坚持如何添加到另一个班级的列表

Question

I have a class, DB, that contains a list:

private List<User> users;

along with an addUser method that allows users to be added to the list

public User addUser(String name) {

Another class, User, needs to create an instance of the User class, so I tried using the following code:

public User(String name) {
        DB.addUser(name);

But it returns the error that I can't make a static reference to the nonstatic method. I'm at a loss, because to create an instance of the User class, I have to put it into the list, but since the list is private, I have to use DB's methods, but I can't do that without getting an error... Is there any way to fix this?

Answer 1

This function here is calling addUser in a static way.

public User(String name) {
    DB.addUser(name);
}

But this function below requires an object to be instantiated in order to call it (non-static)

public User addUser(String name) { ... }

So change it to be

public static User addUser(String name) { ... }

or create an instance of DB and call the method

public User(String name) {
    DB db = new DB();
    db.addUser(name);
}

Answer 2

Seems there are some basics which are not clear to you.

1. Members are always private and to interact them you should create public setter and getter.

   public List getUsers(){ return users; }

   public void getUsers(List users){ this.users=users; }

2. As a good coding standard, the naming should describe the behavior which provides specific action Hence

User addUser(String name)

seems very ambiguous considering it's actual implementation as given adds User to the list.

3. If DB class is a dependency then it should be instantiated Let's assume you are calling this code from main

   User user= new User(); DB dbInstance=new DB(); dbInstance.setUsers(new ArrayList()); dbInstance.getUsers().add(user);

To provide a more specific answer awaiting further requested details