My problem is not so big, but it is annoying.

See my JFrame within my IDE (Netbeans 12.6):

The layout manager for [content pane of] JFrame is BorderLayout. As you can see in the "tree" in the bottom left corner of the above image, JFrame contains three panels, namely topPanel (whose layout manager is BoxLayout), resultPanel and bottomPanel (which both have FlowLayout as their layout manager). Hopefully you will now be able to understand the rest of the "tree".

Now to your problem regarding the space between the JTextFields in the top part of the GUI. As you can see, I used