Question

Hello there I have slight problem and not sure where im going wrong with this.

I have 2 Threads X which prints repeatedly X and Y which prints Y repeatedly. Y needs to be printed after X.

I have derived from Semaphore class BinarySemaphore:

public class BinarySemaphore extends Semaphore {

public BinarySemaphore(int initial){
    value = (initial>0) ? 1 : 0;
}

public synchronized void P() throws InterruptedException{
    while (value==0){
    wait();
    }
value = 0;
    notify();
}

public synchronized void V(){

    value = 1;
    notify();
}
}

X Thread class

public class xThread extends Thread implements Runnable{

private BinarySemaphore xSemaphore;
private BinarySemaphore ySemaphore;


public xThread(String myName, BinarySemaphore nSemaphoreX, BinarySemaphore nSemaphoreY ){
    super(myName);
    xSemaphore = nSemaphoreX;
    ySemaphore = nSemaphoreY;
}

public void run(){
    try{
    xSemaphore.P();
    System.out.println(getName());
    ySemaphore.V();
    }catch(InterruptedException E){
    System.out.println("Thread X was interrupted"); 
    }
}   
}

Y Thread Class

public class yThread extends Thread implements Runnable{

private BinarySemaphore xSemaphore;
private BinarySemaphore ySemaphore;


public yThread(String myName, BinarySemaphore nSemaphoreX, BinarySemaphore nSemaphoreY ){
    super(myName);
    xSemaphore = nSemaphoreX;
    ySemaphore = nSemaphoreY;
}

public void run(){
    try{
    ySemaphore.P();
    System.out.println(getName());
    xSemaphore.V();
    }catch(InterruptedException E){
    System.out.println("Thread Y was interrupted");
    }


}
}

When I run these threads for 10seconds all I get is

X
Y

BUILD SUCCESSFUL (total time: 10 seconds)

What am I missing here ? Why doesn’t it keep alternating them for 10 seconds ?

Answer 1

This is because your threads are printing only once and then are terminated, like @reprogrammer suggested in his comment. You should insert a loop, something like:

public void run(){
  while (needsToRun){
    try{
    xSemaphore.P();
    System.out.println(getName());
    ySemaphore.V();
    }catch(InterruptedException E){
    System.out.println("Thread X was interrupted"); 
    }
  }
}