Fix delete-bug for the wildcard, surname, id

[medkhabt]

fixed this problem "The content of records will not be removed even though you have deleted the course from the course list, although it has no influence." for wildcard, surname query and id. If you still see the bug persist, i would still try to help out.

The solution isn't optimal, just quick fix.

[medkhabt]

@KennardWang would you review the change ? if you find any issue would you mention it ?

[KennardWang]

I had reviewed your change and I was thankful to your effort, but unfortunately the problem still existed. Probably I did not explain it clearly. The 3rd issue I mentioned meant:

1. After Deleting all the courses, the content of display should be cleared.
2. And after each Query, I expect to clear the query condition (e.g. Vona).

At very beginning, I programed this project when I just learned a little Java and Swing so that I did not take too much into consideration. After this semester, I am going to do some structural modification. If you are interested in it, we can work together.

[medkhabt]

@KennardWang Thank you for reply. I look forward to working with you, i wouldn't be really that committed, but i'll look from time to time. I just want to mention i don't really know much about Swing.
I see now the whole issue, I think because i am using mac, the app interface doesn't work as intended, since i always see an empty table, that is supposed to show students instead i only get average grades and other info in an alert when i click on display (the one with the black), but still i 'll try to see how to fix that.
. The part that is working is the one making a query and its result shows as an alert.
I will show you what i thought i fixed.

• I imported 2 files and queried with surname condition "Yu". It resulted in two instances.

• I delete the LP002.txt and query again. And it only showed one result

Before change, the result stayed the same after the delete of the first txt file.

• Going back to the issue, i think one of it facades is making an open file 'button', instead of clicking on the imported courses, because i think it is possible in our current case to open a file which not part of the imported files (imported courses). The second thing, it would be cool if you can link the course class with the Student, they should have a relation btw them, like many-to-many in this case i guess ( many students can take one course, many courses can be taken by a student), actually i think it is better to have an other class "Grade/Exam" (lack of a better word) which links both course and student ( we should remove the grad, score of one course from student class as it doesn't represent him/her, it is something represented by the presence of both student and course ).
  
  // quick graph to describe what i mean.

I didn't read all the code so i might be wrong on many things. I hope it found you well, cheers.

[KennardWang]

@medkhabt Thanks a lot for letting me know about this issue in Mac. Your suggestions are sincere, and I decide to combine Open File button and Import File button together to reduce the redundancy. The original aim of this project is helping me to understand better in Java data structure instead of database, and I will do some modification later on which you have mentioned. By the way, would u mind helping me to test for the next version in Mac because my OS is Windows 10, and I would like to add u into the credit list for testing in README.

[medkhabt]

@medkhabt Thanks a lot for letting me know about this issue in Mac. Your suggestions are sincere, and I decide to combine Open File button and Import File button together to reduce the redundancy. The original aim of this project is helping me to understand better in Java data structure instead of database, and I will do some modification later on which you have mentioned. By the way, would u mind helping me to test for the next version in Mac because my OS is Windows 10, and I would like to add u into the credit list for testing in README.

Yeah, i would like to help with that.