06 Mar 2021

Intro

JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate. It is based on a subset of the JavaScript Programming Language Standard ECMA-262 3rd Edition - December 1999. JSON is a text format that is completely language independent but uses conventions that are familiar to programmers of the C-family of languages, including C, C++, C#, Java, JavaScript, Perl, Python, and many others. These properties make JSON an ideal data-interchange language.

Json-simple API for Java

The Java API to use is JSON.simple

Installation

Download the json-simple-1.1.jar and make it into the project structure.(If using intellj Idea)

Writing to files

Writing to files is quite standard, it mainly includes two different classes. JSONObject and JSONArray

JSONObject

JSONObect usesput() method to add into one Json object.

Example:

import java.io.FileWriter;
import java.io.IOException;
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;

public class WriteJSONExample
{
    @SuppressWarnings("unchecked")
    public static void main( String[] args )
    {
        //First JSONObject
        JSONObject fruitDetails = new JSONObject();
        fruitDetails.put("Name", "Banana");
        fruitDetails.put("Color", "Yellow");
        fruitDetails.put("Price", 10);

        JSONObject fruitObject = new JSONObject();
        fruitObject.put("fruit", fruitDetails);


        //Write JSON file
        try (FileWriter file = new FileWriter("fruits.json")) {
            //We can write any JSONArray or JSONObject instance to the file
            file.write(fruitObject.toJSONString());
            file.flush();

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Result would be

{    
    "fruit": {
            "Name": "Banana",
            "Color": "Yellow",
            "Price": 10
    }
}

JSONArray

JSONArray usesput() method to add into one Json Array.

Example:

import java.io.FileWriter;
import java.io.IOException;
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;

public class WriteJSONExample
{
    @SuppressWarnings("unchecked")
    public static void main( String[] args )
    {
        //First JSONObject
        JSONObject fruitDetails = new JSONObject();
        fruitDetails.put("Name", "Banana");
        fruitDetails.put("Color", "Yellow");
        fruitDetails.put("Price", 10);

        JSONObject fruitObject = new JSONObject();
        fruitObject.put("fruit", fruitDetails);

        //Second JSONObject
        JSONObject fruitDetails2 = new JSONObject();
        fruitDetails2.put("Name", "Orange");
        fruitDetails2.put("Color", "Orange");
        fruitDetails2.put("Price", 5);

        JSONObject fruitObject2 = new JSONObject();
        fruitObject2.put("fruit", fruitDetails2);


        //Add fruits to list
        JSONArray fruitList = new JSONArray();
        fruitList.add(fruitObject);
        fruitList.add(fruitObject2);



        //Write JSON file
        try (FileWriter file = new FileWriter("fruits.json")) {
            //We can write any JSONArray or JSONObject instance to the file
            file.write(fruitList.toJSONString());
            file.flush();

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Result would be

[
    {    
        "fruit": {
                "Name": "Banana",
                "Color": "Yellow",
                "Price": 10
        }
    },

    {
        "fruit": {
                "Name": "Orange",
                "Color": "Orange",
                "Price": 5
        }
    }

]

Reading from files

To read from files, we will need JSONParser, FileReader

There are several Examples online such as GeekForGeek’s tutorial

But it only provides a code example for reading a JSON Object. (and array within one Object). It troubles me for a while when dealing with a JSON file contains one big JSON Array. Yet it turns out to be easy.

Consider this example

import java.io.FileReader;
import java.util.Iterator;

import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
import org.json.simple.parser.*;

class JSONReadExample
{
    public static void main(String[] args) throws Exception
    {
        // parsing file "fruits.json"
        Object obj = new JSONParser().parse(new FileReader("fruits.json"));

        // could also write separately like

        // JSONParser jsonParser = new JSONParser();
        // try (FileReader reader = new FileReader("fruits.json"))


        // typecasting obj to JSONArray since it is a big Array at init
        JSONArray ja = (JSONArray) obj;

        // crafting iterator
        Iterator itr2 = ja.iterator();

        while (itr2.hasNext()) {
            JSONObject curr = (JSONObject) itr2.next();

            String name = (String) curr.get("Name");
            String color = (String) curr.get("Color");

            // PAY ATTENTION The number is type long after parsing
            long price = (long) curr.get("Price");

            /**
             * TODO
             * Use the fields read, store them in ArrayList || system.out.println
             */
        }
    }
}

You could also get number of Objects within one JSONArray using size(), so you can use for loop if needed.

    JSONArray ja = (JSONArray) obj;
    final int SIZE = ja.size();

Ending

Thats all for the Json-simple tutorial, hopefully it is helpful :)

Handle json files in Java