DateField - Work with weeks, dateFormat problem

I’m trying to work with weeks DateField, setting ISOweekNumber.
The problem seems to be formatting, because with passing String
, he does not want to know anything.

Instead, overriding setDateFormat as below, It is formatted correctly.
the problem recurs when the user change the value.
P.S= The field is an String but is treaty as Date.

DateField dateMonthYear = new DateField(){
    private static final long serialVersionUID = 1L;

      public void setDateFormat(String dateFormat) {
         super.setDateFormat(new SimpleDateFormat("w-Y").format(propertyDate));