CS 151 - Lecture 25

Cay S. Horstmann

Puzzler: What is Wrong?

public class BoundedQueue<E>
{
   private Object queueLock = "LOCK";
   . . .
   public void add(E newValue) throws InterruptedException
   { 
      synchronized (queueLock)
      {         
         while (size == elements.length) 
            spaceAvailableCondition.wait();
         elements[tail] = newValue; 
         tail++;
         size++;
         if (tail == elements.length) 
            tail = 0; 
         valueAvailableCondition.signalAll();
      }
   }
}

Puzzler: What is Wrong?

public class BoundedQueue<E>
{
   . . .
   public void add(E newValue) throws InterruptedException
   { 
      synchronized (new String("LOCK"))
      {         
         while (size == elements.length) 
            spaceAvailableCondition.wait();
         elements[tail] = newValue; 
         tail++;
         size++;
         if (tail == elements.length) 
            tail = 0; 
         valueAvailableCondition.signalAll();
      }
   }
}

Puzzler: What is Wrong?

public class BoundedQueue<E>
{
   private Lock queueLock = new ReentrantLock();
   private Condition valueAvailableCondition 
         = queueLock.newCondition();
   . . .
   public void add(E newValue) throws InterruptedException
   { 
      queueLock.lock();
      try
      {         
         while (size == elements.length) 
            spaceAvailableCondition.wait();
         elements[tail] = newValue; 
         tail++;
         size++;
         if (tail == elements.length) 
            tail = 0; 
         valueAvailableCondition.signalAll();
      }
      finally
      {
         queueLock.unlock();
      }
   }
}

Puzzler: What is Wrong?

public class BoundedQueue<E>
{
   . . .
   public void write(String filename) throws IOException
   {
      queueLock.lock();
      try
      {
         PrintWriter out = new PrintWriter(filename);
         int i = head;
         for (int j = 0; j < size; j++)
         {
            out.println(elements[i]);
            i = (i + 1) % elements.length;
         }
      }
      finally
      {
         out.close();
         queueLock.unlock();
      }      
   }
}