Can't Send data to arduino using java

I have been trying to send just a character to arduino using java gnu.io library.It can read data from the serial port sent by the arduino but cant send data to the serial port.Here is my code where I am trying to send ana receive data

public synchronized void serialEvent(SerialPortEvent oEvent) {
    System.out.println("Event Listener Started");
    if (oEvent.getEventType() == SerialPortEvent.OUTPUT_BUFFER_EMPTY) {
        System.out.println("Sending Data");
        try {
            char ch = 'P';
            output.print(ch);
            output.flush();
        } catch (Exception e) {
            System.err.println(e.toString());
        }

    } 
    else if (oEvent.getEventType() == SerialPortEvent.DATA_AVAILABLE) {
        System.out.println("Received Data");
        try {
            String inputLine = input.readLine();
            System.out.println(inputLine);
        } catch (Exception e) {
            System.err.println(e.toString());
        }
    }



}

Here output is a printstream object;Other codes are just like the code I found in the following link http://playground.arduino.cc/Interfacing/Java#.UyWijKiSweg Any suggestion would be very helpful.