A multi-client Chat application with File Transfer

I have been working on this project for 3 months already and I spent most of my time to learn about Socket programming and Threads. I have developed this Multi-Client Chat application using several techniques and it was working perfectly but later I noticed I was having difficulties to integrate the File Transfer function. I am also planning to add Video Call and Audio Chat. I have then added a ChatMessage class.
I have the ServerGUI class, MainGUI class and ChatMessage class. The ChatMessage class helps to create an object to send request to both server and client side. Now my problem is that I am having a NullPointerException error. The problem seems to be in the ServerGUI class. It seems that the username array I have created is wrong or the methods I created to findclient(), senduserlist() are wrong. Each time I am running my application the server accepts two clients and my program stop for the third one.
I would be grateful if anybody could give me a hint on the cause of error. Thank you.

ServerGUI class

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.net.*;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.*;
import java.util.zip.GZIPOutputStream;
import javax.swing.SwingUtilities;

public class ServerGUI extends JFrame implements ActionListener {

    public JList online;
    private JTextField ipaddress, textMessage;
    private JButton send, start, disconnect;
    private JTextArea chatArea;
    private JLabel port;
    int client[] = new int[100];
    private ObjectOutputStream out[] = new ObjectOutputStream[client.length+ 1];
    private ObjectInputStream in[] = new ObjectInputStream[client.length+ 1];
    String username[] = new String[client.length+1];
    static String b;
    public String nm, usm;
    private ServerSocket server;
    private Socket connect;
    boolean success = true;
    int id=0;
   
    ArrayList<String> UserList = new ArrayList<String>();
   

    public ServerGUI() {
        Container c = getContentPane();
        c.setLayout(new BorderLayout());
        c.setPreferredSize(new Dimension(650, 500));


        JPanel p = new JPanel();
        p.setLayout(new FlowLayout());
        p.setBackground(Color.LIGHT_GRAY);
        p.add(port = new JLabel("Port No"));
        p.add(ipaddress = new JTextField("1500"));
        p.add(start = new JButton("START"));
        p.add(disconnect = new JButton("DISCONNECT"));
        disconnect.setEnabled(false);
        start.setBorderPainted(false);
        start.setBackground(Color.blue);
        start.setForeground(Color.WHITE);
        disconnect.setBorderPainted(false);
        disconnect.setBackground(Color.blue);
        disconnect.setForeground(Color.WHITE);
        ipaddress.setCaretPosition(0);

        JPanel p1 = new JPanel();
        p1.setLayout(new FlowLayout());
        p1.setBackground(Color.LIGHT_GRAY);
        p1.add(chatArea = new JTextArea());

        chatArea.setPreferredSize(new Dimension(300, 350));
        chatArea.setLineWrap(true);
        chatArea.setEditable(false);

        JPanel p2 = new JPanel();
        p2.setLayout(new FlowLayout());
        p2.setBackground(Color.LIGHT_GRAY);
        p2.add(textMessage = new JTextField(20));

        p2.add(send = new JButton("SEND"));
        send.setBackground(Color.blue);
        send.setForeground(Color.WHITE);
        send.setBorderPainted(false);

        start.addActionListener(this);
        send.addActionListener(this);


        c.add(p, BorderLayout.NORTH);
        c.add(p1, BorderLayout.CENTER);
        c.add(p2, BorderLayout.SOUTH);


    }
    //current time
    SimpleDateFormat log = new SimpleDateFormat("HH:mm");
    String d = log.format(new Date());

    //Start server
    public void Start() {

        int portNo = 0;
        try {
            
            String no = ipaddress.getText();
            portNo = Integer.parseInt(no);
            chatArea.append("Connection to port " + portNo + "...\n");
            server = new ServerSocket(portNo);
            success = true;

        } catch (Exception ex) {
            chatArea.append("Error cannot bind to port \n");
            success = false;
        }

        if (success == true) {
            addClient ob1 = new addClient("RunServer");
            start.setEnabled(false);
            disconnect.setEnabled(true);
        }
    }

    public class addClient implements Runnable {

        Thread t;

        addClient(String tot) {
            t = new Thread(this, tot);
            t.start();
        }

        public void run() {
            while (true) {
                try {
                    try {
                        WaitClient();
                    } catch (Exception ex) {
                        break;
                    }
                    
                    for (int i = 0; i < client.length; i++) {
                        if (client[i] == 0) {
                            client[i] = i + 1;
                            id = i;
                            break;
                        }
                    }
                  
                  
                    //set stream to send and receive data
                    out[client[id]] = new ObjectOutputStream(connect.getOutputStream());
                    out[client[id]].flush();
                    in[client[id]] = new ObjectInputStream(connect.getInputStream());
                    chatArea.append(d + " Client:[" + client[id] + "] : Connected successful \n");
                   
                    //inform user that connection is successfull                    
                    sendUser(client[id], "Connected to the server");
                    
                    ChatMessage cm = (ChatMessage) in[client[id]].readObject();
                    handle(client[id], cm);
                    
                    
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }

    
    
    public synchronized void handle(int num, ChatMessage cm){
        
        if(cm.type.equals("login")){
             username[client[findClient(num)]] = cm.sender;             
             send(client[findClient(num)], new ChatMessage("login", "SERVER", "TRUE", cm.sender));
             Announce("newuser", "SERVER", cm.sender);
             SendUserList(cm.sender);
        
        }else if(cm.type.equals("message")){
                if(cm.recipient.equals("All")){
                Announce("message", cm.sender, cm.content);
        }
                else{
                    send(findUserThread(cm.recipient), new ChatMessage(cm.type, cm.sender, cm.content, cm.recipient));
                    send(client[findClient(num)], new ChatMessage(cm.type, cm.sender, cm.content, cm.recipient));
                }
            }
    
    
    
    }
    
      public void SendUserList(String toWhom){
        for(int i = 0; i < id; i++){
            send(findUserThread(toWhom), new ChatMessage("newuser", "SERVER", username[client[i]], toWhom));
        }
    }
    
    public int findUserThread(String usr){
        for(int i = 0; i < id; i++){
            
            if(username[client[i]].equals(usr)){
               
                return client[i];
          
        }
        
        }
        
        return id;
    }
     private int findClient(int n){  
    	for (int i = 0; i < id; i++){
        	if (client[i] == n){
                   return client[i];
                   
                }
               
	}
	return id;
    }
    
   
                            
  
//wait for connection, then display connection information
    private void WaitClient() throws IOException {

        chatArea.append(d + " : Waiting for connection... \n");
        connect = server.accept();

        chatArea.append(d + " : Now connected to " + connect.getInetAddress().getHostName() + "\n");
    }

    //send message to specific user
    public void send(int number, ChatMessage cm) {

        try {
            out[number].writeObject(cm);
            out[number].flush();
        } catch (Exception e) {
        }

    }
    
    public void sendUser(int number, String info) {

        try {
            out[number].writeObject(info);
            out[number].flush();
        } catch (Exception e) {
        }

    }
    
     public void Announce(String type, String sender, String content){
        ChatMessage cm = new ChatMessage(type, sender, content, "All");
        for(int i = 0; i < id; i++){
            send(client[i], cm);
        }
    }


    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == send) {
         

        } else if (e.getSource() == start) {
            Start();
        }
        if (e.getSource() == disconnect) {

       
        }
    }
}

MainGUI class

import java.io.*;
import java.net.*;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.io.BufferedOutputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import javax.swing.DefaultListModel;
import javax.swing.JFileChooser;
import javax.swing.JOptionPane;

public class MainGUI extends javax.swing.JFrame {
    
    private ObjectOutputStream out;
    private ObjectInputStream in;
    static String b; //variable for message
    private Socket join;
    boolean success = true;
    private String serverIP = "10.1.1.1"; //set IP Address
    ArrayList<String> userlist = new ArrayList<String>(); //ArrayList to store online users
    //current time
    SimpleDateFormat log = new SimpleDateFormat("HH:mm");
    String d = log.format(new Date());
    public File file;
    public String serverAddr, username, password;
    DefaultListModel model = new DefaultListModel(); 
     

    public MainGUI() {
        initComponents();
        
      
        
        
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {

        jButton3 = new javax.swing.JButton();
        start = new javax.swing.JButton();
        jLabel2 = new javax.swing.JLabel();
        textMessage = new javax.swing.JTextField();
        jScrollPane2 = new javax.swing.JScrollPane();
        chatArea = new javax.swing.JTextArea();
        usernm = new javax.swing.JTextField();
        jLabel1 = new javax.swing.JLabel();
        send = new javax.swing.JButton();
        jScrollPane3 = new javax.swing.JScrollPane();
        online = new javax.swing.JList();
        upload = new javax.swing.JButton();
        filename = new javax.swing.JTextField();
        stop = new javax.swing.JButton();
        login = new javax.swing.JButton();
        jLabel3 = new javax.swing.JLabel();
        ip = new javax.swing.JTextField();
        jLabel4 = new javax.swing.JLabel();
        portnumber = new javax.swing.JTextField();
        jLabel5 = new javax.swing.JLabel();
        passwordfield = new javax.swing.JPasswordField();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("ICARE ");
        setFocusable(false);
        setPreferredSize(new java.awt.Dimension(840, 650));
        setResizable(false);
        getContentPane().setLayout(null);

        jButton3.setText("Send");
        jButton3.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton3ActionPerformed(evt);
            }
        });
        getContentPane().add(jButton3);
        jButton3.setBounds(710, 110, 100, 30);

        start.setBackground(new java.awt.Color(51, 51, 255));
        start.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
        start.setForeground(new java.awt.Color(255, 255, 255));
        start.setText("Start");
        start.setBorder(null);
        start.setBorderPainted(false);
        start.setRequestFocusEnabled(false);
        start.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                startActionPerformed(evt);
            }
        });
        getContentPane().add(start);
        start.setBounds(20, 100, 90, 40);
        getContentPane().add(jLabel2);
        jLabel2.setBounds(0, 0, 820, 0);
        getContentPane().add(textMessage);
        textMessage.setBounds(270, 450, 440, 70);

        chatArea.setColumns(20);
        chatArea.setFont(new java.awt.Font("Arial", 0, 14)); // NOI18N
        chatArea.setRows(5);
        jScrollPane2.setViewportView(chatArea);

        getContentPane().add(jScrollPane2);
        jScrollPane2.setBounds(270, 150, 540, 290);
        getContentPane().add(usernm);
        usernm.setBounds(540, 10, 170, 30);

        jLabel1.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        jLabel1.setText("Enter your nickname :");
        getContentPane().add(jLabel1);
        jLabel1.setBounds(380, 10, 160, 30);

        send.setBackground(new java.awt.Color(51, 51, 255));
        send.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
        send.setForeground(new java.awt.Color(255, 255, 255));
        send.setText("Send");
        send.setBorder(null);
        send.setBorderPainted(false);
        send.setRequestFocusEnabled(false);
        send.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                sendActionPerformed(evt);
            }
        });
        getContentPane().add(send);
        send.setBounds(720, 470, 90, 40);

        online.setBackground(new java.awt.Color(0, 0, 0));
        online.setFont(new java.awt.Font("Tahoma", 0, 12)); // NOI18N
        online.setForeground(new java.awt.Color(102, 255, 0));
        jScrollPane3.setViewportView(online);

        getContentPane().add(jScrollPane3);
        jScrollPane3.setBounds(20, 150, 230, 370);

        upload.setFont(new java.awt.Font("Tahoma", 0, 18)); // NOI18N
        upload.setText("+");
        upload.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                uploadActionPerformed(evt);
            }
        });
        getContentPane().add(upload);
        upload.setBounds(650, 110, 50, 30);
        getContentPane().add(filename);
        filename.setBounds(270, 110, 370, 30);

        stop.setBackground(new java.awt.Color(51, 51, 255));
        stop.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
        stop.setForeground(new java.awt.Color(255, 255, 255));
        stop.setText("Log Out");
        stop.setToolTipText("");
        stop.setBorder(null);
        stop.setBorderPainted(false);
        stop.setRequestFocusEnabled(false);
        stop.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                stopActionPerformed(evt);
            }
        });
        getContentPane().add(stop);
        stop.setBounds(720, 60, 90, 40);

        login.setBackground(new java.awt.Color(51, 51, 255));
        login.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
        login.setForeground(new java.awt.Color(255, 255, 255));
        login.setText("Log In");
        login.setToolTipText("");
        login.setBorder(null);
        login.setBorderPainted(false);
        login.setRequestFocusEnabled(false);
        login.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                loginActionPerformed(evt);
            }
        });
        getContentPane().add(login);
        login.setBounds(720, 10, 90, 40);

        jLabel3.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        jLabel3.setText("Port No      :");
        jLabel3.setToolTipText("");
        getContentPane().add(jLabel3);
        jLabel3.setBounds(20, 60, 100, 30);
        getContentPane().add(ip);
        ip.setBounds(110, 20, 150, 30);

        jLabel4.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        jLabel4.setText("IP Address :");
        jLabel4.setToolTipText("");
        getContentPane().add(jLabel4);
        jLabel4.setBounds(20, 20, 100, 30);
        getContentPane().add(portnumber);
        portnumber.setBounds(110, 60, 150, 30);

        jLabel5.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
        jLabel5.setText("Enter your password :");
        getContentPane().add(jLabel5);
        jLabel5.setBounds(380, 60, 160, 30);
        getContentPane().add(passwordfield);
        passwordfield.setBounds(540, 60, 170, 30);

        pack();
    }// </editor-fold>//GEN-END:initComponents

    private void startActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_startActionPerformed

        Start();
        String group = "All";
        online.setModel(model);
        model.addElement(group);
        online.setSelectedIndex(0);
        send(new ChatMessage("test", "testUser", "testContent", "SERVER"));

       
    }//GEN-LAST:event_startActionPerformed

    private void sendActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_sendActionPerformed
        // TODO add your handling code here:
        String z = textMessage.getText();
        String target = online.getSelectedValue().toString();
       
           
        if(!z.isEmpty() && !target.isEmpty()){
            textMessage.setText("");
            send(new ChatMessage("message", username, z, target));
        }

    }//GEN-LAST:event_sendActionPerformed

    private void uploadActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_uploadActionPerformed
        // TODO add your handling code here:
        //Create a file chooser
        //In response to a button click:
        JFileChooser fileChooser = new JFileChooser();
        fileChooser.showDialog(this, "Select File");
        file = fileChooser.getSelectedFile();

        if (file != null) {
            if (!file.getName().isEmpty()) {
                String str;

                if (filename.getText().length() > 30) {
                    String t = file.getPath();
                    str = t.substring(0, 20) + " [...] " + t.substring(t.length() - 20, t.length());
                } else {
                    str = file.getPath();
                }
                filename.setText(str);
            }
        }
    }//GEN-LAST:event_uploadActionPerformed

    private void jButton3ActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_jButton3ActionPerformed
        // TODO add your handling code here:
        long size = file.length();
        if (size < 120 * 1024 * 1024) {
             send(new ChatMessage("upload_req", username, file.getName(), online.getSelectedValue().toString()));
          

        } else {
            chatArea.append("File is size too large\n");
        }
    }//GEN-LAST:event_jButton3ActionPerformed

    private void stopActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_stopActionPerformed
        // TODO add your handling code here:
        if (!username.isEmpty()) {
            send(new ChatMessage("logout", username, password, "SERVER"));
        }
    }//GEN-LAST:event_stopActionPerformed

    private void loginActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_loginActionPerformed
        // TODO add your handling code here:
        username = usernm.getText();
        password = passwordfield.getText();
        
        if(!username.isEmpty() && !password.isEmpty()){
            send(new ChatMessage("login", username, password, "SERVER"));
        }
    }//GEN-LAST:event_loginActionPerformed

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(MainGUI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(MainGUI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(MainGUI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(MainGUI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new MainGUI().setVisible(true);

            }
        });
    }
    // Variables declaration - do not modify//GEN-BEGIN:variables
    public javax.swing.JTextArea chatArea;
    public javax.swing.JTextField filename;
    public javax.swing.JTextField ip;
    public javax.swing.JButton jButton3;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JLabel jLabel4;
    private javax.swing.JLabel jLabel5;
    private javax.swing.JScrollPane jScrollPane2;
    private javax.swing.JScrollPane jScrollPane3;
    public javax.swing.JButton login;
    public javax.swing.JList online;
    public javax.swing.JPasswordField passwordfield;
    public javax.swing.JTextField portnumber;
    public javax.swing.JButton send;
    public javax.swing.JButton start;
    public javax.swing.JButton stop;
    private javax.swing.JTextField textMessage;
    public javax.swing.JButton upload;
    public javax.swing.JTextField usernm;
    // End of variables declaration//GEN-END:variables

  //Start client program
    public void Start() {
        
        
        try {
            
            start.setEnabled(false);
            chatArea.append(d + " : Attempting connection... \n");
            
            join = new Socket(serverAddr, 10500);
            chatArea.append(d + " : Connected to - " + join.getInetAddress().getHostName() + "\n");
            
            
            success = true;

        } catch (Exception ex) {
            chatArea.append("Error cannot bind to port \n");            
            success = false;
        }

        if (success == true) {

            ClientThread ct = new ClientThread();
            

        }
    }

    class ClientThread implements Runnable {

        ClientThread ct;
        Thread t;

        ClientThread() {
            t = new Thread(this, "RunClient");
            t.start();
        }

        public void run() {
            try {
                try {
                    out = new ObjectOutputStream(join.getOutputStream());
                    out.flush();
                    in = new ObjectInputStream(join.getInputStream());
                    b = (String) in.readObject();
                      
                    chatArea.append(b + "\n");
                    chatArea.setCaretPosition(chatArea.getText().length());
                                    
                } catch (Exception e) {   }

                CThread c1 = new CThread();


            } catch (Exception ex) {   }
        }
    }

    class CThread implements Runnable {

        CThread ob1;
        Thread t;

        CThread() {
            t = new Thread(this, "Message");
            t.start();
        }

        public void run() {
            try {
                do {
                    try {
                       ChatMessage cm = (ChatMessage) in.readObject();
                       System.out.println("Incoming : "+cm.toString());
                       if(cm.type.equals("login")){
                            if(cm.content.equals("TRUE")){
                            chatArea.append("[SERVER > Me] : Login Successful\n"); 
                            } else{
                            chatArea.append("[SERVER > Me] : Login Failed\n");
                            }
                    } else if(cm.type.equals("test")){
                         start.setEnabled(false);
                    }else if(cm.type.equals("message")){
                    if(cm.recipient.equals(username)){
                        chatArea.append("["+cm.sender +" > Me] : " + cm.content + "\n");
                    }
                    else{
                        chatArea.append("["+ cm.sender +" > "+ cm.recipient +"] : " + cm.content + "\n");
                    } 
                    }else if(cm.type.equals("newuser")){
                    
                            online.setModel(model);
                            model.addElement(cm.content); 
                        
                    }else if(cm.type.equals("upload_req")){
                    
                                            
                        JFileChooser jf = new JFileChooser();
                        jf.setSelectedFile(new File(cm.content));
                        int returnVal = jf.showSaveDialog(this);
                       
                        String saveTo = jf.getSelectedFile().getPath();
                        if(saveTo != null && returnVal == JFileChooser.APPROVE_OPTION){
                            Download dwn = new Download(saveTo);
                            Thread t = new Thread(dwn);
                            t.start();
                            //send(new Message("upload_res", (""+InetAddress.getLocalHost().getHostAddress()), (""+dwn.port), msg.sender));
                            send(new ChatMessage("upload_res", username, (""+dwn.port), cm.sender));
                        }
                        else{
                            send(new ChatMessage("upload_res", username, "NO", cm.sender));
                        }
                    }
                    else if(cm.type.equals("upload_res")){
                    if(!cm.content.equals("NO")){
                        int port  = Integer.parseInt(cm.content);
                        String addr = cm.sender;
                        
                    
                        Upload upl = new Upload(serverAddr, port, file);
                        Thread t = new Thread(upl);
                        t.start();
                    }
                    else{
                        chatArea.append("[SERVER > Me] : "+cm.sender+" rejected file request\n");
                    }
                }
                else{
                    chatArea.append("[SERVER > Me] : Unknown message type\n");
    }
    } catch (Exception ex) {
                        chatArea.append("Server is disconnected" + "\n");
                        
                        try {
                            in.close();
                            out.close();
                            join.close();
                        } catch (Exception ex2) {      }
                    }
                } while (!b.equalsIgnoreCase("bye"));
            } catch (Exception ex) {
            }
        }
    }
    
  
    void send(ChatMessage cm) {
        try {
            out.writeObject(cm);
            out.flush();
        } catch (Exception e) {
        }
    }


}

ChatMessage class

import java.io.Serializable;

public class ChatMessage implements Serializable{
    
    private static final long serialVersionUID = 1L;
    public String type, sender, content, recipient;
    
    public ChatMessage(String type, String sender, String content, String recipient){
        this.type = type; this.sender = sender; this.content = content; this.recipient = recipient;
    }
    
    @Override
    public String toString(){
        return "{type='"+type+"', sender='"+sender+"', content='"+content+"', recipient='"+recipient+"'}";
    }
}