Comments by nameJulian (Top 14 by date)

and the continuity:

public static void main(String[] args) {

//ChatApplication program=new ChatApplication();
//program.createInterface();
SwingUtilities.invokeLater(new Runnable(){
public void run(){
String userName=JOptionPane.showInputDialog(window,"Name: "," ",JOptionPane.PLAIN_MESSAGE);
try{
String serverName="localhost";
new ChatApplication(userName, serverName);
window.setTitle(userName);

}catch(IOException ex){
JOptionPane.showMessageDialog(window,"Clientul nu se poate conecta."+ex.getMessage().toString());
}
}

});
}


public void actionPerformed(ActionEvent e) {

if(e.getSource()==sendButton){
sendFlag=true;

try{
String messageToServer=new String("");
messageToServer=messageField.getText();

output.println(messageToServer);
messageField.setText("");

}catch(NullPointerException ex){
System.err.println(ex.getMessage());
ex.printStackTrace();
}
}
}



/*
* clasa se ocupa cu crearea unui thread pentru a putea citi de la server
*/

class ServerMessageThread extends Thread{


public void run(){

String serverResponse=new String();

try {
while(true){
serverResponse=input.readLine();
messagesTextArea.append(serverResponse+"\n");
}
} catch (IOException e) {
messagesTextArea.setForeground(Color.RED);
messagesTextArea.setText("Eroare citire de la server: "+e.getMessage());
}
}
}

/*
* clasa care se ocupa cu crearea firelor de executie a clientilor
*/

}

this is the client class:
import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.border.TitledBorder;


public class ChatApplication implements ActionListener{

static JFrame window;
static Socket clientSocket;
Socket serverClientSocket;
BufferedReader input;
PrintWriter output;
static JTextArea messagesTextArea;
static JTextArea usersTextArea;
static JTextField messageField;
JButton sendButton;
JButton connectButton;
JButton startServerButton;
String userName=new String();
static boolean sendFlag=false;


public ChatApplication(){}

public ChatApplication(String userName,String serverName) throws UnknownHostException, IOException{
this.userName=userName;
clientSocket=new Socket(serverName,9999);
input=new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
output=new PrintWriter(new OutputStreamWriter(clientSocket.getOutputStream()),true);
output.println(userName);
createInterface();
new ServerMessageThread().start();
}

public void createInterface(){
window=new JFrame();
window.setSize(600,600);
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setLayout(new BoxLayout(window.getContentPane(),BoxLayout.Y_AXIS));

sendButton=new JButton("Send");
connectButton=new JButton("Connect");
startServerButton=new JButton("Start");

messageField=new JTextField(10);
messagesTextArea=new JTextArea(40,40);
messagesTextArea.setEditable(false);
usersTextArea=new JTextArea(40,10);
usersTextArea.setEditable(false);

JScrollPane scrollPane=new JScrollPane(messagesTextArea,JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED,
JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);

JPanel initialPanel=new JPanel();
initialPanel.setLayout(new BoxLayout(initialPanel,BoxLayout.X_AXIS));
initialPanel.add(connectButton);
initialPanel.add(startServerButton);

JPanel upPanel=new JPanel();
upPanel.setLayout(new BoxLayout(upPanel,BoxLayout.X_AXIS));
upPanel.setBorder(new TitledBorder(""));
upPanel.add(scrollPane);
upPanel.add(usersTextArea);

JPanel bottomPanel=new JPanel();
bottomPanel.setLayout(new BoxLayout(bottomPanel,BoxLayout.X_AXIS));
bottomPanel.setBorder(new TitledBorder(""));
bottomPanel.add(messageField);
bottomPanel.add(sendButton);

window.add(initialPanel);
window.add(upPanel);
window.add(bottomPanel);

sendButton.addActionListener(this);

// connectButton.addActionListener(new ActionListener(){
//
// public void actionPerformed(ActionEvent e){
//
// String userName=JOptionPane.showInputDialog(window,"Name: "," ",JOptionPane.PLAIN_MESSAGE);
// try{
// String serverName="localhost";
// new ChatApplication(userName, serverName);
// window.setTitle(userName);
//
// }catch(IOException ex){
// JOptionPane.showMessageDialog(window,"Clientul nu se poate conecta."+ex.getMessage().toString());
// }
//
// }
//
// });

startServerButton.addActionListener(new ActionListener(){

public void actionPerformed(ActionEvent e){
Thread thread=new Thread(){
public void run(){

ChatServer server=new ChatServer();
try {
server.process();
} catch (Exception e) {
System.err.println(e.getMessage());
e.printStackTrace();
}
}
};

thread.start();


}

});

}

public static void main(String[] args) {

//ChatApplication

I had given up up the default constructor and called the parameterized constructor directrly intro the main method( in the constroctor calling the createInterface method), like this:
public static void main(String...args){
try{

String userName=JOptionPane.showInputDialog(window,"Name: "," ",JOptionPane.PLAIN_MESSAGE);
try{
String serverName="localhost";
new ChatApplication(userName, serverName); //call to appropriate constructor
window.setTitle(userName);
}catch(IOException e){
System.err.println(e.toString());
}

}//main
But i'm still geting the same exception.

The exception occurs in the actionPerformed method of sendButton, on the line: output.println(messageToServer);