The basic problem is, textArea is null when you try and call updateTextArea, this is because the invokeLater call hasn’t executed yet and constructed your UI, you basically have a race condition.

The normal way to deal with Sockets in Swing is, is to use a SwingWorker

Have a look at Concurrency in Swing and Worker Threads and SwingWorker for more details.

There are a number of ways you could solve you problem. You could use the SwingWorker to read the text from the socket and generate update notifications (through the publish/process) methods, this is commonly known as an “observer pattern”. This is good as it de-couples your code and generates a more reusable solution.

SocketReader

All this class does is reads text from the Socket and generates ActionEvents from the text, simple.

public class SocketReader extends SwingWorker<Void, String> {

    private List<ActionListener> actionListeners;

    public SocketReader() {
        actionListeners = new ArrayList<>(25);
    }

    public void addActionListener(ActionListener listener) {
        actionListeners.add(listener);
    }

    public void removeActionListener(ActionListener listener) {
        actionListeners.remove(listener);
    }

    @Override
    protected Void doInBackground() throws Exception {
        System.out.println("Connected to Server!");

        try (DataInputStream in = new DataInputStream(SocketManager.INSTACNE.getInputStream())) {

            System.out.println("Before setting text area");

            String serverInput = null;
            do {
                // HANDLE INPUT PART HERE
                serverInput = in.readUTF();

                if (serverInput != null) {
                    System.out.println("Read " + serverInput);
                    publish(serverInput);
                }

            } while (!serverInput.equals("/close"));
            System.out.println("Program closed");
        }
        return null;
    }

    @Override
    protected void process(List<String> chunks) {
        for (String text : chunks) {
            ActionEvent evt = new ActionEvent(this, ActionEvent.ACTION_PERFORMED, text);
            for (ActionListener listener : actionListeners) {
                listener.actionPerformed(evt);
            }
        }
    }

}

SocketWriter

This simple writes text to the Socket, simple. Technically, you don’t need to use a SwingWorker for this, but I like the fact that it’s relatively easy to cancel

public class SocketWriter extends SwingWorker<Void, Void> {

    private List<String> messages;
    private ReentrantLock lock;
    private Condition waitCon;

    public SocketWriter() {
        messages = Collections.synchronizedList(new ArrayList<String>(25));
        lock = new ReentrantLock();
        waitCon = lock.newCondition();
    }

    public void write(String text) {
        System.out.println("Write " + text);
        messages.add(text);
        try {
            lock.lock();
            waitCon.signalAll();
        } finally {
            lock.unlock();
        }
    }

    @Override
    protected Void doInBackground() throws Exception {
        try (DataOutputStream out = new DataOutputStream(SocketManager.INSTACNE.getOutputStream())) {
            while (!isCancelled()) {
                while (messages.isEmpty() && !isCancelled()) {
                    try {
                        lock.lock();
                        waitCon.await();
                    } finally {
                        lock.unlock();
                    }
                }
                List<String> cache = new ArrayList<>(messages);
                messages.clear();
                for (String text : cache) {
                    System.out.println("Send " + text);
                    out.writeUTF(text);
                }
            }
        }
        return null;
    }

}

SocketManager

Okay, this is slightly overkill on my part, but I want a central controller for the Socket, you don’t have to use a singleton, you could simply make it a simple class and pass a reference of it to your ChatClient and on down to the SocketReader/Writer, but it’s late and I’m lazy

public enum SocketManager {
    INSTACNE;

    private String host = "localhost";
    private int port = 1337;
    private Socket socket;

    public Socket open() throws IOException {
        if (socket != null) {
            close();
        }
        socket = new Socket(host, port);
        return socket;
    }

    public void close() throws IOException {
        if (socket == null) {
            return;
        }
        socket.close();
    }

    public boolean isOpen() {
        return socket != null
            && socket.isConnected()
            && !socket.isClosed()
            && !socket.isInputShutdown()
            && !socket.isOutputShutdown();
    }

    public InputStream getInputStream() throws IOException {
        Objects.requireNonNull(socket, "Socket is not open");
        return socket.getInputStream();
    }

    public OutputStream getOutputStream() throws IOException {
        Objects.requireNonNull(socket, "Socket is not open");
        return socket.getOutputStream();
    }
}

ChatClient

That’s all nice a awesome and all, but how are you suppose to use it?

Will, very basically, you create an instance of SocketReader and SocketWriter in your ChatClient, you attach an ActionListener to the reader and update the JTextArea when it’s triggered and send the text you want sent to the SocketWriter, for example…

public class ChatClient extends javax.swing.JFrame {

    public ChatClient() {
        initComponents();
        socketReader = new SocketReader();
        socketReader.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String text = e.getActionCommand();
                textArea.append(text);
                textArea.append("\n");
                textArea.setCaretPosition(textArea.getDocument().getLength());
            }
        });
        socketReader.execute();

        socketWriter = new SocketWriter();
        socketWriter.execute();
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        scrollPane = new javax.swing.JScrollPane();
        textArea = new javax.swing.JTextArea();
        btnConnect = new javax.swing.JButton();
        btnDisconnect = new javax.swing.JButton();
        lblStatus = new javax.swing.JLabel();
        lblShowStatus = new javax.swing.JLabel();
        txtInput = new javax.swing.JTextField();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("Chat Client A");

        textArea.setEditable(false);
        textArea.setColumns(20);
        textArea.setRows(5);
        textArea.setText("Welcome to the Chat Server. Type '/close' or Click 'Disconnect' to close.");
        textArea.setWrapStyleWord(true);
        textArea.setCaretPosition(textArea.getDocument().getLength());
        scrollPane.setViewportView(textArea);

        btnConnect.setText("Connect");
        btnConnect.setActionCommand("btnConnect");
        btnConnect.addMouseListener(new java.awt.event.MouseAdapter() {
            public void mouseClicked(java.awt.event.MouseEvent evt) {
                btnConnectMouseClicked(evt);
            }
        });

        btnDisconnect.setText("Disconnect");
        btnDisconnect.setActionCommand("btnDisconnect");
        btnDisconnect.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnDisconnectActionPerformed(evt);
            }
        });

        lblStatus.setText("Status: ");

        lblShowStatus.setFont(new java.awt.Font("Tahoma", 1, 11)); // NOI18N
        lblShowStatus.setForeground(new java.awt.Color(255, 51, 51));
        lblShowStatus.setText("Disconnected");

        txtInput.setToolTipText("");
        txtInput.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                txtInputActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(scrollPane)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(btnConnect)
                        .addGap(18, 18, 18)
                        .addComponent(btnDisconnect)
                        .addGap(42, 42, 42)
                        .addComponent(lblStatus)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(lblShowStatus)
                        .addGap(0, 42, Short.MAX_VALUE))
                    .addComponent(txtInput))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(scrollPane, javax.swing.GroupLayout.PREFERRED_SIZE, 213, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 11, Short.MAX_VALUE)
                .addComponent(txtInput, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(btnConnect)
                    .addComponent(btnDisconnect)
                    .addComponent(lblStatus)
                    .addComponent(lblShowStatus))
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    private void btnConnectMouseClicked(java.awt.event.MouseEvent evt) {
        // TODO add your handling code here:
        lblShowStatus.setFont(new java.awt.Font("Tahoma", 1, 11)); // NOI18N
        lblShowStatus.setForeground(new java.awt.Color(0, 204, 51));
        lblShowStatus.setText("Connected");

        // ADD CODES FOR CONNECTING TO CHAT SERVER
    }

    private void btnDisconnectActionPerformed(java.awt.event.ActionEvent evt) {
        // TODO add your handling code here:
        lblShowStatus.setFont(new java.awt.Font("Tahoma", 1, 11)); // NOI18N
        lblShowStatus.setForeground(new java.awt.Color(255, 51, 51));
        lblShowStatus.setText("Disconnected");

        // ADD CODES FOR DISCONNECTING FROM CHAT SERVER
    }

    private void txtInputActionPerformed(java.awt.event.ActionEvent evt) {
        if (SocketManager.INSTACNE.isOpen()) {
            socketWriter.write(txtInput.getText());
        } else {
            System.out.println("!! Not open");
        }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {

        try {
            /* 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(ChatClient.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
            } catch (InstantiationException ex) {
                java.util.logging.Logger.getLogger(ChatClient.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
            } catch (IllegalAccessException ex) {
                java.util.logging.Logger.getLogger(ChatClient.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
            } catch (javax.swing.UnsupportedLookAndFeelException ex) {
                java.util.logging.Logger.getLogger(ChatClient.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
            }
            //</editor-fold>

            SocketManager.INSTACNE.open();

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

        } catch (IOException ex) {
            ex.printStackTrace();
        }
        //</editor-fold>      

    }

    private SocketWriter socketWriter;
    private SocketReader socketReader;

    // Variables declaration - do not modify                     
    private javax.swing.JButton btnConnect;
    private javax.swing.JButton btnDisconnect;
    private javax.swing.JLabel lblShowStatus;
    private javax.swing.JLabel lblStatus;
    private javax.swing.JScrollPane scrollPane;
    private javax.swing.JTextArea textArea;
    private javax.swing.JTextField txtInput;
    // End of variables declaration                   
}

You’ll note, I used SocketManager#open in the main, sorry, missed you “connect” code. I would suggest moving that to that method instead 😉

ChatServer

I didn’t make to much of change to this, but just in case…

public class ChatServer {

    public static void main(String args[]) {
        int port = 1337;

        try {
            ServerSocket server = new ServerSocket(port);
            String inMessage = "";

            while (true) {
                System.out.println("Waiting");
                Socket clientA = server.accept();
                System.out.println("Connected");
                DataInputStream inA = new DataInputStream(clientA.getInputStream());
                DataOutputStream outA = new DataOutputStream(clientA.getOutputStream());
                // outA.writeUTF("Welcome to the Chat Server. Type '/close' or Click 'Disconnect' to close.");

                // for testing
                // BufferedReader user = new BufferedReader(new InputStreamReader(System.in));
                do {
                    inMessage = inA.readUTF();

                    if (inMessage != null) {
                        outA.writeUTF(inMessage);
                    }

                } while (!inMessage.equals("/close"));
                clientA.close();
            }

        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Normally, when a client connects, you’d start a new Thread and have it process the client Socket, but that wasn’t my focus.

So, based on all that, you have a lot of reading to catch up on, including Concurrency in Java and All About Sockets