初始化尚未聲明的實例變量

Question

我是Java新手，對此感到困惑：UserAccount是另一個類，我如何在此抽象類Person中初始化用戶

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

在您的代碼中，您執行了所謂的“控制反轉” ，盡管在這種情況下其應用可能不是最佳示例。 在Person結構中接收到UserAccount作為參數時，您實際上可能希望將其存儲為Person類的字段/屬性：

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
    }
}

簡而言之：在構建用戶之前，您需要先構建UserAccount ：

// 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);

但是，可以讓Person的構造函數完全處理UserAccount的構造，如下所示：

    // 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
    }

雖然您不能分別調用Person的構造函數（由於該類是abstract ），但是在構造子類時（例如，構造函數被調用）

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;
    }
}

我希望這有幫助。