Reject duplicate attribute entry of an array of objects in java swing

I try to make a program to register users, so I created an array of users with the attributes: Usuario (User ID), nombre (name) and contraseña (password), and everytime I create a new user the program has to ensure that the User ID hasn't registered before, if it's already registered the program must reject the user register until I put a different UserID, every Object created(every user with it's attributes) is saved in it's respective position of the array, so I create a method with these functions (int contador will count the times I register a User, the array has a maximum of 10 so contador<10):

public  void registrarUsuario(int contador){
         int f = contador;
        usuario = t3.getText();
        nombre = t4.getText();
        contraseña = t5.getText();

            for(int i=0; i<users.length; i++){
\line 43 with error if(users[i].getUsuario() == usuario){
                    JOptionPane.showMessageDialog(null, "El nombre de usuario ya ha sido utilizado");

                }
            }



                
        if(this.t6.getText().equals(this.t5.getText())){  
           Usuario user = new Usuario(usuario, nombre, contraseña);
           users[contador] = user;
            
        JOptionPane.showMessageDialog(null, "El usuario se ha registrado con exito!");   
        
            this.t3.setText("");
            this.t4.setText("");
            this.t5.setText("");
            this.t6.setText("");
            
        }else{
            
        JOptionPane.showMessageDialog(null, "ERROR: las contraseñas no coididen");
            this.t5.setText("");
            this.t6.setText("");
            
        }
      
    }

but when I run the program I get this error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at proyecto1.Proyecto1.registrarUsuario(Proyecto1.java:143) 

( see above to line 143)

On this line of code:

for(int i=0; i<users.length; i++){

You never defined the array users - either because you forgot the this identifier, or simply forgot to define it.

I got what you're trying to achieve but I didn't fully understand your method because you didn't send all the classes, so i'll send you how i solved it and if you want you can resend us all your classes and code to have a full view

My code: (of course you can edit it to be interactive with the user, here i'm showing the general idea only)

    //User class
package userArray;
public class User {
private int id;
private String name;
private String password;//there is special classes for password in java, not going to use them in this problem
public User(int id, String name, String password) {
super();
this.id = id;
this.name = name;
this.password = password;
}
//getters and setters + toString()
//UsersList class
package userArray;
import java.util.*;
public class UsersList {
ArrayList<User> users = new ArrayList<User>();
//The original function
public User createUser(int id, String name, String password) throws Exception {
for(User u : this.users) {
if(id == u.getId())
throw new Exception("User already exists...");
}
User u = new User(id,name,password);
this.users.add(u);
return u;
}
//The function used to wrap the original function in try/catch each time executed
public User registerUser(int id, String name, String password) {
try {
return createUser(id,name,password);
} catch (Exception e) {
System.out.println("ID "+ id +" for user "+name+" is for an existing user...");
return null;
}
}
public UsersList() {
super();
this.users = users;
}
// + toString()
//Main class
package userArray;
public class Main {
public static void main(String[] args) {
UsersList list = new UsersList();
list.registerUser(1, "Kevin", "Kevin123");
list.registerUser(2, "Sarah", "Sarah123");
list.registerUser(1, "Steven", "Steven123");
list.registerUser(3, "Stephany", "Stephany123");
list.registerUser(3, "Alex", "Alex123");
System.out.println("The list is: ");
System.out.println(list.toString());
}
}

The execution result:

ID 1 for user Steven is for an existing user... <br>
ID 3 for user Alex is for an existing user... <br>
The list is: <br>
UsersList [users=[User [id=1, name=Kevin, password=Kevin123], User [id=2, name=Sarah, password=Sarah123], User [id=3, name=Stephany, password=Stephany123]]]