Connect pc with arduino using Java Simple Serial Connector

Im trying to make connection between my computer and arduino uno using Java Simple Serial Connector. Im trying to do it using code listed below. Somehow its not working ( the led diode connected to pin 7 of arduino is not turning on while running my programm, but when im using serial monitor of artuino software it does. ). Does anyone know why?

Java project code :

    import jssc.SerialPort;
import jssc.SerialPortException;

public class Main {

    public static void main(String[] args) {
        //In the constructor pass the name of the port with which we work
        SerialPort serialPort = new SerialPort("COM3");
        try {
            //Open port
            serialPort.openPort();
            //We expose the settings. You can also use this line - serialPort.setParams(9600, 8, 1, 0);
            serialPort.setParams(SerialPort.BAUDRATE_9600,
                                 SerialPort.DATABITS_8,
                                 SerialPort.STOPBITS_1,
                                 SerialPort.PARITY_NONE);
            //Writes data to port
            serialPort.writeBytes("Test".getBytes());
            //Closing the port
            serialPort.closePort();
        }
        catch (SerialPortException ex) {
            System.out.println(ex);
        }
    }
}`

Arduino code:

void setup() {
  Serial.begin(9600); //Ustawienie prędkości transmisji
  pinMode(7, OUTPUT);
  digitalWrite(7, LOW);
}

void loop() {
  if( Serial.available() > 0){
    digitalWrite(7, HIGH);

  }

}

I think, your Arduinocode is wrong.

I do it like this.

https://www.arduino.cc/en/Serial/Write

Serial.write(val)

Serial.write(str) Serial.write(buf, len)

val: a value to send as a single byte str: a string to send as a series of bytes buf: an array to send as a series of bytes len: the length of the buffer