How to improve my code?

Can you please take a look at my code? I apologize for its length. Is there any way that I can shorten it? This is my final project in my course. I'm a first year student. I only used what my professor taught me but I'm really eager to learn something new. I'm finished with my project but I'm not really satisfied with my work. I want to improve it. In the admin frame, I want to know how to remove an existing item, let the user add his own category and add items in an existing category.I also have a problem with my code whhich is that whenever I close the program and open it, it doesn't record the previous transactions. The stocks will revert to its original amount. I don't know how to do it.

Please help me. Even if it would require me to rewrite my entire code, I'm willing to do it. I would be really thankful of any help.

I uploaded it on my dropbox account. Here's the link. It contains one java file. I can't seem to post the code here because it's too long. http://ift.tt/1wG91BE

Thank you in advance.