• the sample tests aren't even runnable
• the initial setup isn't explained
• the initial setup is just awfully bad, with a static value to mutate along the way. this is defnitely "worst practices". If state is needed, use instances, not static methods.
• the private interface doesn't make any sense without any explanation (void!?)
• input constraints aren't given

Could the people of your group restrain themselves and stop authoring mostly... "several not good kata per day". I'm starting to think about locking every single account...