Java ArrayList 不允许我从外部构造函数添加

Question

public class Start {
    public Register theReg = new Register();
    public static Start go = new Start();

    public static void main(String[] args) {
        Register theReg = new Register();
        go.regUsers();

        if(theReg.logIn("jsmith","password")) {
            System.out.println("You're logged in as " +
                               theReg.userLoggedIn.getName());
        } else {
            System.out.println("dang");
        }
    }

    public void regUsers() {
        Student regJoe =
            theReg.regSeniorStaff("Joe smith", "password", "jsmith", 1);
    }
}

public class Register {
    public ArrayList<People> users;
    public People userLoggedIn;

    public Register () {
        users = new ArrayList<People>();
        users.add(new Student("john","password","jo",1));
        userLoggedIn = null;
    }

    public Student regStudent(String name, String password,
            String username, int stuId) {
        Student s = new Student(name, password, username, stuId);
        users.add(s);
        return s;
    }
}

我想我刚刚错过了一些愚蠢的事情。就像....

start 方法将创建一个新的 Register 对象，该对象具有其 ArrayList。然后，它开始使用 regStudent 等方法注册用户。但是，只有 Register 上的构造函数允许我将对象添加到 ArrayList 中；稍后调用这些方法来做同样的事情只是不会添加它们。它创建对象但无法添加它们。也无法删除东西，只有 .get 可以在其中工作。

任何帮助都会很棒，谢谢!

Answer 1

看起来您正在隐藏您的 theReg 全局变量。您很可能查询全局并看到只有构造函数添加的项位于您的 ArrayList 中。

public class Start {
    public Register theReg = new Register(); //<<---- your global var
    public static Start go = new Start();

    public static void main(String[] args) {
        Register theReg = new Register();  // <<---- hiding the global
        go.regUsers();

        if(theReg.logIn("jsmith","password")) 
          System.out.println("You're logged in as " + theReg.userLoggedIn.getName());
        else 
          System.out.println("dang");
    }
}