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What does "stops" mean?
The run method exits killing the thread and requiring a new thread to be created
or the thread blocks waiting for an event
or ???

Did you read the API doc for that exception? Are you doing the thing it is thrown for?

Not sure what you mean by "idle" and "still runs"

Not sure what you mean by "idle". There are many ways to block/suspend execution of a thread.
For example the wait() method.