/*

* A museum can accommodate up to 500 people at a time and has one entrance,

* which only allows one person to pass through at a time.

* Synchronization between visitors.

*/

public class Exit {

public static void main(String[] args) {

Semaphores semaphores = new Semaphores();

Thread visitor1 = new Thread(new Visitor(semaphores, 1));

Thread visitor2 = new Thread(new Visitor(semaphores, 2));

Thread visitor3 = new Thread(new Visitor(semaphores, 3));

Thread visitor4 = new Thread(new Visitor(semaphores, 4));

Thread visitor5 = new Thread(new Visitor(semaphores, 5));

visitor1.start();

visitor2.start();

visitor3.start();

visitor4.start();

visitor5.start();

}

}

class Semaphores {

public int empty = 500;

public boolean mutex = true;

public synchronized boolean pEmpty() {

if (empty > 0) {

empty--;

return true;

} else {

return false;

}

}

public synchronized boolean pMutex() {

if (mutex) {

mutex = false;

return true;

} else {

return false;

}

}

public synchronized void vEmpty() {

empty++;

}

public synchronized void vMutex() {

mutex = true;

}

}

class Visitor implements Runnable {

public Semaphores semaphores;

public int rank;

public Visitor(Semaphores semaphores, int rank) {

this.semaphores = semaphores;

this.rank = rank;

}

public void run() {

while (!semaphores.pEmpty()) {

}

while (!semaphores.pMutex()) {

}

System.out.println("Visitor " + rank + " enters the museum.");

semaphores.vMutex();

System.out.println("Visitor " + rank + " visits the museum.");

while (!semaphores.pMutex()) {

}

System.out.println("Visitor " + rank + " leaves the museum.");

semaphores.vMutex();

semaphores.vEmpty();

}

}