// Fig. 23.20: CircularBuffer.java
// Synchronizing access to a shared three-element bounded buffer.
public class CircularBuffer implements Buffer
{
   private final int[] buffer = { -1, -1, -1 }; // shared buffer

   private int occupiedCells = 0; // count number of buffers used
   private int writeIndex = 0; // index of next element to write to
   private int readIndex = 0; // index of next element to read
   
   // place value into buffer
   public synchronized void set( int value ) throws InterruptedException
   {
      // output thread information and buffer information, then wait;
      // while no empty locations, place thread in waiting state
      while ( occupiedCells == buffer.length ) 
      {
         System.out.printf( "Buffer is full. Producer waits.\n" );
         wait(); // wait until a buffer cell is free
      } // end while

      buffer[ writeIndex ] = value; // set new buffer value

      // update circular write index
      writeIndex = ( writeIndex + 1 ) % buffer.length;

      ++occupiedCells; // one more buffer cell is full
      displayState( "Producer writes " + value );
      notifyAll(); // notify threads waiting to read from buffer
   } // end method set
    
   // return value from buffer
   public synchronized int get() throws InterruptedException
   {
      // wait until buffer has data, then read value;
      // while no data to read, place thread in waiting state
      while ( occupiedCells == 0 ) 
      {
         System.out.printf( "Buffer is empty. Consumer waits.\n" );
         wait(); // wait until a buffer cell is filled
      } // end while

      int readValue = buffer[ readIndex ]; // read value from buffer

      // update circular read index
      readIndex = ( readIndex + 1 ) % buffer.length;

      --occupiedCells; // one fewer buffer cells are occupied
      displayState( "Consumer reads " + readValue );
      notifyAll(); // notify threads waiting to write to buffer

      return readValue;
   } // end method get
    
   // display current operation and buffer state
   public void displayState( String operation )
   {
      // output operation and number of occupied buffer cells
      System.out.printf( "%s%s%d)\n%s", operation, 
         " (buffer cells occupied: ", occupiedCells, "buffer cells:  " );

      for ( int value : buffer )
         System.out.printf( " %2d  ", value ); // output values in buffer

      System.out.print( "\n               " );

      for ( int i = 0; i < buffer.length; i++ )
         System.out.print( "---- " );

      System.out.print( "\n               " );

      for ( int i = 0; i < buffer.length; i++ )
      {
         if ( i == writeIndex && i == readIndex )
            System.out.print( " WR" ); // both write and read index
         else if ( i == writeIndex )
            System.out.print( " W   " ); // just write index
         else if ( i == readIndex )
            System.out.print( "  R  " ); // just read index
         else
            System.out.print( "     " ); // neither index
      } // end for

      System.out.println( "\n" );
   } // end method displayState
} // end class CircularBuffer


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 * (C) Copyright 1992-2007 by Deitel & Associates, Inc. and               *
 * Pearson Education, Inc. All Rights Reserved.                           *
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