如何在java中合并两个嵌套的HashMap

Question

I have two nested maps and I try to merge two nested maps and produce a output of two merged HashMap . Below is the code that I use:

HashMap<String, Object> map = new HashMap<>();

HashMap<String, Object> map1 = new HashMap<>();
map1.put("location", "A");
HashMap<String, Object> map2 = new HashMap<>();
map2.put("geocoordinates", map1);
HashMap<String, Object> map3 = new HashMap<>();
map3.put("TEST", map2);
map.putAll(map3); 

HashMap<String, Object> map11 = new HashMap<>();
map11.put("longitude", "B");
HashMap<String, Object> map12 = new HashMap<>();
map12.put("geocoordinates", map11);
HashMap<String, Object> map13 = new HashMap<>();
map13.put("TEST", map12);

map.putAll(map13); 
System.out.println(map);

The output that I get is:

   {TEST={geocoordinates={longitude=B}}}

But I expected both longitude and location key to be nested inside geocoordinates key but only longitude B is there. So, How can I get the combined. How achieve this?

Answer 1

Do it as follows:

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Object> map = new HashMap<>();

        HashMap<String, Object> map1 = new HashMap<>();
        map1.put("location", "A");
        HashMap<String, Object> map2 = new HashMap<>();
        map2.put("geocoordinates", map1);
        HashMap<String, Object> map3 = new HashMap<>();
        map3.put("TEST1", map2);
        map.putAll(map3);

        HashMap<String, Object> map11 = new HashMap<>();
        map11.put("longitude", "B");
        HashMap<String, Object> map12 = new HashMap<>();
        map12.put("geocoordinates", map11);
        HashMap<String, Object> map13 = new HashMap<>();
        map13.put("TEST2", map12);

        map.putAll(map13);

        System.out.println(map);
    }
}

Output:

{TEST2={geocoordinates={longitude=B}}, TEST1={geocoordinates={location=A}}}

Reason : a Map replaces the old value when you put a new value on the same key (in your case, it is TEST ). Note that HashMap.putAll() copies all of the mappings from one map into another. In your code, map.putAll(map3) is equivalent of map.put("TEST",map3) . And, map.putAll(map13) is equivalent of map.put("TEST",map13) which replaces the earlier value, map3 because of the same key, TEST .

Update: Adding the following update based on the new requirement mentioned in your comment

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

public class Main {
    public static void main(String[] args) {
        HashMap<String, Object> map = new HashMap<>();
        List<HashMap> list=new ArrayList<HashMap>();

        HashMap<String, Object> map1 = new HashMap<>();
        map1.put("location", "A");
        list.add(map1);

        HashMap<String, Object> map11 = new HashMap<>();
        map11.put("longitude", "B");
        list.add(map11);

        HashMap<String, Object> map2 = new HashMap<>();
        map2.put("geocoordinates", list);       

        map.put("TEST",map2);

        System.out.println(map);
    }
}

Output:

{TEST={geocoordinates=[{location=A}, {longitude=B}]}}

Another way of doing it as follows:

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Object> map1 = new HashMap<>();
        map1.put("location", "A");
        map1.put("longitude", "B");

        HashMap<String, Object> map2 = new HashMap<>();
        map2.put("geocoordinates", map1);

        HashMap<String, Object> map = new HashMap<>();
        map.put("TEST", map2);

        System.out.println(map);
    }
}

Output:

{TEST={geocoordinates={location=A, longitude=B}}}

Answer 2

The behavior you're seeing is correct – namely, if you have a map and store a new value for an existing key, the old value will be lost.

Here's a simple example that isolates what you're doing. In this code, the initial value "one" will be overwritten by "two" . This is how maps work. In your case, you're using other maps instead of simple strings, but the behavior is the same – you have one value and you're replacing it with another value.

HashMap<String, Object> map = new HashMap<>();
map.put("TEST", "one");
map.put("TEST", "two");

To retain both "one" and "two", you need to either use different keys (ex: "TEST1" and "TEST2"), or alter one of the nested maps stored under "TEST", or introduce an altogether different data structure (such as a java.util.Set ).