title: How to get list of properties using apache.commons
body: <p>I need to get the list of properties which are in the .properties file. For example, if have the following .properties file:</p>
  
  <pre><code>users.admin.keywords = admin
  users.admin.regexps = test-5,test-7
  users.admin.rules = users.admin.keywords,users.admin.regexps
  
  users.root.keywords = newKeyWordq
  users.root.regexps = asdasd,\u0432[\u044By][\u0448s]\u043B\u0438\u0442[\u0435e]
  users.root.rules = users.root.keywords,users.root.regexps,rules.creditcards
  
  users.guest.keywords = guest
  users.guest.regexps = *
  users.guest.rules = users.guest.keywords,users.guest.regexps,rules.creditcards
  
  rules.cc.creditcards = 1234123412341234,11231123123123123,ca
  rules.common.regexps = pas
  rules.common.keywords = asd
  </code></pre>
  
  <p>And as a result I'd like to get an ArrayList which consists of names of fields like this:
  <code>users.admin.keywords, users.admin.regexps, users.admin.rules</code> and so on. And as you have noticed, I need to do this using apache.commons.config </p>