Java ComboBox

public class MyForm extends JFrame {

    public MyForm() {

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setBounds(100, 100, 343, 273);

        setTitle("Kittisay Java");

        setLayout(null);

        // Combobox

        String[] petStrings = {"Bird", "Cat", "Dog", "Rabbit", "Pig"};

        JComboBox comboBox = new JComboBox(petStrings);

        comboBox.setSelectedIndex(4);

        comboBox.setBounds(113, 76, 107, 20);

        add(comboBox);

        // Button

        JButton btn = new JButton("Button");

        btn.addActionListener((e) -> {

            JOptionPane.showMessageDialog(null, 

             "You selected : " + comboBox.getSelectedItem());

        });

        btn.setBounds(126, 123, 81, 23);

        add(btn);

    }

    public static void main(String[] args) {

      MyForm frm = new MyForm();

      frm.setVisible(true);

    }