I get this code from the internet which read from comm port of the computer & modify is as below:-

package COM;

/**
*
* @author kwngeaw
*/
import javax.comm.*;
import java.io.*;
import java.util.*;

public class ComControl implements Runnable, SerialPortEventListener {
static CommPortIdentifier portId1;
//static CommPortIdentifier portId2;

InputStream inputStream;
OutputStream outputStream;
SerialPort serialPort1;//, serialPort2;
Thread readThread;
protected String divertCode = "10";
static String TimeStamp;
static String kwngeaw = "";
Integer y = 0;

public void main() {
//Integer x = new Integer(0);
try {
portId1 = CommPortIdentifier.getPortIdentifier("COM8");
//portId2 = CommPortIdentifier.getPortIdentifier("COM8");
ComControl reader = new ComControl();

//sleep few sec
// Thread.sleep(5000);


// x = Integer.parseInt(kwngeaw.split(" ")[kwngeaw.split(" ").length - 1]);
//close running thread
//Thread.interrupted();

}

catch
(Exception e) {
TimeStamp = new java.util.Date().toString();
System.out.println(TimeStamp + ": COM8 " + portId1);
//System.out.println(TimeStamp + ": COM8 " + portId2);
System.out.println(TimeStamp + ": msg1 - " + e);
}


};


public ComControl() {

//========
try {
TimeStamp = new java.util.Date().toString();
serialPort1 = (SerialPort) portId1.open("ComControl", 2000);
System.out.println(TimeStamp + ": " + portId1.getName() + " opened for scanner input");
//serialPort2 = (SerialPort) portId2.open("ComControl", 2000);
//System.out.println(TimeStamp + ": " + portId2.getName() + " opened for diverter output");

} catch (PortInUseException e) {}

//========
try {
inputStream = serialPort1.getInputStream();
} catch (IOException e) {}

//========
try {
serialPort1.addEventListener(this);
} catch (TooManyListenersException e) {}

//========
serialPort1.notifyOnDataAvailable(true);

//========
try {

serialPort1.setSerialPortParams(2400,
SerialPort.DATABITS_7,
SerialPort.STOPBITS_1,
SerialPort.PARITY_EVEN);

serialPort1.setDTR(false);
serialPort1.setRTS(false);

//.setSerialPortParams(2400,
//SerialPort.DATABITS_7,
//SerialPort.STOPBITS_1,
//SerialPort.PARITY_EVEN);

} catch (UnsupportedCommOperationException e) {}


//========
readThread = new Thread(this);
readThread.start();
//========

}
public void run() {
try {
Thread.sleep(100);
} catch (InterruptedException e) {}
}
public void serialEvent(SerialPortEvent event) {
Integer x = new Integer(0);
switch(event.getEventType()) {
case SerialPortEvent.BI:
case SerialPortEvent.OE:
case SerialPortEvent.FE:
case SerialPortEvent.PE:
case SerialPortEvent.CD:
case SerialPortEvent.CTS:
case SerialPortEvent.DSR:
case SerialPortEvent.RI:
case SerialPortEvent.OUTPUT_BUFFER_EMPTY:
break;


case SerialPortEvent.DATA_AVAILABLE:
StringBuffer readBuffer = new StringBuffer();
int c;
try {
while ((c=inputStream.read()) != 10){
if(c!=13) readBuffer.append((char) c);
}
String scannedInput = readBuffer.toString();
TimeStamp = new java.util.Date().toString();
//System.out.println(TimeStamp + ": scanned input received:" + scannedInput);
kwngeaw = scannedInput;
x = Integer.parseInt(kwngeaw.split(" ")[kwngeaw.split(" ").length - 1]);
inputStream.close();
if(scannedInput.substring(0,1).equals("F")){
outputStream = serialPort1.getOutputStream();
outputStream.write(divertCode.getBytes());
//System.out.println(TimeStamp + ": diverter fired");
outputStream.close();
} else {
// System.out.println(TimeStamp + ": diverter not diverted");
}
} catch (IOException e) {}

break;
}
y=x;
System.out.println(y);
}
public Integer getWeight()
{
return y;
}
}
=========================================================================
In my other class, I have my desktop application which called the getWeight() function from this ComControl class when i click the Jbutton2:

package desktopapplication1;

import COM.ComControl;
import java.util.logging.Level;
import java.util.logging.Logger;
import org.jdesktop.application.Action;
import org.jdesktop.application.ResourceMap;
import org.jdesktop.application.SingleFrameApplication;
import org.jdesktop.application.FrameView;
import org.jdesktop.application.TaskMonitor;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.Timer;
import javax.swing.Icon;
import javax.swing.JDialog;
import javax.swing.JFrame;
import java.sql.*;
import java.io.*;
/**
* The application's main frame.
*/
public class DesktopApplication1View extends FrameView {




public DesktopApplication1View(SingleFrameApplication app) {
super(app);

initComponents();




// status bar initialization - message timeout, idle icon and busy animation, etc
ResourceMap resourceMap = getResourceMap();
int messageTimeout = resourceMap.getInteger("StatusBar.messageTimeout");
messageTimer = new Timer(messageTimeout, new ActionListener() {
public void actionPerformed(ActionEvent e) {
statusMessageLabel.setText("");
}
});
messageTimer.setRepeats(false);
int busyAnimationRate = resourceMap.getInteger("StatusBar.busyAnimationRate");
for (int i = 0; i < busyIcons.length; i++) {
busyIcons[i] = resourceMap.getIcon("StatusBar.busyIcons[" + i + "]");
}
busyIconTimer = new Timer(busyAnimationRate, new ActionListener() {
public void actionPerformed(ActionEvent e) {
busyIconIndex = (busyIconIndex + 1) % busyIcons.length;
statusAnimationLabel.setIcon(busyIcons[busyIconIndex]);
}
});
idleIcon = resourceMap.getIcon("StatusBar.idleIcon");
statusAnimationLabel.setIcon(idleIcon);
progressBar.setVisible(false);

// connecting action tasks to status bar via TaskMonitor
TaskMonitor taskMonitor = new TaskMonitor(getApplication().getContext());
taskMonitor.addPropertyChangeListener(new java.beans.PropertyChangeListener() {
public void propertyChange(java.beans.PropertyChangeEvent evt) {
String propertyName = evt.getPropertyName();
if ("started".equals(propertyName)) {
if (!busyIconTimer.isRunning()) {
statusAnimationLabel.setIcon(busyIcons[0]);
busyIconIndex = 0;
busyIconTimer.start();
}
progressBar.setVisible(true);
progressBar.setIndeterminate(true);
} else if ("done".equals(propertyName)) {
busyIconTimer.stop();
statusAnimationLabel.setIcon(idleIcon);
progressBar.setVisible(false);
progressBar.setValue(0);
} else if ("message".equals(propertyName)) {
String text = (String)(evt.getNewValue());
statusMessageLabel.setText((text == null) ? "" : text);
messageTimer.restart();
} else if ("progress".equals(propertyName)) {
int value = (Integer)(evt.getNewValue());
progressBar.setVisible(true);
progressBar.setIndeterminate(false);
progressBar.setValue(value);
}
}
});
}




/** 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() {

mainPanel = new javax.swing.JPanel();
btnCapture = new javax.swing.JButton();
jTextField1 = new javax.swing.JTextField();
jComboBox1 = new javax.swing.JComboBox();
jLabel1 = new javax.swing.JLabel();
jLabel2 = new javax.swing.JLabel();
jButton2 = new javax.swing.JButton();
jButton3 = new javax.swing.JButton();
jLabel3 = new javax.swing.JLabel();
menuBar = new javax.swing.JMenuBar();
javax.swing.JMenu fileMenu = new javax.swing.JMenu();
javax.swing.JMenuItem exitMenuItem = new javax.swing.JMenuItem();
javax.swing.JMenu helpMenu = new javax.swing.JMenu();
javax.swing.JMenuItem aboutMenuItem = new javax.swing.JMenuItem();
statusPanel = new javax.swing.JPanel();
javax.swing.JSeparator statusPanelSeparator = new javax.swing.JSeparator();
statusMessageLabel = new javax.swing.JLabel();
statusAnimationLabel = new javax.swing.JLabel();
progressBar = new javax.swing.JProgressBar();

mainPanel.setName("mainPanel"); // NOI18N

javax.swing.ActionMap actionMap = org.jdesktop.application.Application.getInstance(desktopapplication1.DesktopApplication1.class).getContext().getActionMap(DesktopApplication1View.class, this);
btnCapture.setAction(actionMap.get("showAboutBox")); // NOI18N
org.jdesktop.application.ResourceMap resourceMap = org.jdesktop.application.Application.getInstance(desktopapplication1.DesktopApplication1.class).getContext().getResourceMap(DesktopApplication1View.class);
btnCapture.setText(resourceMap.getString("btnCapture.text")); // NOI18N
btnCapture.setName("btnCapture"); // NOI18N

jTextField1.setColumns(20);
jTextField1.setName("jTextField1"); // NOI18N

jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
jComboBox1.setName("jComboBox1"); // NOI18N

jLabel1.setText(resourceMap.getString("jLabel1.text")); // NOI18N
jLabel1.setName("jLabel1"); // NOI18N

jLabel2.setText(resourceMap.getString("jLabel2.text")); // NOI18N
jLabel2.setName("jLabel2"); // NOI18N

jButton2.setText(resourceMap.getString("jButton2.text")); // NOI18N
jButton2.setName("jButton2"); // NOI18N
jButton2.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton2ActionPerformed(evt);
}
});

jButton3.setLabel(resourceMap.getString("jButton3.label")); // NOI18N
jButton3.setName("jButton3"); // NOI18N

jLabel3.setText(resourceMap.getString("jLabel3.text")); // NOI18N
jLabel3.setName("jLabel3"); // NOI18N

javax.swing.GroupLayout mainPanelLayout = new javax.swing.GroupLayout(mainPanel);
mainPanel.setLayout(mainPanelLayout);
mainPanelLayout.setHorizontalGroup(
mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, mainPanelLayout.createSequentialGroup()
.addContainerGap()
.addComponent(jButton3, javax.swing.GroupLayout.PREFERRED_SIZE, 119, javax.swing.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
.addGroup(mainPanelLayout.createSequentialGroup()
.addComponent(jLabel1)
.addGap(27, 27, 27))
.addGroup(mainPanelLayout.createSequentialGroup()
.addComponent(jLabel2)
.addGap(18, 18, 18)))
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(mainPanelLayout.createSequentialGroup()
.addComponent(jLabel3, javax.swing.GroupLayout.PREFERRED_SIZE, 84, javax.swing.GroupLayout.PREFERRED_SIZE)
.addContainerGap())
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, mainPanelLayout.createSequentialGroup()
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(jComboBox1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGroup(mainPanelLayout.createSequentialGroup()
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)
.addComponent(jTextField1, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(btnCapture, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(jButton2, javax.swing.GroupLayout.PREFERRED_SIZE, 68, javax.swing.GroupLayout.PREFERRED_SIZE)))
.addContainerGap())))
);
mainPanelLayout.setVerticalGroup(
mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(mainPanelLayout.createSequentialGroup()
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(mainPanelLayout.createSequentialGroup()
.addGap(22, 22, 22)
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jComboBox1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(jLabel1))
.addGap(18, 18, 18)
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(jTextField1)
.addComponent(jLabel2))
.addGap(13, 13, 13))
.addGroup(mainPanelLayout.createSequentialGroup()
.addContainerGap()
.addComponent(jButton3, javax.swing.GroupLayout.PREFERRED_SIZE, 82, javax.swing.GroupLayout.PREFERRED_SIZE)))
.addGap(13, 13, 13)
.addComponent(jLabel3, javax.swing.GroupLayout.PREFERRED_SIZE, 33, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(81, 81, 81)
.addGroup(mainPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(btnCapture)
.addComponent(jButton2, javax.swing.GroupLayout.PREFERRED_SIZE, 23, javax.swing.GroupLayout.PREFERRED_SIZE))
.addContainerGap())
);

menuBar.setName("menuBar"); // NOI18N

fileMenu.setText(resourceMap.getString("fileMenu.text")); // NOI18N
fileMenu.setName("fileMenu"); // NOI18N

exitMenuItem.setAction(actionMap.get("quit")); // NOI18N
exitMenuItem.setName("exitMenuItem"); // NOI18N
fileMenu.add(exitMenuItem);

menuBar.add(fileMenu);

helpMenu.setText(resourceMap.getString("helpMenu.text")); // NOI18N
helpMenu.setName("helpMenu"); // NOI18N

aboutMenuItem.setAction(actionMap.get("showAboutBox")); // NOI18N
aboutMenuItem.setName("aboutMenuItem"); // NOI18N
helpMenu.add(aboutMenuItem);

menuBar.add(helpMenu);

statusPanel.setName("statusPanel"); // NOI18N

statusPanelSeparator.setName("statusPanelSeparator"); // NOI18N

statusMessageLabel.setName("statusMessageLabel"); // NOI18N

statusAnimationLabel.setHorizontalAlignment(javax.swing.SwingConstants.LEFT);
statusAnimationLabel.setName("statusAnimationLabel"); // NOI18N

progressBar.setName("progressBar"); // NOI18N

javax.swing.GroupLayout statusPanelLayout = new javax.swing.GroupLayout(statusPanel);
statusPanel.setLayout(statusPanelLayout);
statusPanelLayout.setHorizontalGroup(
statusPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(statusPanelSeparator, javax.swing.GroupLayout.DEFAULT_SIZE, 439, Short.MAX_VALUE)
.addGroup(statusPanelLayout.createSequentialGroup()
.addContainerGap()
.addComponent(statusMessageLabel)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 269, Short.MAX_VALUE)
.addComponent(progressBar, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(statusAnimationLabel)
.addContainerGap())
);
statusPanelLayout.setVerticalGroup(
statusPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(statusPanelLayout.createSequentialGroup()
.addComponent(statusPanelSeparator, javax.swing.GroupLayout.PREFERRED_SIZE, 2, javax.swing.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.addGroup(statusPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
.addComponent(statusMessageLabel)
.addComponent(statusAnimationLabel)
.addComponent(progressBar, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(3, 3, 3))
);

setComponent(mainPanel);
setMenuBar(menuBar);
setStatusBar(statusPanel);
}// </editor-fold>

private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
try
{
Integer kwngeaw=0;

ComControl reader = new ComControl();
reader.main();

kwngeaw = d.getWeight();
System.out.println(kwngeaw);

}

catch(Exception e)
{
System.out.println(e);
}

finally

{

}

}

// Variables declaration - do not modify
private javax.swing.JButton btnCapture;
private javax.swing.JButton jButton2;
private javax.swing.JButton jButton3;
private javax.swing.JComboBox jComboBox1;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JTextField jTextField1;
private javax.swing.JPanel mainPanel;
private javax.swing.JMenuBar menuBar;
private javax.swing.JProgressBar progressBar;
private javax.swing.JLabel statusAnimationLabel;
private javax.swing.JLabel statusMessageLabel;
private javax.swing.JPanel statusPanel;
// End of variables declaration

private final Timer messageTimer;
private final Timer busyIconTimer;
private final Icon idleIcon;
private final Icon[] busyIcons = new Icon[15];
private int busyIconIndex = 0;

private JDialog aboutBox;
}
======================================================================
This system always printout null pointer exception. Is there anything wrong with the program? It don't have any compilation error using netbean 6.9