How to Create a Chat Room in Java

import org.jgroups.JChannel;
import org.jgroups.Message;
import org.jgroups.ReceiverAdapter;
import org.jgroups.View;
import org.jgroups.util.Util;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.LinkedList;
import java.util.List;

public class GUIChat extends ReceiverAdapter {

    final List<String> history = new LinkedList<String>();

    private JChannel channel;
    private String userName = System.getProperty("user");
    private JTextField textField;
    private JTextArea textArea;

    public static void main(String[] args) throws Exception {
        new GUIChat().start();
    }

    public void viewAccepted(View newView) {
        appendText("** in room: " + newView + "\n");
    }

    public void receive(Message msg) {
        String line = msg.getObject().toString();
        synchronized(history) {
            history.add(line);
            appendText(line);
        }
    }

    public void getState(OutputStream output) throws Exception {
        synchronized(history) {
            Util.objectToStream(history, new DataOutputStream(output));
        }
    }

    public void setState(InputStream input) throws Exception {
        List<String> list = (List<String>) Util.objectFromStream(new DataInputStream(input));
        synchronized(history) {
            history.clear();
            history.addAll(list);
        }
        appendText("received " + list.size() + " messages in chat history:\n");
        for(String str: list) appendText(str);
    }

    private void start() throws Exception {
        final JPanel panel = createPanel();
        channel = new JChannel();
        channel.setReceiver(this);
        channel.connect("ChatCluster");
        channel.getState(null, 10000);
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame("Simple Chat");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(panel);
                frame.pack();
                frame.setVisible(true);
            }
        });
    }

    private JPanel createPanel() {
        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();
        c.gridwidth = GridBagConstraints.REMAINDER;
        c.fill = GridBagConstraints.HORIZONTAL;
        c.weightx = c.weighty = 1.0;
        textArea = new JTextArea(20, 40);
        textArea.setEditable(false);
        panel.add(new JScrollPane(textArea), c);

        c.weightx = c.weighty = 0.0;
        c.fill = GridBagConstraints.HORIZONTAL;

        JPanel messageContainer = new JPanel(new FlowLayout());
        messageContainer.add(new JLabel("Message:"));
        textField = new JTextField(40);
        textField.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                String line="[" + userName + "] " + textField.getText() + "\n";
                try {
                    channel.send(new Message(null, null, line));
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
        messageContainer.add(textField);

        JButton button = new JButton("Bye!");
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                channel.close();
                System.exit(0);
            }
        });
        messageContainer.add(button);

        panel.add(messageContainer, c);
        return panel;
    }

    private void appendText(String newline) {
        textArea.append(newline);
        textField.setText("");
        textArea.setCaretPosition(textArea.getDocument().getLength());
    }

}