初始化尚未声明的实例变量

Question

I'm new to java and I'm confused by this: UserAccount is another class how do i initialize user in this abstract class Person

public abstract class Person {
private String name;
private String email;
public Person(String name, String email, UserAccount user) {
//initalize user

    this.name = name;
    this.email = email;
//user?


}
public class UserAccount {
private String username;
private String password;
public UserAccount(String username, String password) {

    this.username = username;
    this.password = password;
}

Answer 1

In your code, you did what is known as Inversion of Control , although its application in this scenario may be not the best example. When receiving a UserAccount as a parameter in the construct of Person , you may actually want to store it as a field/attribute of class Person :

public abstract class Person {
    private String name;
    private String email;
    private User user; // Add field user

    public Person(String name, String email, UserAccount user) {
        this.name = name;
        this.email = email;
        this.user = user; // Inversion of Control: assume user is already constructed
    }
}

In short: you would construct the UserAccount before constructing the user like this:

// first construct the account...
UserAccount user = new UserAccount("John", "123secret321");
// ... then pass this account to a person
Person person = new Person("John", "john@doe.com", user);

There is, however, the possibility to let the constructor of Person fully handle the construction of a UserAccount like this:

    // signature has changed, pass all necessary information to Person, let it construct a UserAccount for us.
    public Person(String name, String email, String userName, String password) {
        this.name = name;
        this.email = email;
        this.user = new UserAccount(userName, password); // construct the user with the given information
    }

While you cannot call the constructor of Person dicretly (since the class is abstract ), the constructor gets called, when a sub-class is constructed, eg

public class FinishedPerson extends Person {
    private Date birthDate;

    public FinishedPerson(String name, String email, Date birthDate, String username, String password) {
        // call Person's constructor that, amongst other things, initializes the field user.
        super(name, email, username, password); 
        this.birthDate = birthDate;
    }
}

I hope this helps.