/* using a Java monitor to implement semaphores */

public class Semaphore
{
    private int semval;
    private int maxval;
    
    public Semaphore(int m) 
    { 
       maxval = m; semval = 1;  
    } // constructor

    public synchronized void proberen() 
    {
	try   // wait can cause exception
        {
	  while (semval == 0) wait();
	  semval--;
	} catch (InterruptedException ie) { ie.printStackTrace(); }
    }

    public synchronized void verhogen()
    {
  	  if (semval<maxval) semval++;
	  if (semval==1) notifyAll();  // everyone will have chance to compete
    }
    
}  //class Semaphore


/*
  Usage: a separate semphore should be created to protect each critical
  section:

  Semaphore sem = new Semaphore(3);

  ...

  sem.proberen();

  ... critical section ...

  sem.verhogen();


*/