为什么我的Java GUI中没有显示任何内容？

I am currently working on a calculator. Right now, I am trying to gain experience with coding Java GUI, by making a simple program that makes a window with a text field. The code can compile without errors, but when I execute the program, the window appears, but without the text field. How do I make the text field visible? The code is shown as follows:I am currently working on a calculator. Right n