Как я могу обновить файл json, созданный с помощью Gson, в java?

Question

Я сомневаюсь в своем коде. Я создаю приложение java, в котором оно проверяет, существует ли файл json, и если он существует, он будет обновлен (в противном случае он будет создан и отредактирован). Он будет обновлен с помощью моего класса пользователя

класса пользователя:

public class User {
    //user's variables
    String name;
    String second_name;
    int age;
    String email;
    String user_id; //automatically generated
    String password;
    //constructor without parameters
    public User() {}
    //coonstructor with parameters
    public User(String name, String second_name, int age, String email, String user_id, String password) {
        this.name = name;
        this.second_name = second_name;
        this.age = age;
        this.email = email;
        this.user_id = user_id;
        this.password = password;
    }
    //Setters and getters
    public void setName(String name) {
        this.name = name;
    }
    public void setSecond_name(String second_name) {
        this.second_name = second_name;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public void setUser_id(String user_id) {
        this.user_id = user_id;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    public String getName() {
        return name;
    }
    public String getSecond_name() {
        return second_name;
    }
    public int getAge() {
        return age;
    }
    public String getEmail() {
        return email;
    }
    public String getUser_id() {
        return user_id;
    }
    public String getPassword() {
        return password;
    }
    ////////////////////////////////////////////////////////////////////////////////////////////////
    //FUNCTIONS
    public String toString(){
        String personal_data =
                "Id:"+this.getUser_id()+
                "\nEmail:"+this.getEmail()+
                "\nPassword: "+this.getPassword()+
                "\nName: "+this.getName()+
                "\nSecond name: "+this.getSecond_name()+
                "\nAge: "+this.getAge();
        return personal_data;
    }
    ////////////////////////////////////////////////////////////////////////////////////////////////

}

И моего основного класса, в котором я обновляю свой json файл:

import com.google.gson.Gson;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;
import java.util.*;

public class main {
    public static void main(String[] args) throws IOException {
        Scanner scanner = new Scanner(System.in);
        //menu
        System.out.println("Welcome to Vault's App.");
        System.out.println("================================");
        System.out.println("Please, choose an option:");
        System.out.println("1.Sign up");//Sign up
        System.out.println("2.Log in");//Log in
        System.out.println("3.View Security's Log");//View Security's Log
        System.out.println("4.Print data");
        int option = scanner.nextInt();
        menu(option);
    }
    public static void menu(int opt) throws IOException {
        switch (opt){
            case 1:
                signUp(); //sign up function
                break;
            case 2:
                logIn();//log in function
                break;
            case 3:
                securityLog();//security log (check password and python script[root access])
                break;
            case 4:
                printData();//print hashmap
                break;
        }
    }
    public static void signUp() throws IOException {// add user
        String path = "E:\\1PROGRAMMING\\AccountsGestor\\src\\users.json";
        User new_user = new User();//new user's instance
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter data: ");
        System.out.println("==================");
        String id = generateUserId() ;
        System.out.println("Enter your email: ");
        String emailUser = scanner.nextLine();
        System.out.println("Enter your password: ");
        String passwordUser = ingressPassword();
        System.out.println("Enter your name: ");
        String nameUser = scanner.nextLine();
        System.out.println("Enter your second name: ");
        String second_name_User = scanner.nextLine();
        System.out.println("Enter your age: ");
        int ageUser = scanner.nextInt();

        new_user.setUser_id(id);
        new_user.setEmail(emailUser);
        new_user.setPassword(passwordUser);
        new_user.setName(nameUser);
        new_user.setSecond_name(second_name_User);
        new_user.setAge(ageUser);

        //Test
        //System.out.println(new_user.toString());
        //json array
        ArrayList<User> allItems = new ArrayList<>();
        Writer writer = new FileWriter(path);
        File temp = new File(path);
        try
        {
            temp.createTempFile("users", ".json");
            boolean exists = temp.exists();
            if (!exists){
                System.out.println("No json file detected. Creating...");
                new Gson().toJson(jsonArray(allItems,new_user,path),writer);
                writer.close();
            }else{
                System.out.println("Temp file exists ");
                //read json data
                //add allItems element
            }
        } catch (IOException e)
        {
            e.printStackTrace();
        }

    }

    public static ArrayList jsonArray(ArrayList allItems, User user, String path) throws IOException {
        //adding arraylist new element
        allItems.add(new User(user.getName(),user.getSecond_name(),user.getAge(),user.getEmail(),user.getUser_id(),user.getPassword()));
        return allItems;
    }
    public static void logIn(){System.out.println("PENDING");}
    public static void securityLog(){//check passwords saved (root)
        System.out.println("PENDING");
    }
    public static void printData() {
        System.out.println("PENDING");
    }


    public static String generateUserId(){
        int MAX = 20;
        String elements = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
        Random random = new Random();
        StringBuilder idGenerated = new StringBuilder(MAX);
        for (int i =0 ;i < MAX; i++){
            int randomInt = random.nextInt(elements.length());
            idGenerated.append(elements.charAt(randomInt));
        }
        return idGenerated.toString();
    }
    public static String ingressPassword(){//CALLING TO PYTHON SCRIPT PASSWORDCHECKER
        Scanner scanner = new Scanner(System.in);
        String psswd = scanner.nextLine();
        //export info from python
        return (psswd);
    }
}

Я могу создавать и помещать новые данные, но мне нужно знать, как я могу обновить файл json, если он существует.