How to Read a JSON Array File into Java Using the JSON.simple Library

Question

How can I effectively read a JSON array file in Java using the JSON.simple library?

[{
    "name":"John",
    "city":"Berlin",
    "cars":[
        "audi",
        "bmw"
    ],
    "job":"Teacher"
},
{
    "name":"Mark",
    "city":"Oslo",
    "cars":[
        "VW",
        "Toyota"
    ],
    "job":"Doctor"
}]

Answer

To read a JSON file that contains an array of objects using the JSON.simple library in Java, you need to correctly parse the structure of the JSON. The provided code mistakenly treats the entire JSON array as a single JSON object, which leads to a ClassCastException. Below is a detailed explanation and corrected code to read the JSON array properly.

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.Iterator;
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;

public class JavaApplication1 {
    public static void main(String[] args) {
        JSONParser parser = new JSONParser();
        try {  
            JSONArray jsonArray = (JSONArray) parser.parse(new FileReader("c:\\file.json"));
            for (Object obj : jsonArray) {
                JSONObject jsonObject = (JSONObject) obj;
                String name = (String) jsonObject.get("name");
                String city = (String) jsonObject.get("city");
                String job = (String) jsonObject.get("job");
                System.out.println("Name: " + name);
                System.out.println("City: " + city);
                System.out.println("Job: " + job);
                JSONArray cars = (JSONArray) jsonObject.get("cars");
                Iterator<String> iterator = cars.iterator();
                System.out.print("Cars: ");
                while (iterator.hasNext()) {
                    System.out.print(iterator.next() + " ");
                }
                System.out.println();  // New line for clarity
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

Causes

The JSON file begins with a `[` character, indicating that the root element is an array, not an object. As a result, the file should be parsed into a JSONArray, not a JSONObject.

Solutions

Modify the parsing logic to handle the JSON array by directly casting the parsed object to a JSONArray.
Iterate through the JSONArray to access individual JSONObjects.

Common Mistakes

Mistake: Directly casting the parsed file to JSONObject when it is an array.

Solution: Change the casting to JSONArray and iterate over its elements.

Mistake: Not handling file path correctly in different operating systems.

Solution: Use double backslashes (\\) or forward slashes (/) to ensure compatibility across different platforms.

Helpers

read JSON file in Java
JSON.simple library Java
parse JSON array Java
Java read JSON example
Java JSON parsing errors