code: private static String forEachFields(final Set<String> fields, OnFieldListener listener) {
  		StringBuilder builder = new StringBuilder();
  		{
  			String comma = "";
  			for (String item : fields) {
  				builder.append(comma + listener.onField(item));
  				comma = ", ";
  			}
  		}
  
  		return builder.toString();
  	}
docs: For each fields.
  
  @param fields
  the fields
  @param listener
  the listener
  @return the string