java 8将具有相同键和值的映射分组，并生成一个键-值对

Question

我有一个方法fetchConfigScont（），它返回列表

结果如下：

                    {
                        "web_id": "abc",
                        "count": 1
                    },
                    {
                        "web_id": "xyz",
                        "count": 1
                    },
                    {
                        "web_id": "pqr",
                        "count": 1
                    }

我们还有另一个方法fetchWebSiteScont（），它也会以相同的格式返回响应。

web_id是键，它的值是所有者名。count是另一个键，它的值是类型int。

                    {
                        "web_id": "abc",
                        "count": 2
                    },
                    {
                        "web_id": "xyz",
                        "count": 3
                    },
                    {
                        "web_id": "pqr",
                        "count": 4
                    }

我们有一个ArrayList类型的所有者列表

List<String> listOfOwners = ["abc","xyz","pqr"];

我想收集所有这些信息，并创建如下结果：

result:[
    {"owner": "abc",
    "ownerPresence":{
    "configCount":1,
    "websiteCount":2
    },
    {"owner": "xyz",
    "ownerPresence":{
    "configCount":1,
    "websiteCount":3
    },
    {"owner": "pqr",
    "ownerPresence":{
    "configCount":1,
    "websiteCount":4
    }]

我不确定我怎么能在Java8做到这一点。




Answer 1

1. 将地图的初始列表重新映射为Map

static Map<String, Integer> convert(List<Map<String, Object>> list) {
    return list
        .stream()
        .collect(Collectors.toMap(
            m -> (String) m.get("web_id"),
            m -> (Integer) m.get("count")
        ));
}

List<Map<String, Object>> configs  = fetchConfigsCount();
List<Map<String, Object>> websites = fetchWebsitesCount();

Map<String, Integer> configMap  = convert(configs);
Map<String, Integer> websiteMap = convert(websites);

class Pojo {
    String owner;
    Map<String, Integer> ownerPresence;
    // boilerplate: constructor(s), getters/setters
}

List<String> listOfOwners = Arrays.asList("abc","xyz","pqr");

List<Pojo> result = listOfOwners
    .stream()
    .map(owner -> new Pojo(
        owner,
        Map.of(
            "configCount",  configMap.get(owner),
            "websiteCount", websiteMap.get(owner)
        )
    ))
    .collect(Collectors.toList());