The code is too messy and I don't know where your professor taught JAVA, so I can't give you a clear answer, but I can give you an answer in the abstract.

MenuAdditional problems 1.

Use data structures: use arraylist 2.
Use arraylist as Instance Variable ( I think you're learning this level by Korean university standards, so you should be fine.)

Additional advice

• Objectify and methodize repetitive code

- Avoid hard coding
( Your code seems too inefficient to me. You don't use object orientation in an object-oriented language).