Java How to Program Fourth Edition - DCC

Chapter 15 Multithreading 867
157
158 // place buffer elements in outputBuffer
159 for ( int count = 0; count < buffer.length; count++ )
160 outputBuffer.append(
161 " " + formatNumber.format( buffer[ count ] ) );
162
163 // update Swing GUI component
164 SwingUtilities.invokeLater( new UpdateThread( outputArea,
165 "\tbuffer: " + outputBuffer ) );
166 }
167
168 } // end class HoldIntegerSynchronized
Fig. Fig. 15.15 15.15 15.15 Class HoldIntegerSynchronized monitors access to a shared array
of integers (part 5 of 5).
Method setSharedInt (lines 34–87) performs the same tasks as it did in
Fig. 15.10, with a few modifications. When execution continues at line 55 after the while
loop, setSharedInt places the produced value in the circular buffer at location
writeLocation. Next, readable is set to true (line 58), because there is at least one
value in the buffer that the client can read. Lines 61–63 use SwingUtilities method
invokeLater to append the value produced and the cell where the value was placed to
the JTextArea (method run of class UpdateThread performs the actual append operation).
Then, line 66 updates writeLocation for the next call to setSharedInt. The
output continues with the current writeLocation and readLocation values and the
values in the circular buffer (lines 69–73). If the writeLocation is equal to the read-
Location, the circular buffer is currently full, so writeable is set to false (line 77)
and the program displays the string BUFFER FULL (lines 80–81). Finally, line 85 invokes
method notify to indicate that a waiting thread should move to the ready state.
Method getSharedInt (lines 92–149) also performs the same tasks in this example
as it did in Fig. 15.10, with a few minor modifications. When execution continues at line
115 after the while loop, writeable is set to true because there will be at least one
open position in the buffer in which the producer can place a value. Next, line 118 assigns
value the value at readLocation in the circular buffer. Lines 121–123 append to the
JTextArea the value consumed and the cell from which the value was read. Then, line
126 updates readLocation for the next call to method getSharedInt. Lines 129–
131 continue the output in the JTextArea with the current writeLocation and
readLocation values and the current values in the circular buffer. If the readLocation
is equal to the writeLocation, the circular buffer is currently empty, so readable
is set to false (line 137), and lines 140–141 display the string BUFFER EMPTY.
Finally, line 145 invokes method notify to place the next waiting thread into the ready
state, and line 147 returns the retrieved value to the calling method.
In this version of the program, the outputs include the current writeLocation and
readLocation values and the current contents of the buffer sharedInt. The elements
of the sharedInt array were initialized to –1 for output purposes so that you can see each
value inserted in the buffer. Notice that after the program places the fifth value in the fifth
element of the buffer, the program inserts the sixth value at the beginning of the array—
thus providing the circular buffer effect. Method displayBuffer (lines 152–166) uses