How i can allow user to input "empty" for integer columns in Jtable and mysql table?

I want allow the user to pass some optional data in the input like:

  1. salary, Graduation year are int type
  2. Hiring Date, End Date, Birth Date are str type

How can I avoid this, and allow mysql Entry?

public class Employee implements mainData{
    private int EmpNo;
    private String EmpName;
    private String HireDate;
    private String EndDate;
    private Double Salary;
    private int GraduationYear;
    private String BirthDate;
    private String Nationality;
    private int DespNo;
    private String Note;

    public int getEmpNo() {
        return EmpNo;
    }

    public void setEmpNo(int EmpNo) {
        this.EmpNo = EmpNo;
    }
//and continue setters & getters
Employee emp = new Employee();

private void setValues(){
        //Main Entry
            emp.setEmpNo(Integer.parseInt(txtEmpNo.getText()));
            emp.setEmpName(txtEmpName.getText());

        // optional input (have problem as type is integer)
            emp.setSalary(Double.parseDouble(txtSalary.getText()));
            emp.setGraduationYear(Integer.parseInt(txtGradyear.getText()));

        // optional input (no problem as type is string)
            emp.setNationality(txtNationality.getText());
            emp.setNote(txtNote.getText());

        //optional input ((have problem type is string)
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

            String HDate = String.valueOf(sdf.format(dteHire.getDate()));
            String EDate = String.valueOf(sdf.format(dteEnd.getDate()));
            String BDate = String.valueOf(sdf.format(dteBirth.getDate()));

            emp.setHireDate(HDate);
            emp.setHireDate(EDate);
            emp.setBirthDate(BDate);

        //Main Entry    
            String dspNam = cmbDespline.getSelectedItem().toString();
            String dspNo = desp.getValueByName(dspNam);

            emp.setDespNo(Integer.parseInt(dspNo));

    }

    private void btnAddActionPerformed(java.awt.event.ActionEvent evt) {                                       
        setValues();
        emp.add();
        clearData();
        emp.getAllRows(tblEmployee);

If there is an empty entry by the user for the optional input, the row entry is not added to the jtable or mysql table and get the following errors:

  1. The following error occurs for Salary:

    Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: empty String

  2. The following error occurs for Dates:

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException Thanks for any help