class User implements Serializable{

    public User(String username, String password) {
        this.username=username;
        this.password=password;
    }

    private static final long serialVersionUID = 1L;
    String username;
    String password;

}

public class SerializableExample {

    public static void main(String[] args) {
        User user = new User("userB","passwordB");

        String filename = "E:\Proj-docs\userFile.txt";

         FileOutputStream file;
        try {
            file = new FileOutputStream(filename);
            ObjectOutputStream out = new ObjectOutputStream(file);

            out.writeObject(user);

            out.close();
            file.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        User user2=null;

        try {
            FileInputStream file2 = new FileInputStream(filename);
            ObjectInputStream in = new ObjectInputStream(file2);


            user2= (User) in.readObject();


            Optional checkNull = Optional.ofNullable(user2);
            if(checkNull.isPresent()) {
                System.out.println(user2.username + " "+user2.password);

            }else {
                System.out.println("Null Object");
            }

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

}

public class SerializableExample {

    public static void main(String[] args) {
        User user = new User("userD","passwordD");
        String filename = "E:\Proj-docs\userFile.txt";

         FileOutputStream file;
        try {
            file = new FileOutputStream(filename);
            ObjectOutputStream out = new ObjectOutputStream(file);

            out.writeObject(user);
            out.close();
            file.close();
            :
            :
            :