Try doing this:
public void serialEvent(SerialPortEvent event){
switch(event.getEventType()) {
case SerialPortEvent.DATA_AVAILABLE:
byte[] readBuffer = new byte[8];
try {
while (inputStream.available()>0) {
int numBytes = inputStream.read(readBuffer);
peso += new String (readBuffer, 0, numBytes, "UTF-8").replaceAll("\\s+","").replaceAll("\\n", "").replaceAll("\\r", "");
}
} catch (IOException ex) {}
break;
}
}
Inicialize the String peso OUTSIDE of this SerialEvent method.