How can I add data to a JSON on Java?

Question

I was wondering already having an existing .json file how can I add more information to it without having to delete it and write it again. For now I have this code, but I cannot add new information to it.

public static void main( String[] args )
    {
        //First Employee
        JSONObject employeeDetails = new JSONObject();
        employeeDetails.put("firstName", "Lokesh");
        employeeDetails.put("lastName", "Gupta");
        employeeDetails.put("website", "howtodoinjava.com");

        JSONObject employeeObject = new JSONObject(); 
        employeeObject.put("employee", employeeDetails);

        //Second Employee
        JSONObject employeeDetails2 = new JSONObject();
        employeeDetails2.put("firstName", "Brian");
        employeeDetails2.put("lastName", "Schultz");
        employeeDetails2.put("website", "example.com");

        JSONObject employeeObject2 = new JSONObject(); 
        employeeObject2.put("employee", employeeDetails2);

        //Add employees to list
        JSONArray employeeList = new JSONArray();
        employeeList.add(employeeObject);
        employeeList.add(employeeObject2);

        //Write JSON file
        try (FileWriter file = new FileWriter("employees.json")) {

            file.write(employeeList.toJSONString());
            file.flush();

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