap which takes a mergeFunction as a third argument to resolve the values that collide on the same key.

So you need to place toMap() as a second argument (downstream collector) into the groupingBy. That will be a syntactically correct structure of the collectors and functions:

Collectors.groupingBy(classifier, Collectors.toMap(keyMapper, valueMapper))

And that how the code might look like:

return events.stream()
        .sorted(Comparator.comparing(Events::getMajor)
            .thenComparing(Events::getMinor))
        .collect(Collectors.groupingBy(Events::getId,
            Collectors.toMap(cae -> cae.getMajor() + "." + cae.getMinor(),
                cae -> cae.getDates().get(i).getEndDate() != null ? 
                    "Predicted" : "Not Predicted")));

Note, Collectors.mapping() isn't meant to produce a Map but to transform the elements of the stream, you can think of it as an analog of the map() operation that is done not is the middle of the stream pipeline, but during the process of collecting the data.

And if I understood your logic correctly, there's no need to apply mapping() here.

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