code: public static String join(String separator, Collection<String> strings) {
  		if(strings != null) {
  			String[] array = new String[strings.size()];
  			return join(separator, strings.toArray(array)); 
  		}
  		return null;			
  	}
docs: Joins the given set of strings using the provided separator.
  
  @param separator
  a character sequence used to separate strings.
  @param strings
  the set of strings to be joined.
  @return
  a string containing the list of input strings, or null if no valid input
  was provided.