如何通过流java8中的键获取所有不同的值 [英] How to get all the distinct values by key inside stream java8

问题描述

我目前正在学习一些有关流的知识. 我具有以下JSONArray，并且希望能够检索所有不同的xvalue.

I am currently learning a bit about streams. I have the following JSONArray, and I want to be able to retrieve all the distinct xvalues.

 datasets: {
    ds1: {
     xvalues: [
        "(empty)",
        "x1",
        "x2"
      ]
    },
    ds2: {
    xvalues: [
        "(empty)",
        "x1",
        "x2",
        "x3"
      ]
    }
}

我正在尝试以下代码，但似乎不太正确....

I am trying the following code but it doesn't seem quite right....

List<String> xvalues = arrayToStream(datasets)
                .map(JSONObject.class::cast)
                .map(dataset -> {
                    try {
                         return dataset.getJSONArray("xvalues");
                    } catch (JSONException ex) {

                    }
                    return;
            })
            .distinct()
            .collect((Collectors.toList()));

private static Stream<Object> arrayToStream(JSONArray array) {
    return StreamSupport.stream(array.spliterator(), false);
}

推荐答案

我相信使用json库(Jackson，Gson)是处理json数据的最佳方法.如果可以使用Jackson，这可能是一个解决方案

I believe using a json library(Jackson, Gson) is the best way to deal with json data. If you can use Jackson, this could be a solution

public class DataSetsWrapper {   
    private Map<String, XValue> datasets;
    //Getters, Setters
}

public class XValue {
    private List<String> xvalues;
    //Getters, Setters
}  



ObjectMapper objectMapper = new ObjectMapper();
DataSetsWrapper dataSetsWrapper = objectMapper.readValue(jsonString, DataSetsWrapper.class);

List<String> distinctXValues = dataSetsWrapper.getDatasets()
                .values()
                .stream()
                .map(XValue::getXvalues)
                .flatMap(Collection::stream)
                .distinct()
                .collect(Collectors.toList()); 

将jsonString替换为您的json.我用json

Replace jsonString with your json. I tested this with this json

String jsonString = "{\"datasets\": {\n" +
        "    \"ds1\": {\n" +
        "     \"xvalues\": [\n" +
        "        \"(empty)\",\n" +
        "        \"x1\",\n" +
        "        \"x2\"\n" +
        "      ]\n" +
        "    },\n" +
        "    \"ds2\": {\n" +
        "    \"xvalues\": [\n" +
        "        \"(empty)\",\n" +
        "        \"x1\",\n" +
        "        \"x2\",\n" +
        "        \"x3\"\n" +
        "      ]\n" +
        "    }\n" +
        "}}";

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