How to make an ArrayList inside a class and why like that?

I creating a simple game where I have a User class where I store user data. There is a specific kind of data that I want to store in a ArrayList and where I want to add things to during the game. Below you can see how I thought I had to write it, but apparently it is not correct like that. Why not? How do I do it correctly and why like that? I searched on the internet but couldn't find it.

public Class User {
    List positions;
}

User x = new User;
x.positions = new ArrayList;

x.positions.add(1);
x.positions.add(2);

First, you should make positions private.
Second, you need to make the appropriate public getter and setter methods of the positions.

See some sample below:
Class User (package is omitted):

import java.util.ArrayList;
import java.util.List;

public class User {
    private List<Integer> positions = new ArrayList<>();

    public List<Integer> getPositions() {
        return positions;
    }

    public void setPositions(List<Integer> positions) {
        this.positions = positions;
    }
}

Class Main (package is omitted):

import java.util.ArrayList;
import java.util.List;

public class MainApp {
    public static void main(String[] args) {
        User user = new User();
        user.getPositions().add(1);
        user.getPositions().add(2);

        // another way
        List<Integer> positions = user.getPositions();
        positions.add(3);

        // another way
        List<Integer> myNewList = new ArrayList<>();
        myNewList.add(4);
        myNewList.add(5);
        myNewList.add(6);
        // below will replace list 123 with 456
        user.setPositions(myNewList);
    }
}

I would change List to ArrayList<T>, where T is a type of object, so it has a constraint of what can be stored in it. Also, in your class, you want to set it up so you have private variables, with setters and getters to access the variables.

I also included an initialzer block that constructs the ArrayList<T>, and this initializer block gets called on every creation of a new User object, even if you have multiple constructors in the class. Without constructing the ArrayList<T>, it would be null.

public class User {
    private ArrayList<T> positions;

    public ArrayList<T> getPositions() {
        return this.positions;
    }

    public void setPosition(ArrayList<T> position) {
        this.position = position;
    }

    // Initializer block
    {
        this.position = new ArrayList<>();
    }
}

With this, you can then call x.getPosition.add() to add positions, view positions, remove positions etc.

Few things:

1. Instantiate your ArrayList with a correct constructor (at the moment, your code won't compile, as you are not calling the constructor. It should be new ArrayList<T>(););

2. Define more specific granularity for your List, and declare it with a Generic type parameter, like List<TypeYouWantToStore>. Otherwise, your list is of the raw type, and will not have any constraint on what you store in it;

3. Define accessors (getters) and mutators (setters) for your List field, instead of accessing it directly as a public field. You'll then have:

   x.getPositions().add(1);
   

4. I'd initialize your List field, from the constructor of User class.