contract with date validation

[DataContractAttribute]
class NW_WHTContract implements SysOperationValidatable
{
    FromDate    fromDate;
    ToDate      toDate; 


    [DataMemberAttribute('FromDate')]
    public FromDate parmFromDate(FromDate _formDate = fromDate)
    {
        fromDate = _formDate;
        return fromDate;
    }

    [DataMemberAttribute('ToDate')]
    public ToDate parmToDate(ToDate _toDate = toDate)
    {
        toDate = _toDate;
        return toDate;
    }

    public boolean validate()
    {
        boolean isValid = true;
        if(this.fromDate && this.toDate && (this.fromDate > this.toDate))
        {
            Error('@SYS91020');
            isValid=false;
        }
        return isValid;
    }

}