Question : GPP Java Program

Hi people, i am a newbie in this things of Java.
I have one program, which i pretend to modify, but first i need to know how it works. Well, i know what he does, but i dont know how is structured, like variables, classes etc etc etc. So, what i want is if someone can comment all the next code. To have and idea how to work it out after. Thanks a lot.


import java.io.*;
import java.util.*;
import javax.comm.*;

public class GpsReader implements SerialPortEventListener {


   private InputStream        inputStream;
   private SerialPort         sPort;

   private int                bcount = 0;

   static OutputStreamWriter  outW = null;
   static CommPortIdentifier  portId;
   static Enumeration         portList;

   public static final String FILENAME = "NMEA.txt";
   public static final String PORTNAME = "COM5";
   public static final int    BAUDRATE = 4800;
   public static final int    TIMEOUT  = 2000;    

   public static void main(String[] args) {

      GpsReader g = new GpsReader();
      g.mymain();              
   }

   void mymain(){

       try {
          outW = new OutputStreamWriter(
                 new FileOutputStream (FILENAME));
       }
       catch (IOException e) {
          System.err.println("Ficheiro de escrita não aberto\n" + e.toString());
          System.exit(1);
       }

        portList = CommPortIdentifier.getPortIdentifiers();

        while (portList.hasMoreElements()) {
            portId = (CommPortIdentifier) portList.nextElement();
            if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {
                if (portId.getName().equals(PORTNAME)) {


        try   {sPort = (SerialPort) portId.open("GarApp", TIMEOUT);}
        catch (PortInUseException e) {}

        try   {inputStream = sPort.getInputStream();}
        catch (IOException e) {}

     try   {sPort.addEventListener(this);}
        catch (TooManyListenersException e) {}

        sPort.notifyOnDataAvailable(true);
        try {
           sPort.setSerialPortParams(BAUDRATE,
                          SerialPort.DATABITS_8,
                          SerialPort.STOPBITS_1,
                          SerialPort.PARITY_NONE);
           }
        catch (UnsupportedCommOperationException e) {}
                }
            }
        }

   }

   public void serialEvent(SerialPortEvent event) {

      int  newData = 0;
      char mybyte;

      switch(event.getEventType()) {
         case SerialPortEvent.BI:            
         case SerialPortEvent.OUTPUT_BUFFER_EMPTY:
            break;

      case SerialPortEvent.DATA_AVAILABLE:
            while(newData != -1) {
               try {
                  newData = inputStream.read();
               if (newData == -1) {break;}
                  mybyte = (char)newData;
                  outW.write(mybyte);
                  bcount = bcount + 1;
                  if(mybyte == '$') {System.out.print("\r"+bcount+" bytes");}
               } catch(IOException ex) {
                  System.err.println(ex);
                  return;
               }
            }
            break;
      }
   }
}

Answer : GPP Java Program

Here's the code with some comments and references to online documentation. Ask if you need specific info on some constructs. In order to really understand this, you'll have to read the documentation of the classes used. I would think that this code is still "work in progress"; it will do what's required but needs further work.


HTH, J


/* make names available, avoid full qualification: http://java.sun.com/docs/books/jls/second_edition/html/packages.doc.html#70209 */
import java.io.*;
import java.util.*;
import javax.comm.*;

/* class to copy GPS info from serial port into file */
public class GpsReader implements SerialPortEventListener {


   private InputStream        inputStream; /* http://java.sun.com/j2se/1.4.2/docs/api/java/io/InputStream.html */
   private SerialPort         sPort; /* http://java.sun.com/products/javacomm/javadocs/javax/comm/SerialPort.html */

   private int                bcount = 0; /* byte count */

   static OutputStreamWriter  outW = null; /* class to write an output file: http://java.sun.com/j2se/1.5.0/docs/api/java/io/OutputStreamWriter.html */
   static CommPortIdentifier  portId; /* which serial port to use: http://java.sun.com/products/javacomm/javadocs/javax/comm/CommPortIdentifier.html */
   static Enumeration         portList; /* list of all available serial ports */

   public static final String FILENAME = "NMEA.txt";  // file to write output to; full path would be better
   public static final String PORTNAME = "COM5";  // which serial port to use; probably an USB serial device
   public static final int    BAUDRATE = 4800;  // baud rate - NMEA standard
   public static final int    TIMEOUT  = 2000;  //

   /* main program: Create a new GPSReader object and run its mymain method */
   public static void main(String[] args) {

      GpsReader g = new GpsReader();
      g.mymain();              
   }

   /* mymain method - main method of GPSReader class */
   void mymain(){

       /* create/open output file FILENAME = NMEA.TXT */
       try {
          outW = new OutputStreamWriter(
                 new FileOutputStream (FILENAME));
       }
       /* if this fails, issue an error message and exit with error code 1 */
       catch (IOException e) {
          System.err.println("Ficheiro de escrita não aberto\n" + e.toString());
          System.exit(1);
       }

        // find all available communication ports and copy them to port list */
        portList = CommPortIdentifier.getPortIdentifiers();

        // iterate through all ports and check whether it's a serial port
        while (portList.hasMoreElements())
        {
            portId = (CommPortIdentifier) portList.nextElement();
            // if it's a serial port, check whether it's the port specified as PORTNAME (COM5:)
            if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {
                if (portId.getName().equals(PORTNAME)) {

        // yes, it's our port; try to open it and assign name "GarApp"
        try   {sPort = (SerialPort) portId.open("GarApp", TIMEOUT);}
        // no further error handling
        catch (PortInUseException e) {}

        // make the port available as stream to easy further processing
        try   {inputStream = sPort.getInputStream();}
        catch (IOException e) {}

     // assign an event handler - in this case, the SerialEvent method further down.
     // check http://java.sun.com/products/javacomm/javadocs/javax/comm/SerialPortEvent.html
     try   {sPort.addEventListener(this);}
        catch (TooManyListenersException e) {}

        // we're interested only if there's new data available
        sPort.notifyOnDataAvailable(true);
        /* a number of events is not handled, including error events and protocol handling events. This should really be done for
            production quality code.
        */

        // set comm parameters
        try {
           sPort.setSerialPortParams(BAUDRATE,
                          SerialPort.DATABITS_8,
                          SerialPort.STOPBITS_1,
                          SerialPort.PARITY_NONE);
           }
        catch (UnsupportedCommOperationException e) {}
                }
            }
        }
   // we're done initializing; the event handler will process all data coming from the serial port.
   }

   /* process all serial port events. In this case, write the received data to the NMEA.TXT file. */
   /* Check out http://java.sun.com/products/javacomm/javadocs/javax/comm/SerialPortEvent.html for all events */
   public void serialEvent(SerialPortEvent event) {

      int  newData = 0;
      char mybyte;

      /* a number of events is not handled, including error events and protocol handling events. This should really be done for
          production quality code.
      */

      // we're not interested in BREAK signals or if the output buffer is empty. Code probably isn't necessary since these
      switch(event.getEventType()) {
         case SerialPortEvent.BI:            
         case SerialPortEvent.OUTPUT_BUFFER_EMPTY:
            break;

      // more interesting: if there's new data available
      case SerialPortEvent.DATA_AVAILABLE:
            // while there's more data avilable on the serial port
            while(newData != -1) {

               // read one byte from the InputStream associated with the serial port
               try {
                  newData = inputStream.read();

               // stop if there's no more data
               if (newData == -1) {break;}
                  // translate into a byte
                  mybyte = (char)newData;

                   // write to file
                  outW.write(mybyte);

                  bcount = bcount + 1;

                  // if the byte is a dollar sign, issue a progress message to the console; for debugging only
                  if(mybyte == '$') {System.out.print("\r"+bcount+" bytes");}

               // if something goes wrong, issue an error message and exit
               } catch(IOException ex) {
                  System.err.println(ex);
                  return;
               }
            }
            break;
      }
   }
}