在 Java 中检索嵌套的键值对-解网

问：

我想知道如何构建嵌套的地图，然后检索嵌套的键：值对。这就是我创建要嵌套的地图的方式。

//Create List of Nested HashMaps
List<Map> nestedMap = new ArrayList<>();
nestedMap.add(building3Map);
nestedMap.add(building31Map);
nestedMap.add(buildingHFTFMap);
System.out.println("The nested map is: " + nestedMap);

这是以下代码的系统输出：

The nested map is: [{buildingname=[Building 3], buildingid=[3]}, {buildingname=[Building 31], buildingid=[1]}, {buildingname=[HFTF], buildingid=[4]}]

这是正确的，因为我想要一个地图列表。但下一步是让我感到困惑的。当我尝试构建外层地图时，如下所示：

HashMap<String, List<Map>> queryMap = new HashMap<>();
queryMap.put("buildings", nestedMap);
System.out.println(queryMap.get("buildings.buildingid"));

尝试该方法时，我得到的系统输出为 null。理想情况下，我需要输出如下所示：.get("buildings.buildingid")

[[3, 1, 4]]

其中，它返回数组中键为的所有值。我是编码新手，所以如果我犯了任何基本错误，请告诉我，以及如何创建一个嵌套的地图，我可以访问内层，或者是否有其他方法应该使用。"buildings.buildingid"

Java 嵌套列表

import java.util.ArrayList;
import java.util.List;

public class Example {

    public static void main(String args[]) {

        List<Building> buildings = new ArrayList<>();
        buildings.add(new Building(3, "Building 3"));
        buildings.add(new Building(31, "Building 31"));
        buildings.add(new Building(4, "HFTF"));

        List<Integer> buildingIds = new ArrayList<>();
        buildings.forEach(building -> buildingIds.add(building.getBuildingId()));

        List<String> buildingNames = new ArrayList<>();
        buildings.forEach(building -> buildingNames.add(building.getBuildingName()));

        System.out.println(buildingIds);
        System.out.println(buildingNames);

    }

    public static class Building {
        int buildingId;
        String buildingName;

        public Building(final int buildingId, final String buildingName) {
            this.buildingId = buildingId;
            this.buildingName = buildingName;
        }

        public int getBuildingId() {
            return buildingId;
        }

        public void setBuildingId(final int buildingId) {
            this.buildingId = buildingId;
        }

        public String getBuildingName() {
            return buildingName;
        }

        public void setBuildingName(final String buildingName) {
            this.buildingName = buildingName;
        }
    }
}

0赞 magicmn 8/30/2022 #2

queryMap.get("buildings.buildingid")返回 null，因为键下仅包含一个值。A 只能使用存储该值的同一键访问该值。键和值不会以任何进一步的方式进行处理。queryMapbuildingsHashMap

一个简单的替代方案可能是

queryMap.get("buildings").stream() // Create a Stream<Map>
    .map(building -> building.get("buildingid")) // Create Stream<String> by extracting the buildingid values.
    .collect(Collectors.toList()); // Collect Stream into a List<String> which contains the buildingid's

如果你不喜欢这个解决方案，可以更深入地研究属性表达式，并编写你自己的映射实现来解析这些表达式。但是要让它正常工作需要做很多工作......

在 Java 中检索嵌套的键值对

Retrieving nested key:value pairs in Java

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