question: private void handleArgumentField ( int begin , int end , int argIndex , FieldPosition position , List < FieldContainer > fields ) { if ( fields != null ) { fields . add ( new FieldContainer ( begin , end , Field . ARGUMENT , Integer . valueOf ( argIndex ) ) ) ; } else { if ( position != null && position . getFieldAttribute ( ) == Field . ARGUMENT && position . getEndIndex ( ) == 0 ) { position . setBeginIndex ( begin ) ; position . setEndIndex ( end ) ; } } }
target:  Adds a new FieldContainer with MessageFormat.Field.ARGUMENT field, argIndex, begin and end index to the fields list, or sets the position's begin and end index if it has MessageFormat.Field.ARGUMENT as its field attribute.