How to convet reader to double?

I have this set of coding which read the input from the com1 port. I wan to ask how to convert the input (the reader) into Double so that the input can perform arithmetic operation?

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package javaapplication1;
import java.io.*;
import java.util.*;
import javax.comm.*;
/**
*
* @author kwngeaw
*/
public class SimpleRead implements Runnable, SerialPortEventListener {
static CommPortIdentifier portId;
static Enumeration portList;

InputStream inputStream;
SerialPort serialPort;
Thread readThread;

public static void main(String[] args) {
portList = CommPortIdentifier.getPortIdentifiers();


while (portList.hasMoreElements()) {
portId = (CommPortIdentifier) portList.nextElement();
if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {
if (portId.getName().equals("COM1")) {
// if (portId.getName().equals("/dev/term/a")) {
SimpleRead reader = new SimpleRead();

}
}
}

}

public SimpleRead() {
try {
serialPort = (SerialPort) portId.open("SimpleReadApp", 2000);
} catch (PortInUseException e) {System.out.println(e);}
try {
inputStream = serialPort.getInputStream();
} catch (IOException e) {System.out.println(e);}
try {
serialPort.addEventListener(this);
} catch (TooManyListenersException e) {System.out.println(e);}
serialPort.notifyOnDataAvailable(true);
try {
serialPort.setSerialPortParams(2400,
SerialPort.DATABITS_7,
SerialPort.STOPBITS_1,
SerialPort.PARITY_EVEN);
} catch (UnsupportedCommOperationException e) {System.out.println(e);}
readThread = new Thread(this);
readThread.start();
}

public void run() {
try {
Thread.sleep(20000);
} catch (InterruptedException e) {System.out.println(e);}
}

public void serialEvent(SerialPortEvent event) {
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:
byte[] readBuffer = new byte[20];

try {
while (inputStream.available() > 0) {
int numBytes = inputStream.read(readBuffer);
}
System.out.print(new String(readBuffer));
} catch (IOException e) {System.out.println(e);}
break;
}
}
}