Need HELP coding Threads to read a file.

Question

Hello, I am reading an incomming file (from a device called /dev/gps) of latitude and longitude positions. It updates a few times every second, so I would like to read only the last line when the getPosition function is called. Here is a sample of the file being read:
Latitude: xxxx.xxxxxx,Longitude: -xxxx.xxxxx1
Latitude: xxxx.xxxxxx,Longitude: -xxxx.xxxxx2
Latitude: xxxx.xxxxxx,Longitude: -xxxx.xxxxx3
MY QUESTION:
how would the code look for a Thread that does the basic reading from my device and update a string variable called currentCoordinates? Then have another thread which is the main program that just reads that variable whenever it's ready for the next coordinate, and it will only read the last value stored in the variable. The first Thread will take care of discarding the unused ones.
Also, should the updating and reading of the variable should be protected by synchronization?
thanks for your help!

Answer 1

Hi, i hope i understood your problem correctly....
also, im new to threads, so this might not be text book
Below is the code for 4 classes
CoordHolder - class that holds the current coord data
CoordReader - class that reads the file and updates the coordholder when it finds a new line
CoordUser - class that reads coordholder and gets the coord data
CoordTest - class to test the above
When CoordTest is running try changing the text in the file (in this example called output.txt) and you will see the last line output to teh console.
I think this would also be better done using a javax.swing.timer but the SDK im using doesnt have it.
Hope this helps a little!!
public class CoordHolder
     private boolean alreadyRead = false;
     private String coords = "";
     public synchronized void setCoords(String inCoords)
          while (alreadyRead == false)
               try
                    wait(); //wait for value to be read
               } catch (InterruptedException e){}
          coords = inCoords;
          alreadyRead = false;
          notifyAll();
     public synchronized String getCoords()
          while (alreadyRead == true)
               try
                    wait(); //wait for value to ne written
               } catch (InterruptedException e){}
          alreadyRead = true;
          notifyAll();
          return coords;
     public boolean isAlreadyread()
          return alreadyRead;
import java.io.*;
import java.util.*;
public class CoordReader extends Thread
     private String data;
     private String lastLine = "";
     private String lastLine2 = "";
     private CoordHolder ch;
     private File file;
     private int tick=1;
     public CoordReader(CoordHolder c, String f)
          ch = c;
          file = new File(f);
     public void run()
          while (true)
               tick++;
               if ((tick % 500000) == 0)
                    BufferedReader LINE;
                    try
                         LINE = new BufferedReader(new FileReader(file)); //input file handle
                         while((data = LINE.readLine()) != null)
                              lastLine = data;
                              //do nothing, get to end of file
                         LINE.close();
                         if (!(lastLine2.equals(lastLine)))
                              ch.setCoords(lastLine);
                              lastLine2 = lastLine;
                              System.out.println("Reader: "+lastLine);
                    }catch(IOException e){System.out.println("ACK! IO problem.");}
               if (tick>4000000)
                    tick = 1;
          yield();
public class CoordUser extends Thread
     private CoordHolder ch;
     private String coord;
     public CoordUser(CoordHolder c)
          ch = c;
     public void run()
          int value = 0;
          while (true)
               coord = ch.getCoords();
               System.out.println("User: "+coord);
public class CoordTest
     public static void main(String[] args)
          CoordHolder c = new CoordHolder();
          CoordReader r = new CoordReader(c, "C:\\output.txt");
          CoordUser u = new CoordUser(c);
          r.start();
          u.start();
}