Why can't I access data from my static list?

I am trying to access data from my list but it always end up with null or nothing in it. I run my Class Server That have public static ArrayList<String> clientUsernameList = new ArrayList<>(); In the code. If my server accept a socket this code below should occur.

package multi.threaded_server_application;

import java.io.DataInputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.time.Clock;
import java.util.ArrayList;
import java.util.logging.Level;
import java.util.logging.Logger;
import jdk.nashorn.internal.codegen.CompilerConstants;

/**
*
* @author Seeya
*/
public class Server {

public static ArrayList<String> clientUsernameList = new ArrayList<>();

/**
 * @param args the command line arguments
 * @throws java.io.IOException
 */
public static void main(String[] args) throws IOException {

    int port = 8900;    //Port number the ServerSocket is going to use
    ServerSocket myServerSocket = null;  //Serversocket for sockets to connect
    Socket clientSocket = null;    //Listerner for accepted clients
    ClientThread[] clientsConnected = new ClientThread[20]; //Max clients in this server
    DataInputStream IN = null;

    try {
        myServerSocket = new ServerSocket(port);
        System.out.println("Server waiting for clients on port:" + port);
    } catch (IOException ex) {
        Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
    }

    while (true) {            
        try {   //Freezes while-loop untill a socket have been accepted or if some failure occur
            clientSocket = myServerSocket.accept();
            IN = new DataInputStream(clientSocket.getInputStream());
            String userName = IN.readUTF();
            Server.clientUsernameList.add(userName);

            System.out.println("Client have connected from:" + clientSocket.getLocalAddress().getHostName());
            System.out.println("Username:" + userName);
            System.out.println(Server.clientUsernameList);

        } catch (Exception e) {
            //Print out exception
            System.out.println(e);
        }
        //For-loop that counts every element in Array-clientsConnected
        for (int i = 0; i < clientsConnected.length; i++) {
            //If-statement checks if current element is null 
            if(clientsConnected[i] == null){
                //If current element in the Array is null then create a new object from ClientThread class
                //With a socket and the object of itself as parameter.
                (clientsConnected[i] = new ClientThread(clientSocket, clientsConnected)).start();

                //Must have a break otherwise it will create 20 objects (Exit for-loop)
                break;
            }   //Exit if-statement
        }   //Exit for-loop
    }   //Exit while-loop

}

} I recieve the username from my Class Client when it is running. I print it out to see if I added a username to the list. Now when I try to access the data from my Class Client I recieve nothing/null.

package Server_application;

import java.io.DataInputStream;
import java.io.DataOutput;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.Scanner;
import java.util.TimeZone;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
*
* @author Seeya
*/
public class Client extends Thread {

ClientGUI GUI = null;
DataInputStream IN = null;
DataOutputStream OUT = null;
Socket SOCKET = null;
String Message = null; 
String userName = null;
String ipAdress = "localhost";
int port = 8900;

public Client(ClientGUI gui,String username) {
    this.userName = username;
    this.GUI = gui;
}



@Override
public void run() {
    try {
            //Handle Input/Output from client
            SOCKET = new Socket(ipAdress, port);
            IN = new DataInputStream(SOCKET.getInputStream());
            OUT = new DataOutputStream(SOCKET.getOutputStream());

            //Sends username to server!
            OUT.writeUTF(userName);                

            System.out.println(Server.clientUsernameList);
            GUI.addToList(Server.clientUsernameList.toString());
            while (true) {
                CheckForMessages(IN);
            }

    } catch (IOException ex) {
        Logger.getLogger(Client.class.getName()).log(Level.SEVERE, null, ex);
    }

 }
//-----------------------------------------------------------------
//Method for sending Messages!
public void write(String s) throws IOException{
    //Writes message to server so it can send it to other clients.
    //Also sends to clients self GUI
    String A = s;
    OUT.writeUTF(A);
    GUI.writeGUI("You said: " + A);
}
//------------------------------------------------------------------
//Method for listening on messages
private void CheckForMessages(DataInputStream in) throws IOException {
    try {
        String IncomingMessage = in.readUTF();        
        System.out.println(IncomingMessage);
        GUI.writeGUI(IncomingMessage);
    } catch (Exception e) {
        System.err.println(e);
    }
}
}

All that is printed out is [] from my Class Client. Is there something wrong with how I try to access my static variable?