JTextfield does not show korean charaters properly. It shows empty boxes in instead of characters. Here is the screenshot of my application.
Thats because of the fonts that you used.
According to this oracle document Korean letters is not supported by Lucida font.
Note that of the writing systems that are generally fully supported by the JRE, the Lucida fonts do not support Chinese (Simplified), Chinese (Traditional), Japanese, and Korean.
So if you use the fonts that support the korean language, it will work properly.
来源：https://stackoverflow.com/questions/42244385/korean-characters-are-displayed-as-empty-boxes-on-jtextfield