Java Never Ends - Absolute Java - page 1096

In this particular case, we can add the keyword synchronized to either the method
or around the specific code. If we add synchronized to the increment() method in
the Counter class, then it looks like this:
public synchronized void increment()
{
int local;
local = counter;
local++;
counter = local;
}
If we add synchronized inside the code, then we can write
public void increment()
{
int local;
synchronized ( this )
{
local = counter;
local++;
counter = local;
}
}
Either version will result in a counter whose final value is always 30,000. There are
many other issues involved in thread management, concurrency, and synchronization.
These concepts are often covered in more detail in an operating systems or parallel
programming course.
Self-Test Exercises
3. In the run() method of Display 19.5, make the thread sleep a random amount
of time between one and fi ve milliseconds. You should see an increase in the
number of problems caused by race conditions. Can you explain why?
4. Here is some code that synchronizes thread access to a shared variable. How
come it is not guaranteed to output 30,000 every time it is run?
public class Counter
{
private int counter;
public Counter()
{
counter = 0;
}