JLabel声明解决办法

    public SwingWorkerTest() {                setTitle("SwingWorker\u4F7F\u7528");        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        setBounds(100, 100, 300, 150);        contentPane = new JPanel();        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));        setContentPane(contentPane);        contentPane.setLayout(new GridLayout(2, 1, 5, 5));                JPanel labelPanel = new JPanel();        contentPane.add(labelPanel);        labelPanel.setLayout(new BorderLayout(0, 0));         [color=#FF0000]       label = new JLabel("");[/color]        label.setHorizontalAlignment(SwingConstants.CENTER);        labelPanel.add(label);                JPanel buttonPanel = new JPanel();        contentPane.add(buttonPanel);                JButton button = new JButton("\u751F\u6210\u968F\u673A\u6570");        button.addActionListener(new ActionListener() {            public void actionPerformed(ActionEvent e) {                do_button_actionPerformed(e);            }        });        buttonPanel.add(button);            }        protected void do_button_actionPerformed(ActionEvent e)    {        RandomNumber random = new RandomNumber();        random.execute();    }        private class RandomNumber extends SwingWorker<Void, Integer>{        @Override        protected Void doInBackground() throws Exception {            TreeSet<Integer> set = new TreeSet<Integer>();// 创建一个TreeSet类型对象            for (int i = 0; i < 1000; i++) {                set.add(new Random().nextInt());// 在TreeSet中存入1000个随机整数            }            label.setText("最大随机数是：" + set.last());// 在标签上显示最大的整数            return null;        }    }