How can I refer the objcet that calls a method in the method in Java?

Question

I have the below code

User u1 = new User()
User u2 = new User()

u1.addUser(u2)

How can I refer to the object u1 in method addUser? Below is the code of class User

public class User{
    private ArrayList<User> users;
    public void addUser(User a){
         users.add(a);
         // Here I want to do the opposite. 
    }
}