用相同的附加字段包装不同的子类

Question

Let's say I have some abstract class User , and then subclasses Owner and Client . Each of the two subclasses have some specific fields, but they both inherit common fields from the User . This is all OK, but in a specific situation I'd like to be able to wrap any instance of User with another field. Here is the example.

User.java

public abstract class User {

    private String email;

    private String password;

    private String name;

}

Client.java

public class Client extends User {

    private String salary;

    private String profit;

}

Owner.java

public class Owner extends User {

    private int numberOfClients;

    private double averageClientSalary;

}

Now in a specific case, I'd like to be able to wrap such instances with a login token. So no matter if it is a Client or the Owner instance, it should receive another field loginToken on top of all it's fields.

The use-case would be something like this.

Client client = new Client(email, password, name, salary, profit);

String loginToken = "123abc456";

UserLogin clientLogin = new UserLogin(client);
clientLogin.setLoginToken(loginToken);

// now this should be possible
clientLogin.getEmail(); // base class field
clientLogin.getSalary(); // derived class field
clientLogin.getLoginToken(); // field from a wrapper

So this UserLogin should be able to receive any subclass of User as constructor argument, and remain all of it's fields, just add the extra one, ie. loginToken .

Would something like this be possible and how?

Basically, what would the UserLogin class look like?

Answer 1

For base class fields and wrapper class fields (or to be precise getters as private fields are not accessible from subclasses) , you can add delegation methods in the wrapper UserLogin class.
But you could not use getters of the subclass as the wrapper manipulates a User declared variable.

public class UserLogin {

    private User user;
    private String token;

    public UserLogin(User user, String token) {
        this.user = user;
        this.token = token;
    }

    public String getEmail() {
        return user.getEmail();
    }    

    public String getLoginToken() {
        return token;
    }

    // not possible for derived fields
    public String getSalary() {
        return user.getSalary();
    }

}

As workaround to do such a thing you could make UserLogin a generic class that provides a method that returns an instance of the specific User class.

For example :

public class UserLogin<T extends User> {

    private T user;
    private String token;

    public UserLogin(T user, String token) {
        this.user = user;
        this.token = token;
    }

    public String getEmail() {
        return user.getEmail();
    }

    public T getConcreteUser() {
        return user;
    }

    public String getLoginToken() {
        return token;
    }    

}

You can now invoke :

UserLogin<Client> clientLogin = new UserLogin<>(new Client(), "token");

clientLogin.getEmail(); 
clientLogin.getConcreteUser()
           .getSalary(); 
clientLogin.getLoginToken();