PrintWriter append method not appending

The fact that PrintWriter‘s method is called append() doesn’t mean that it changes mode of the file being opened.

You need to open file in append mode as well:

PrintWriter pw = new PrintWriter(new FileOutputStream(
    new File("persons.txt"), 
    true /* append = true */)); 

Also note that file will be written in system default encoding. It’s not always desired and may cause interoperability problems, you may want to specify file encoding explicitly.