I am wondering how to build a nested Map and then retrieve the nested key:value pairs. This is how I created the map to be nested.

//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);

This is the system output for the following code:

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

This is correct as I want a list of maps. But the next step is what is confusing to me. When I try to build the outer layer Map as below:

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

I get a system output of null when attempting the .get("buildings.buildingid") method. Ideally, I need the output to look like this:

[[3, 1, 4]]

Where it returns all values with a key of "buildings.buildingid" in an array. I am new to coding so please let me know if I'm making any fundamental errors and how I can create a nested Map where I can access the inner layer or if there is another method I should be using.

CodePudding user response：

I think you are making it way too complicated than it should be. you can store your data in a simple map where the ids are the keys for example and the names are the values. So you only need to read the keys or the values from the map to get your result.

Map<Integer, String> myMap = new HashMap<>();
myMap.put(3, "Building 3");
myMap.put(31, "Building 31");
myMap.put(4, "HFTF");

System.out.println(myMap.keySet());
System.out.println(myMap.values());

However, Java is an object-oriented language. If it makes sense for your use case you might want to create a custom class "Building" instead of frantically trying to store your data in nested data structures. See below for an example of how it might look using a custom class:

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;
        }
    }
}

CodePudding user response：

queryMap.get("buildings.buildingid") returns null, because queryMap only contains a value under the key buildings. A HashMap can only access the value using the same key it was stored under. The key and the value is not processed in any further way.

A simple alternative could be

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

If you don't like this solution you could take a deeper look into property expressions, and write your own map implementation that can resolve these expressions. But it will be a lot of work to get it working correctly...