为什么不将String重置为null？

Question

im trying to create a login system with an server and a client. right now almost every thing works as i want to but there is one problem.. When i login with the client it sends the Username and password that the persion have inputed.if you login with correct information you will get logged in ofcurse. but if you put in a wrong username after the first persion logged in. it will still work.? heres an exempel.

//Correct Login info

Username: xoleo
Password: 123
 = LOGGED IN

//Wrong Login Info but still works after the correct login info

Username: xoleo123
Password: 123
 = LOGGED IN

//Wrong Login Info, this dosent work, and i dont know why the second way worked and this one dident..

Username: xoleo
Password: 1234
 = LOGGED FAILED

here is some code from the client side that sends the info after you press login:

    button2.addActionListener(new ActionListener() {
        @SuppressWarnings("deprecation")
        public void actionPerformed(ActionEvent arg0) {
            send(username2.getText(), password2.getText(), InGame.address, InGame.port);
        }
    });

}

private void send(String name, String pass, String address, int port) {
    new LoginSend(name, pass, address, port);
}

Here is the code that for the 'send(String name, String pass, String address, int port)':

package ca.sidez.main;

import javax.swing.JFrame;
import javax.swing.JTextField;
import ca.sidez.menu.InGame;
import ca.sidez.menu.Main;
import ca.sidez.menu.Main.STATE;
import ca.sidez.screen.ClientLogin;

public class LoginSend extends JFrame implements Runnable {
private static final long serialVersionUID = 1L;

private JTextField txtMessage;
private Thread run, listen;
private ClientLogin clientLogin;

private boolean running = false;

public LoginSend(String name, String pass, String address, int port) {
    clientLogin = new ClientLogin(name, address, port);
    boolean connect = clientLogin.openConnection(address);
    if (!connect) {
        System.err.println("Connection failed!");
        System.out.println("Connection failed!");
    }
    System.out.println("Försöker att ansluta til " + address + ":" + port + ", username: " + name);
    String connection = "/u/" + name + "/u/" + "/p/" + pass + "/p/";
    clientLogin.send(connection.getBytes());
    running = true;
    run = new Thread(this, "Running");
    run.start();
}

public void run() {
    listen();
}

public void listen() {
    listen = new Thread("Listen") {
        public void run() {
            while (running) {
                String message = clientLogin.receive();
                if (message.startsWith("/xp/")) {
                    InGame.Xp = message.split("/xp/|/xp1/")[1];
                    InGame.Gold = message.split("/g/|/e/")[1];
                    Main.State = STATE.LOADING;
                    System.out.println("Successfully Received Game Data To Player!");
                }
            }
        }
    };
    listen.start();
}
}

Here is some code from the server that will check the username and password:

    } else if (string.startsWith("/u/")) {
        user = string.split("/u/|/u/")[1];          
        pass = string.split("/p/|/p/")[1];

        passx = TextFile.readFile("/" + user + ".txt");
            System.out.println(user);
            System.out.println(pass);
            System.out.println(passx);

        if(pass.equals(passx)) {
            System.out.println(user);
            System.out.println(pass);
            System.out.println(passx);
            System.out.println("LOGGED IN");
            passx = "";
            user = null;
            pass = null;

        } else {
            System.out.println(user);
            System.out.println(pass);
            System.out.println(passx);
            System.out.println("LOGGED FAILED");
            passx = "";
            user = null;
            pass = null;
        }

The Server Console:

//this is the correct first one:

User input: xoleo
User input: 123
Server Checks: 123

LOGGED IN

//this is the wrong second time:

user input: xoleo
user input: 1234
correct password for the username: 123 (checks for a file with that name)
LOGGED FAILED

//this username doent exist, but the password is the old one

user input: leo
user input: 123
server checks: 123
LOGGED IN

Answer 1

Reference data type parameters, such as objects, are also passed into methods by value. This means that when the method returns, the passed-in reference still references the same object as before. However, the values of the object's fields can be changed in the method, if they have the proper access level.