Semaphore.java

package semaphore;
/*
This class implements a samafor through the use of wait and notify
 */

public class Semaphore {
    //Maximum number of threads
    private int maxNumber;
    //Number of threads in the critical region
    private int threadsIn;

    Semaphore(int max) {
        maxNumber = max;
        threadsIn = 0;
    }

    public synchronized void enter() {
        System.out.println("" + threadsIn + " in the critical region...");

        // When the maximum number of threads is reached, new threads have to wait
        while (threadsIn >= maxNumber) {
            try {this.wait();}
            catch(InterruptedException ie) {ie.printStackTrace();}
        }

        threadsIn++;
    }

    public synchronized void exit() {
        threadsIn--;
        // When a thread exits the critical region, it wakes another threads (if there is one) that's waiting to enter the critical region
        this.notify();
    }
}