Java multi thread errors

I have been trying to get a seperate thread going to do some background processing in my SourceForge project FileExplorer but after following this tutorial : http://www.exampledepot.com/egs/java.lang/WorkQueue.html?l=rel

I keep getting this error:
Exception in thread "Thread-5" java.lang.IllegalMonitorStateException: current thread not owner at java.lang.Object.wait(Native Method)

Which was caused by:
missing the 'synchronized' keyword from the definition of the methods that call wait() & notify()