rfidreader | Use rfid reader with a raspberry pi

After a lot of fiddeling around I found that the error thrown in the script was caused by access rights to the NuGetScratch folder my Windows account did not have access to that folder. Since Nuget restores it packages from that lock folder it could not restore any packages, thus the build failed. After altering the access rights the error disappeared so my only guess is that that the "Value cannot be null. (Parameter 'path1') " error is a somewhat generic error that doesn't really show the under lying problem.

From the HashSet docs:

The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the Iterator throws a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Basically, when you're iterating over a collection, you shouldn't modify that same collection - you'll see this same warning under HashMap, LinkedList etc. It causes unpredictable behaviour, and there are different changes you could make that would affect the current state in different ways, so they just make it fail and say "so don't do that!"

As for why it happens with large batches and not a single scan:

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis.

So it's probably just more likely that a large set of changes will get noticed. It goes on to say "don't rely on this behaviour" so you should probably do things in a different way! Usually you would just create a separate Set (an empty one, or a copy of your original) and make changes to that, and then return it or swap the original out for it when you're done.

Just to be clear, the problem here is that you're creating an iterator on a collection (with the for loop) and then modifying that collection while the iterator is running on it. I noticed you have synchronized on your method, so if you're also accessing the same collection on different threads, you have to make sure you're doing that in a thread-safe way too - the HashSet link talks a bit about that stuff

Yes, you can intercept the back button and do anything you want there (even cancel the back navigation), but then you'll miss the cases where the user leaves by the Home button, or an incoming call, or through a notification...

What you should do instead is to override the onPause, onStop, and/or, onDestroy methods for your activity. They are all callbacks related to the activity life cycle, which you will soon need to learn about.

In short: onPause happens when you lose focus, onStop when you're no longer visible, and onDestroy when the activity is fully deleted (e.g. the user pressed "back", but usually not when they pressed "home").

In your case, onStop is probably the most suitable one.

You can not simply cast a Set to a List you have to convert it like:

(java 8)