Java - pq no me funciona el getSelectedItem mi combo box tiene solo datos String :c

private void btn_aceptarActionPerformed(java.awt.event.ActionEvent evt) {

    String cadena2,cadena3,cadena4,cadena5,cadena6,cadena7,cadena8,cadena9,cadena10;

    cadena2 = txt_codigo.getText();

    cadena3 = txt_numeroMotor.getText();

    cadena4 = cmb_tipoVehiculo.getSelectedItem().toString();

    cadena5 = cmb_marca.getSelectedItem().toString();

    cadena6 = txt_anio.getText();

    cadena7 = txt_color.getText();

    cadena8 = txt_precio.getText();

    cadena9 = cmb_estado.getSelectedItem().toString();

    cadena10 =cmb_revision.getSelectedItem().toString();

    if (txt_codigo.getText().equals("") || (txt_numeroMotor.getText().equals("")) || (cmb_tipoVehiculo.getSelectedItem().equals(null))) || (cmb_marca.getSelectedItem().equals(null))

       || (txt_anio.getText().equals("")) || (txt_color.getText().equals("")) || (txt_precio.getText().equals("")) ||(cmb_estado.getSelectedItem().equals(null)) ||(cmb_revision.getSelectedItem().equals(null))) {

        javax.swing.JOptionPane.showMessageDialog(this,"Debe llenar todos los campos \n","AVISO!",javax.swing.JOptionPane.INFORMATION_MESSAGE);

        txt_codigo.requestFocus();

    }

public class ComboBox extends JFrame {

	private static final long serialVersionUID = -4009559269919494234L;

	private JComboBox<String> combo;

	public ComboBox() {

		super("Validar Combo");

		setSize(350, 90);

		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		setLocationRelativeTo(null);

		setResizable(false);

		init();

	}

	public void init() {

		combo = new JComboBox<String>();

		combo.addItem("Seleccione");

		loadCombo();

		JButton boton = new JButton("Validar");

		boton.addActionListener(new ActionListener() {

			public void actionPerformed(ActionEvent e) {

				validateCombo();

			}

		});

		Container container = getContentPane();

		container.setLayout(new FlowLayout());

		container.add(new JLabel("Eliga una opción:"));

		container.add(combo);

		container.add(boton);

	}

	private void loadCombo() {

		combo.addItem("Valor 1");

		combo.addItem("Valor 2");

		combo.addItem("Valor 3");

	}

	private void validateCombo() {

		if(combo.getSelectedItem().equals("Seleccione")) {

			JOptionPane.showMessageDialog(null, "Seleccione una opción.","¡Error!", JOptionPane.ERROR_MESSAGE);

		} else {

			JOptionPane.showMessageDialog(null, "El valor seleccionado es: " + combo.getSelectedItem().toString(),"Información", JOptionPane.INFORMATION_MESSAGE);

		}

	}

	public static void main(String[] args) {

		ComboBox ventana = new ComboBox();

		ventana.setVisible(true);

	}

}