Brute force password cracker
Well here is a good idea:
You start with 1 character long passwords. Then you move on to two character passwords. You can define these loops manually because there will only be so many. If not, you can make an if function and an index to clarify how many characters are in the password. Then, here is a rough explanation of how it would work:
You set the first character to the first letter in a list. Then you iterate through the character list with another character. You could iterate by accessing the second character with indexes and changing the value. Once you have run through all of the second character values', you move the first character to the next character in the list.
Now you will just how to figure out how to iterate through the following values. I would look up a python brute force password cracker and look at how they do it, and attempt to replicate it into Java. A sample one could be: https://github.com/CyanCoding/Brute-Force-Password-Cracker/blob/master/Python/main.py - this shows (in a way) how to interate through passwords using python, which can be easily translated into Java.
To me, one thing that would make your script extraordinary is if you are able to make multiple different index loops, and have them all operate at once using a bash threading feature.