This is from the sample chapter on Threads ( for
SCJP ) from A Programmer's Guide to
Java SCJP Certification - 3rd Ed book.
Threads have been explained very well in this chapter.
In section 13.3 The Main
Thread there's a mention of what causes IllegalThreadStateException, most of the text is very easy to follow and try out with code
Based on what is mentioned in the sample chapter, I tried
See
https://coderanch.com/t/432566/Programmer-Certification-SCJP/certification/FYI-Website-Programmer-s-Guide
for sample chapter on Threads (for SCJP)