The background threads will keep running, even if the MAIN thread completes.
If you want MAIN to stop them (example, when MAIN is complete), have your MAIN set a "keep running" flag variable (which you must set as "volatile"), which the threads occasionally look at. MAIN sets it to false (variable) or null (object) when MAIN wants to stop them. When it's false or null, then the thread must "return;".
This is somewhat complex to implement, there are many ways, but easiest is to make your Runnable an inner class, so that your Runnable has easy sharing of the flag.
For the best implementations, look up this technique in Java applets' start/stop routines.
