JAVA Concurrency (Monitors)

SteveB_NI

SteveB_NI

SteveB_NI

Associate
Joined
24 Sep 2005
Posts
209
Would anyone be able to help me with a little problem I'm experiencing while producing a concurrent scenario in Java (using monitors).

Basically I have a variable, numPumps (in PumpMon.java) which is decremented each time a car enters the petrol station. I have attempted to make this variable global and call it in car.java as in:

System.out.println("There are" + numPumps + "available at the station");

Although this presents the error that a non-static variable cannot be used in a static expression. This makes sense given that numPumps value can change so many time during the program execution.

How would I go about assigning the current value of numPumps to a static variable so I can display the value of numPumps at different times throughout execution?

Cheers for any pointers.
Code: 
public class PumpMon {

private int numPumps = 2;



PumpMon(int num) {
numPumps = num;
}

synchronized void enter() {

while (numPumps <=0) {
try { wait(); }
catch (InterruptedException e) {}
}

numPumps--;


}

synchronized void exit() {
numPumps++;
notify();
}
}

Code: 
public class Car extends Thread {

private String name;
private PumpMon mon;


public Car(String name, PumpMon mmm) {
this.name = name;
mon = mmm;

}


public void run() {

System.out.println("There are available at the station");
System.out.println(name + " about to try entering the forecourt");
mon.enter();
System.out.println(name + " is at the petrol pump");
PetrolStationActivity.idleQuietly(8000);


System.out.println(name + " driver has paid, and has left the petrol pump");
mon.exit();
System.out.println(name + " has left the forecourt");
}

}

Code: 
public class PetrolStationActivity {

public static void idle(int millisecs) {

Thread thisThread = Thread.currentThread();
System.out.println(thisThread.getName() + ": About to sleep");

try {
thisThread.sleep(millisecs);
}
catch (InterruptedException e) { }
System.out.println(thisThread.getName() + ": Woken");
}

public static void idleQuietly(int millisecs) {
Thread thisThread = Thread.currentThread();

try {
thisThread.sleep(millisecs);
}
catch (InterruptedException e) { }
}
}


This is the runnable program that shows you the scenario being demonstrated.

Code: 
public class PetrolStationSolution {

public static void main(String[] args) {

PumpMon mon = new PumpMon(4);

Car c1 = new Car("Volvo Reg. Number IKZ4463", mon);
Car c2 = new Car("Peugeot Reg. Number IAZ4563", mon);
Car c3 = new Car("BMW Reg. Number AAZ6474", mon);
Car c4 = new Car("Audi Reg. Number IIA43", mon);
Car c5 = new Car("Ford Reg. Number IKZ5453", mon);
Car c6 = new Car("Porsche Reg. Number AAB1", mon);
Car c7 = new Car("Jaguar Reg. Number BIG4423", mon);


c1.start();
c2.start();
c3.start();
c4.start();
c5.start();
c6.start();
c7.start();

}
}
 
Why not write an accessor method in the PumpMon class to get the number of pumps availiable?

Code: 
public int getNumPumps(){
return numpumps;
}

Then in the car class make a call to the method:

Code: 
System.out.println("There are "+mon.getNumPumps()+" available at the station");
 
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