DoctorComboBoxListener.java

public class DoctorComboBoxListener implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent e) {
        Doctor doctor = (Doctor) MainFrame.cmbDoctor.getSelectedItem();
        System.out.println(doctor.getId());
    }
    
}

KonstruktorAplikasi.java

public MainFrame() {
        initComponents();
        
        disableAll();
        
        // Make fullscren startup
        setExtendedState(MainFrame.MAXIMIZED_BOTH);
        
        getPatientData();
        getDoctorData();
        cmbDoctor.addActionListener(new DoctorComboBoxListener());

        visitDataTable();
        getCaptureImgData(panelCapture);
}

MethodgetDoctorData.java

public void getDoctorData() {
        DoctorService doctorService = new DoctorService();
        List<Doctor> list = doctorService.selectAll();        
        
        cmbDoctor.removeAllItems();
        for (Doctor doctor : list) {
            cmbDoctor.addItem(doctor);
        }
}