Threading: wait /notify...please help !

[ozmanbozman]

Hi guys...
my question is about implementation of wait and notify to simulate threading...here is the Problem description:
I have an application to simulate 2 trains travelling in the same direction on a loop from station, through a tunnel and back to the station ;normaly on 2 seperate tracks.however both tracks pass through the tunnel wich has one single track: so there should be one train at a time in the tunnel...

what i need is to rewrite method useTunnel() given below so that train 1 and 2 are allowed to enter tunnel in strict rotation:train 2 should be allowed to enter first then train 1 then 2 and so on...method useTunnel should contain code to enforce the ordring of which train can enter at a given time...for this we should use wait and notify and maybe change the Tunnel constractor...how can i do this?

the code is below (classes Train.java , Tunnel.java ans Simulate.java)

many thanks for helping

//Train class
public class Train extends Thread
{

private int id; // id identifies each train

public Train(int id) {
// each train has a different identifing number
this.id = id;
}

private static Tunnel tun = new Tunnel(); // only one tunnel

public void run() {
// each train makes 2 loops
for (int j=0; j < 2; j++) {
System.out.println("Train " + id + " leaves station");

travel(10); // train takes 10 time units to travel from station to tunnel
System.out.println("Train " + id + " arrives at tunnel");


tun.useTunnel(this); // current train uses tunnel


travel(15); // train takes 15 time units to travel from tunnel to station

System.out.println("Train " + id + " arrives at station");
travel(5); // train stays in station for 5 time units
}
}

public int getId () {
return id;
}

public void travel(int time) {
int limit = 500000*id;
for (int j=0; j < time; j++) {
for (int k=0; k < limit; k++) {double r = j / 2 * 2;}
}
}

}

//Tunnel class
public class Tunnel
{
public void Tunnel () {}

public void useTunnel (Train t) {
/* the tunnel is a shared resource
between Train 1 and 2 .so we should use
a synchronized block to lock the object "Tunnel"
from being accessed simultaneousely by Train 1 and 2
*/
synchronized(this){


System.out.println("Train " + t.getId() + " enters tunnel");
t.travel(20); // train takes 20 time unit to go through tunnel
System.out.println("Train " + t.getId() + " exits tunnel");
}
}
}
//Simulate class
public class Simulate
{
public static void main(String [] args) {

// ***************************************
Train t1=new Train(1);
t1.start();
Train t2=new Train(2);
t2.start();

// Keeps output on screen until Return is pressed

System.out.println("Type return to terminate program");
try {
System.in.read();
}
catch (IOException e){}
}

}

[doredson]

How about this?

package simulation;

import java.io.IOException;

//Tunnel class
class Tunnel {
  
  public int allowed;

  public Tunnel(int first) {
    allowed = first;
  }

  public void useTunnel(Train t) {

    /*
     * the tunnel is a shared resource between Train 1 and 2 .so we should use a
     * synchronized block to lock the object "Tunnel" from being accessed
     * simultaneousely by Train 1 and 2
     * 
     * if t is not allowed, wait
     */
    if (t.getId() != allowed) {
      System.out.println("Train " + t.getId() + " not allowed to enter tunnel.  Waiting.");
      synchronized (this) {
        try {
          this.wait();
        } catch (InterruptedException ie) {
          ie.printStackTrace();
        }
      }
    }

    //original code: synchronized (this) {
    System.out.println("Train " + t.getId() + " enters tunnel");
    t.travel(20); // train takes 20 time unit to go through tunnel
    System.out.println("Train " + t.getId() + " exits tunnel");
    //}

    // Change allowed and wake up waiter
    synchronized (this) {
      allowed = (allowed == 1) ? 2 : 1;
      System.out.println("Train " + allowed + " is ok to enter tunnel.");
      this.notify();
    }     
  }
}

//Train class

class Train extends Thread {

  private static Tunnel tun = new Tunnel(1); // only one tunnel

  //  Simulate class

  private int id; // id identifies each train

  public Train(int id) {
    // each train has a different identifing number
    this.id = id;
  }

  public void run() {
    // each train makes 2 loops
    for (int j = 0; j < 3; j++) {
      System.out.println("Train " + id + " leaves station");

      travel(10); // train takes 10 time units to travel from station to tunnel
      System.out.println("Train " + id + " arrives at tunnel");

      tun.useTunnel(this); // current train uses tunnel

      travel(15); // train takes 15 time units to travel from tunnel to station

      System.out.println("Train " + id + " arrives at station");
      travel(5); // train stays in station for 5 time units
    }
  }

  public int getId() {
    return id;
  }

  public void travel(int time) {
    int limit = 500000 * id;
    for (int j = 0; j < time; j++) {
      for (int k = 0; k < limit; k++) {
        double r = j / 2 * 2;
      }
    }
  }

}

public class Simulate {

  public static void main(String[] args) {

    // ***************************************
    Train t1 = new Train(1);
    t1.start();
    Train t2 = new Train(2);
    t2.start();

    // Wait for both Trains to finish.
    try {
	    t1.join();
	    t2.join();
    } catch (InterruptedException ie ) {
      ie.printStackTrace();
    }

    // Keeps output on screen until Return is pressed
    System.out.println("Type return to terminate program");
    try {
      System.in.read();
    } catch (IOException e) {
    }
  }
}

[ozmanbozman]

wow !
this is very cool and it's working fine :-)

many thanks for you man...i'm very gratefull ...
i had only to make few changes if i want the Train 2 to be allowed first (new Tunnel(2))...and that's it
I will keep on testing it for all cases (big number of loops) and will notify you...but i'm sure this is the wright solution .

many thanks :-)

[doredson]

you're welcome.

Happy Holidays

[ozmanbozman]

thanks and happy hollidays for you too :-)