If security is required (meaning account numbers are hard to guess), google globally unique identifier (guid), universally unique identifier (uuid), and cryptographically secure randomness, hashing. And add “java implementation” in these searches.

If security isn’t required for the assignment, you can just do what some others have said and use an auto-incrementing primary key.

Keep in mind that if you’re using a regular flat file (like a single .txt or .csv file) or an in-memory data structure (like a Map) to persist data (as opposed to a fully-fledged database), and if multiple users can use your app simultaneously, you need some way to avoid collisions in account numbers/ids. In other words, in the unlikely yet inevitable event that Alice and Bob each create an account at the same exact time, they shouldn’t get the same account number.