Another classical problem takes place in a barber shop.  This
problem is due to Dijkstra.  The barber shop has one barber,
a barber chair, and //n// chairs for waiting customers, if any, to
sit on.  If there are no customers present, the barber sits down
in the barber chair and falls asleep.  When a customer arrives,
he has to wake up the sleeping barber.  If additional customers
arrive while the barber is cutting a customer's hair, then they
sit down (if there are empty chairs) or leave the shop (if all
chairs are full).  The problem is how to program the barber
and the customers without getting into race conditions.                                                 In your solution, you may want to introduce a class ''Shop''. The ''Shop'' object has-a ''Barber'' object.  A ''Barber''
object is-a ''Thread'' object.  The ''run'' method of the
''Barber'' class should give rise to the output
''The barber sits down and falls asleep'' and
''The barber wakes up'', whenever appropriate.  In your
solution, you may also want to introduce a class
''Customer''.  A ''Customer'' object is-a ''Thread''.
The ''run'' method of the ''Customer'' class should
give rise to the output ''Customer i enters'',
''Customer i sits down'' (if there is an empty
chair), ''Customer i gets a haircut'' (if there
is an empty chair and once it is his turn), and
''Customer i leaves''.

An applet of this problem can be found
[[http://colos1.fri.uni-lj.si/%7Ecolos/Colos/EXAMPLES/MARIBOR/barber|here]].

<code java>
public class Barber extends Thread 
{
  public void run()
  {
    while (true)
    {
      while (shop.isEmpty())
      {
        try
        {
          this.wait();
        }
        catch (InterruptedException e) {}
      }
    }
  }
}
</code>
<code java>
public class Customer extends Thread 
{
  private int id;
  private Shop shop;

  public Customer(int id)
  {
    super();
    this.id = id;
  }

  public void run()
  {
    while (true)
    {
      System.out.println("Customer " + this.id + " enters");
      int number = shop.getChair();
      if (number != -1)
      {
        System.out.println("Customer " + this.id + " sits down");
        while (!shop.isFirst(number))
        {
          try
          {
            this.wait();
          }
          catch (InterruptedException e) {}
        }
        System.out.println("Customer " + this.id + " gets a haircut");
        while (!shop.isDone())
        {
          try
          {
            this.wait();
          }
          catch (InterruptedException e) {}
        }
      }
      System.out.println("Customer " + this.id + " leaves");
    }
  }
}
</code>

<code java>
public class Shop 
{
  private Barber barber;
	
  public final int NUMBER_OF_CHAIRS = 8;
	
  private int first;
  private int next;
  private boolean done;
	
  public synchronized int getChair()
  {
    if (this.next - this.first > Shop.NUMBER_OF_CHAIRS)
    {
      return -1;
    }
    else
    {
      this.notifyAll();
      return last++;
    }
  }
	
  public synchronized boolean isFirst(int number)
  {
    return this.first == number;
  }

  public synchronized boolean isDone()
  {
    return this.done;
  }	
}
</code>

Another sketch of a solution can be found
[[http://www.cse.yorku.ca/~franck/teaching/2002-03/6490A/0502.html|here]].

Slawomir's running (!!!) solution can be found [[discussion|here]].