It is even more complicated than that.

You must assume that all actions that aren't otherwise synchronized
might appear in some unexpected order to another thread.

For example:
if you have
public int a,b,c;

and Thread 1 code is:
a = 10;
b = 3*a;
c = a*b;

Thread 2 might see at any time a=0, b=0, c=0. Or a=10, b=0, c=300. Even
if thread 2 waits for c != 0, changes to a and b might not be visible to
it, so you would see a=0, b=0, c=300.

Understanding concurrency isn't as difficult as some people make it
sound, but there are quite a few subtleties to learn.  My usual
suggestion is to read the book Java Concurrency In Practice:
<http://virtualinfinity.net/wordpress/technical-book-recommendations/java-concurr
ency-in-practice/>
This book if full of a complete and intuitive explanation of all of the
current Java Concurrency features, including Java 1.5 concurrency framework.

Hope this helps,
Daniel.