Setting the selected file in a the filename textfield JFileChooser when it starts up

You just have to create a new File and set it as selectedFile before starting the FileChooser. Depending on what you want to do with the selected file you should check the file with File.canRead(), File.canWrite() or File.exists(). Here is an example:

File file = new File(date + " " + user + ".pdf");
JFileChooser fc = new JFileChooser();
int returnValue = fc.showSaveDialog(this);

This must be wrapped in a try-catch-block, because creating a File could throw an exception.