send command to arduino via serial (usb) port.

SerialSwitch.java

import gnu.io.CommPortIdentifier;
import gnu.io.PortInUseException;
import gnu.io.SerialPort;
import gnu.io.UnsupportedCommOperationException;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.util.Enumeration;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class SerialSwitch implements ActionListener, Runnable {
    private static final String PORT_NAMES[] = {
        "/dev/tty.usbserial-A9007UX1", // Mac OS X
        "/dev/ttyUSB0", // Linux
        "COM3", // Windows
    };
    private static final int TIME_OUT = 2000;
    private static final int DATA_RATE = 9600;

    private SerialPort serialPort;
    private OutputStream output;
    private ByteArrayOutputStream bout;
    private OutputStreamWriter writer;

    private boolean on;

    public SerialSwitch() {
        bout = new ByteArrayOutputStream();
        writer = new OutputStreamWriter(bout);
    }

    @SuppressWarnings("CallToThreadDumpStack")
    public void initialize() {
        CommPortIdentifier portId = findPortId();
        if (portId == null) {
            System.out.println("Could not find COM port.");
            return;
        }
        try {
            serialPort = (SerialPort)portId.open(this.getClass().getName(), TIME_OUT);
            serialPort.setSerialPortParams(DATA_RATE,
                                           SerialPort.DATABITS_8,
                                           SerialPort.STOPBITS_1,
                                           SerialPort.PARITY_NONE);
            output = serialPort.getOutputStream();
            serialPort.notifyOnDataAvailable(true);
        } catch (PortInUseException
                 | UnsupportedCommOperationException
                 | IOException e) {
            e.printStackTrace();
        }
    }

    private CommPortIdentifier findPortId() {
        CommPortIdentifier portId = null;
        Enumeration portEnum = CommPortIdentifier.getPortIdentifiers();
        while (portEnum.hasMoreElements()) {
            CommPortIdentifier currPortId = (CommPortIdentifier)portEnum.nextElement();
            for (String portName : PORT_NAMES) {
                if (currPortId.getName().equals(portName)) {
                    portId = currPortId;
                    break;
                }
            }
        }
        return portId;
    }

    public synchronized void close() {
        if (serialPort == null) {
            return;
        }
        serialPort.close();
    }

    private void createAndShowGui() {
        JFrame frame = createFrame();
        frame.setVisible(true);
    }

    private JFrame createFrame() {
        JFrame frame = new JFrame("SerialSwitch");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setContentPane(createPane());
        frame.pack();
        return frame;
    }

    private JPanel createPane() {
        JPanel pane = new JPanel();
        pane.add(createButton());
        return pane;
    }

    private JButton createButton() {
        JButton button = new JButton("ON");
        button.addActionListener(this);
        return button;
    }

    @Override
    public void actionPerformed(ActionEvent event) {
        JButton button = (JButton)event.getSource();
        onButton(button);
    }

    @SuppressWarnings("CallToThreadDumpStack")
    private void onButton(JButton button) {
        try {
            String command = getCommand();
            writer.write(command, 0, command.length());
            writer.flush();
            bout.writeTo(output);
            button.setText(getButtonText());
            on = !on;
            bout.reset();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private String getCommand() {
        return on ? "OFF\n" : "ON\n";
    }

    private String getButtonText() {
        return on ? "ON" : "OFF";
    }

    @Override
    public void run() {
        createAndShowGui();
    }

    public static void main(String[] args) throws Exception {
        SerialSwitch main = new SerialSwitch();
        main.initialize();
        SwingUtilities.invokeLater(main);
        System.out.println("Started");
    }
}

SerialWithLED.cpp

// arduino sketch (need Arduino IDE 1.0)
class Control {
public:
  int pin;

public:
  Control(int pin) : pin(pin) {}
  ~Control() {}

  void pinMode(int mode) {
    ::pinMode(pin, mode);
  }

  void pinModeIn() {
    pinMode(INPUT);
  }

  void pinModeOut() {
    pinMode(OUTPUT);
  }

  void digitalWrite(int value) {
    ::digitalWrite(pin, value);
  }

  void digitalWriteHigh() {
    digitalWrite(HIGH);
  }

  void digitalWriteLow() {
    digitalWrite(LOW);
  }
};

class App {
public:
  Control* led;
  bool status;
  char buffer[128];
  String line;

public:
  App() : led(NULL), status(false) {
    led = new Control(13);
    clearBuffer();
  }

  ~App() {
    delete led;
  }

  void clearBuffer() {
    for (size_t i = 0; i < sizeof(buffer); ++i) {
      buffer[i] = 0;
    }
  }

  void setup() {
    led->pinModeOut();
    Serial.begin(9600);
    Serial.setTimeout(1000);
  }

  void loop() {
    int n = readLine(buffer, sizeof(buffer), line);
    if (n <= 0) {
      return;
    }
    if (line == "ON") {
      led->digitalWriteHigh();
    } else if (line == "OFF") {
      led->digitalWriteLow();
    }
  }

  int readLine(char* buf, int bufsiz, String& ret) {
    if (Serial.available() <= 0) {
      return -1;
    }
    int n = Serial.readBytesUntil('\n', buf, bufsiz - 1);
    if (n >= 0) {
      buf[n] = '\0';
      ret = buf;
    }
    return n;
  }
};

void* gApp;

void setup() {
  App* app = new App();
  app->setup();
  gApp = app;
}

void loop() {
  ((App*)gApp)->loop();
}

Please can u help me to code in java like if I give the input 'A' it will send the result to arduino UNO and on the LED of 'A'