Java Never Ends - Absolute Java - page 1094

Display 19.4 The Counter Class

1 public class Counter
2 {
3 private int counter;
4 public Counter()
5 {
6 counter = 0;
7 }
8 public int value()
9 {
10 return counter;
11 }
12 public void increment()
13 {
14 int local;
15 local = counter;
16 local++;
17 counter = local;
18 }
19 }

The only new tool that we need for our demonstration program is a way to wait
for all the threads to finish. If we do not wait, then our program might output the
counter before all the threads have had a chance to increment the value. We can wait
by invoking the join() method for every thread we create. This method waits for the
thread to complete. The join() method throws InterruptedException . This is a
checked exception so we must use the try/catch mechanism.
The class RaceConditionTest in Display 19.5 illustrates the race condition. You
may have to run the program several times before you get a value less than 30,000.
Problems as a result of race conditions are often rare occurrences. This makes them
extremely hard to find and debug!

VideoNote
Walkthrough
of a Program
with Race
Conditions

Display 19.5 The RaceConditionTest Class (part 1 of 2)

1 public class RaceConditionTest extends Thread
2 {
3 private Counter countObject;

Stores a reference to a
single Counter object.

4 public RaceConditionTest(Counter ctr)
5 {
6 countObject = ctr;
7 }