How to develop graphical user interface like Getter?

I plan to develop a IDEA plugin to add a annotation for some method in a java file. It can be executed as same as Getter and Setter. I need to develop a UI for this plugin. I had developed a UI by swing, but it's ugly. I have been searched from source of IDEA community, but I don't found. So, I want to know how to develop UI as same as Getter or toString.