programming with java RXTX library.. Accessing the data through usb

i'm trying to access the data from an RFID reader connected to PC through USB, i came to know that i need to work with RXTX library and after a long study came up to code with rxtx but couldn't cope up with what i expected. My code goes like this..

import gnu.io.CommPort;
import gnu.io.CommPortIdentifier;
import gnu.io.SerialPort;
import java.io.FileDescriptor;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class mainrdr
{
						/** Milliseconds to block while waiting for port open */
		private static final int TIME_OUT = 2000;
		private static final int DATA_RATE = 9600;		/** Default bits per second for COM port. */
		public mainrdr()
		{
			super();
		}
		void connect(String portName) throws Exception
		{
		    CommPortIdentifier portIdentifier = CommPortIdentifier.getPortIdentifier(portName);
			if(portIdentifier.isCurrentlyOwned())
				{
		              		System.out.println("Error: Port is currently in use");
				}
			else
			{
			   CommPort commPort = portIdentifier.open(this.getClass().getName(),TIME_OUT);
				if(commPort instanceof SerialPort)
				{
			        	SerialPort serialPort = (SerialPort) commPort;
					serialPort.setSerialPortParams(DATA_RATE,
				                                             	SerialPort.DATABITS_8,
				 		                            SerialPort.STOPBITS_1,
					                               SerialPort.PARITY_NONE);
					InputStream in = serialPort.getInputStream();
					(new Thread(new SerialReader(in))).start(); //									
				}
				else
				{
				System.out.println("Error: Only serial ports are handled by this example.");
				}
			}
		}
		public static class SerialReader implements Runnable
		{
			InputStream in;
			public SerialReader(InputStream in)
			{
				this.in = in;
			}
			public void run()
			{
			byte[] buffer = new byte[1024];
			int len = -1;
			try
			{
            			while((len = this.in.read(buffer))>-1)
				{
	           			System.out.print(new String(buffer,0,len));
				}
			}
		        catch(IOException e)
			{
				e.printStackTrace();
			}
			}
		}
 
  public static void main(String[] args)
  {
       try
          {
            [size="5"][b](new mainrdr()).connect(args[0]);[/b][/size]
          }
       catch(Exception e)
       {
                                           // TODO Auto-generated catch block
          e.printStackTrace();
       }
  }
}

program has been compiled successfully, but at runtime it showed an exception in main block where i have high-lighted. I couldn't know why it couldn't access the data from the reader. Is there any problem with code..