[Java] Try editing the elements of the Json string using the library

Thing you want to do

String jsonString = "{\"Apple\":{" 
						+ "\"cost\":\"0\"},"
					+ "\"Pen\":{"
						+ "\"cost\":\"0\"}}";

I want to do the following two points.

• I want to edit the part of the above Json string that corresponds to " 0 ".
• I want to add a new key " Apple Pen ".

This time, I will use the following library for operating typical (?) Json.

• Gson
• Jackson

Execution environment

Android Studio: 3.4.1 gradle: 5.1.1

Library addition

Describe the library to be added in the dependencies block of build.gradle.

dependencies {
	...
    implementation 'com.google.code.gson:gson:2.8.5'
    implementation 'com.fasterxml.jackson.core:jackson-databind:2.9.8'
    implementation 'org.glassfish:javax.json:1.1.4'
}

Gson Create Json string with the created method (editJson) Replaced with Map <String, MAP <String, String >> (Gson.fromJson) and assigned the desired key and value. After the substitution, the Map is converted back to a Json string (Gson.toJson).

Although it deviates from the main subject, Convert [Json => other class] to "Deserialization", The conversion of [other class => Json] is called "Serialization".

import com.google.gson.Gson;
...

public class GsonExample {

	String jsonString = "{\"Apple\":{" +
							"\"cost\":\"0\"}," +
						"\"Pen\":{" + 
							"\"cost\":\"0\"}}";
	
	public void main() {
		editJson("Apple", "100");
		/*
			{
				"Apple": {
					"cost": "100"
				},
				"Pen": {"
					"cost": "0"
				}
			}"
		*/
		editJson("ApplePen", "");
		/*
			{
				"Apple": {
					"cost": "100"
				},
				"Pen": {"
					"cost": "0"
				}
				"ApplePen": {"
					"cost": "0"
				}
			}"
		*/
	}

	private void editJson(String key, String str) {
		if (str.isEmpty()) str = "0";
        Map<String, String> innerMap = new HashMap<>();
        innerMap.put("cost", str);

    	Gson gson = new Gson();
    	//Convert Json string to Map
        Map<String, Map<String,String>> jsonMap = gson.fromJson(jsonString, Map.class);
        jsonMap.put(key, innerMap);
    	//Convert Map to Json string again
        jsonString = gson.toJson(jsonMap);
    }
}

Jackson The replacement process is performed in almost the same way as for Gson. Jackson translates Json via the ʻObjectMapper` method.

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
...

public class JacksonExample {
	// main()Is omitted

    private void editJson(String str, String key) {
		if (str.isEmpty()) str = "0";
        Map<String, Object> map = null;
        Map<String, String> innerMap = new HashMap<>();
        innerMap.put("cost", str);

        ObjectMapper mapper = new ObjectMapper();
        try {
        	//Convert Json string to Map
            map = mapper.readValue(json, 
									new TypeReference<Map<String, Object>>() {});
        } catch (IOException e) {
            e.printStackTrace();
        }
        map.put(key, innerMap);
        try {
        	//Convert Map to Json string again
            json = mapper.writeValueAsString(map);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
	}
}

About javax.json

I tried to implement the same with the javax.json library, but I got stuck.

• Can't convert Json string to Map?
• JsonObject cannot be rewritten. (UnsupportedOperationException occurred)

Impressions

• Gson is easy to use.
• I don't know anything about Type Reference.

20.09.27 postscript (about org.json.JSONObject)

I realized that if you just want to convert Json to Map and add a value, you can use ʻorg.json.JSONObject`.

import org.json.JSONException;
import org.json.JSONObject;
...

public class JSONExample {
	// main()Is omitted

    private void editJson(String str, String key) {
		if (str.isEmpty()) str = "0";
        Map<String, String> innerMap = new HashMap<>();
        innerMap.put("cost", str);
        try {
            JSONObject jsonObject = new JSONObject(json);
            jsonObject.put(key, new JSONObject(innerMap));
            json = jsonObject.toString();
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }
}

References

I referred to the following article.

• How to use Gson, Jackson
  • https://qiita.com/riversun/items/cdd990e96c2bbf9cb043
• Gson
  • https://sites.google.com/site/gson/gson-user-guide
  • https://www.codeflow.site/ja/article/gson-json-to-map
• Jackson
  • https://salumarine.com/how-to-convert-json-string-to-map-in-java/
• javax.json
  • https://spring.pleiades.io/specifications/platform/8/apidocs/javax/json/JsonObject.html