public class BoundedBuffer {
  private Object[] buffer = new Object[20]; // arbitrary size
  private int numOccupied = 0;
  private int firstOccupied = 0;

  /* invariant: 0 <= numOccupied <= buffer.length
     0 <= firstOccupied < buffer.length
     buffer[(firstOccupied + i) % buffer.length]
     contains the (i+1)th oldest entry,
     for all i such that 0 <= i < numOccupied  */

  public synchronized void insert(Object o)
    throws InterruptedException
  {
    while(numOccupied == buffer.length)
      // wait for space
      wait();
    buffer[(firstOccupied + numOccupied) % buffer.length] = o;
    numOccupied++;
    // in case any retrieves are waiting for data, wake them
    notifyAll();
  }

  public synchronized Object retrieve()
    throws InterruptedException
  {
    while(numOccupied == 0)
      // wait for data
      wait();
    Object retrieved = buffer[firstOccupied];
    buffer[firstOccupied] = null; // may help garbage collector
    firstOccupied = (firstOccupied + 1) % buffer.length;
    numOccupied--;
    // in case any inserts are waiting for space, wake them
    notifyAll();
    return retrieved;
  }
}