JAVA中类名不是抽象的,并且未覆盖抽象方法

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class ComboBoxFrame extends JFrame{
    private String []citys = {"HongKong","Shanghai","Beijing","Xi'an"};
    private JComboBox combCity = new JComboBox();
    private JButton b = new JButton("Add City");
    private JTextField t = new JTextField(15);
    private int count = 0;
    public ComboBoxFrame(){
        setSize(new Dimension(200,175));
        combCity.addItem("Shenzhen");
        b.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                if(count < citys.length){
                    combCity.addItem(citys[count++]);
                }
            }
        });
        combCity.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                int index = ((JComboBox)e.getSource()).getSelectedIndex();
                t.setText("index:" + index);
            }
        });

        getContentPane().setLayout(new FlowLayout());
        getContentPane().add(t);
        getContentPane().add(combCity);
        getContentPane().add(b);

    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable(){
            public void run(){
                try{
                    UIManager.setLookAndFeel(
                        UIManager.getSystemLookAndFeelClassName());
                }catch(Exception exception){
                    exception.printStackTrace();
                }
                ComboBoxFrame comboboxframe = new ComboBoxFrame();
                comboboxframe.setVisible(true);
            }
        });
    }
}

COPY过去运行吧！

actionPerformed