Building ordered JSON String from LinkedHashMap

Question

I had a need for having Key/Value pairs in the order of my insertion, so I opted to use LinkedHashMap over HashMap. But I need to convert the LinkedHashMap into a JSON String where the order in the LinkedHashMap is maintained in the string.

But currently I'm achieving it by:

1. First converting the LinkedHashMap into JSON.

2. Then converting the JSON into a string.

   import java.util.LinkedHashMap;
   import java.util.Map;
   
   import org.json.JSONObject;
   
   public class cdf {
       public static void main(String[] args) {
           Map<String,String > myLinkedHashMap =  new LinkedHashMap<String, String>();
           myLinkedHashMap.put("1","first");
           myLinkedHashMap.put("2","second");
           myLinkedHashMap.put("3","third");
   
           JSONObject json = new JSONObject(myLinkedHashMap);
           System.out.println(json.toString());
       }
   }
   

The output is:

{"3":"third","2":"second","1":"first"} . 

But I want it in the order of insertion of the keys, like this:

{"1":"first","2":"second","3":"third"}

Once I convert the LinkedHashMap into a JSON it loses it order (it's obvious that JSON doesn't have the notion of order) and hence the string too is out of order. Now, how do I generate a JSON string whose order is same as the LinkedHashMap?

Answer 1

Gson if your friend. This will print the ordered map into an ordered JSON string.

If you want to preserve insertion order, use a LinkedHashMap.

I used the latest version of Gson (2.8.5), you can can download it via the following options at the bottom of this post.

import java.util.*;
import com.google.gson.Gson;

public class OrderedJson {
    public static void main(String[] args) {
        // Create a new ordered map.
        Map<String,String> myLinkedHashMap = new LinkedHashMap<String, String>();

        // Add items, in-order, to the map.
        myLinkedHashMap.put("1", "first");
        myLinkedHashMap.put("2", "second");
        myLinkedHashMap.put("3", "third");

        // Instantiate a new Gson instance.
        Gson gson = new Gson();

        // Convert the ordered map into an ordered string.
        String json = gson.toJson(myLinkedHashMap, LinkedHashMap.class);

        // Print ordered string.
        System.out.println(json); // {"1":"first","2":"second","3":"third"}
    }
}

If you want the items to always be inserted at the right place, use a TreeMap instead.

import java.util.*;
import com.google.gson.Gson;

public class OrderedJson {
    public static void main(String[] args) {
        // Create a new ordered map.
        Map<String,String> myTreeHashMap = new TreeMap<String, String>();

        // Add items, in any order, to the map.
        myTreeHashMap.put("3", "third");
        myTreeHashMap.put("1", "first");
        myTreeHashMap.put("2", "second");

        // Instantiate a new Gson instance.
        Gson gson = new Gson();

        // Convert the ordered map into an ordered string.
        String json = gson.toJson(myTreeHashMap, TreeMap.class);

        // Print ordered string.
        System.out.println(json); // {"1":"first","2":"second","3":"third"}
    }
}

Dependency Options

Maven

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.5</version>
</dependency>

Gradle

compile 'com.google.code.gson:gson:2.8.5'

Or you can visit Maven Central for more download options.