What happens if you call run() directly instead of calling start() on a Thread?

Calling run() directly simply executes the Runnable's logic synchronously on whichever thread made the call -- it behaves exactly like calling any ordinary method, and no new thread is created. start() is what actually creates and launches a new operating-system thread, which then calls run() asynchronously on that new thread. This is a common beginner mistake because it doesn't throw an exception or fail loudly: the code appears to work correctly, but it silently runs single-threaded on the calling thread, completely defeating the purpose of introducing threading in the first place.