in Java, I can't assign values to variables while i'm in the run() function of a thread-extended class.

Question

This is my code. As you see in the run method, I assign values to tStart, tEnd, tAround and wTime. But when the Thread ends, they still have the default values of -1. I try printing out their values while the run() is running, and I have the correct values. But they are not 'writing' those values back to the variables when the thread ends.

public class PCB extends Thread{
 public int id, arrivalTime, cpuBurst, ioBurst;
 public int tStart, tEnd, tAround, wTime;
 public PCB(){
  id = -1;
  arrivalTime = -1;
  cpuBurst = -1;
  ioBurst = -1;

  tStart = -1;
  tEnd = -1;
  tAround = -1;
  wTime = -1;
 }

 public void run(){
  try{
 .........

   //calculation for FCFS 
   if (id == 1){ //special case for the first one
    tStart = arrivalTime;
   }
   else tStart = lastEndTime;

   tEnd = tStart + cpuBurst + ioBurst;
   tAround = tEnd - arrivalTime;
   wTime = tStart - arrivalTime;

                            PCBThreadStopFlag = true;   

  }
  catch(InterruptedException e){
   e.printStackTrace();
  }
 }
}

When the thread ends, this is how I print out the values:

     // now we print out the process table
 String format = "|P%1$-10s|%2$-10s|%3$-10s|%4$-10s|%5$-10s|%6$-10s|%7$-10s|%8$-10s|\n";
 System.out.format(format, "ID", "ArrTime", "CPUBurst", "I/OBurst", "TimeStart", "TimeEnd","TurnAround","WaitTime");
 ListIterator<PCB> iter = resultQueue.listIterator();
 while(iter.hasNext()){
  PCB temp = iter.next();
  System.out.format(format, temp.id, temp.arrivalTime, temp.cpuBurst, temp.ioBurst, temp.tStart, temp.tEnd, temp.tAround, temp.wTime );
 }

And here is how I waited for the thread to stop first:

while(!rq.values.isEmpty()){
   //System.out.println("Ready queue capacity now: " + rq.values.size());
   currentProcess = new PCB(rq.values.getFirst());
   currentProcess.start();

   while(PCBThreadStopFlag == false) {}
   //currentProcess.stop();
   PCBThreadStopFlag = false;

   //after everything is done, remove the first pcb
   // and add it to the result queue (just to print the report)
   resultQueue.addLast(rq.values.removeFirst());   
  }

I use the flag PCBThreadStopFlag in the run() up top (at the end when all the assignments are done) then in this function, I use while(PCBThreadStopFlag == false) {} to do the "busy-wait" task. may be this is the cause??

Answer 1

This is just a guess, but I'll bet you're not joining on the threads before you print the results. In other words I suspect you're starting the threads and then immediately printing the result without waiting for the threads to complete.

EDIT: OK, try this...

Idea #1: Declare the PCBThreadStopFlag as volatile, and try again. Tell us if that works.

Idea #2: Get rid of the whole stop flag thing altogether, and replace the busy wait with

currentProcess.join();

and tell us if that works.