小编Zac*_*tte的帖子

好吧,我正在点击游戏和Blah Blah Blah ...点是,我不能让JLabel更新......我很困惑......看到我已经完成了这个......这里是我的守则.

            import javax.swing.*;
            import java.awt.*;
            import java.awt.event.*;
            public class ClickingGame extends JPanel implements ActionListener {
      private static final long serialVersionUID = 1L;

    static JFrame frame;
    static JButton startbutton, clickingbutton, timerstop;
    static JLabel timelabel, scorelabel;
    static int time = 0;
    static JTextField entertime;
    static Timer clock;
    static Timer countdown;

public ClickingGame() {
    setLayout(new GridLayout(3, 2, 5, 5));

    startbutton = new JButton("Start CountDown");

    timelabel = new JLabel("Time Left = NULL", SwingConstants.CENTER);

    entertime = new JTextField();

    clickingbutton = new JButton("Click …

File f = new File(System.getProperty("user.home/Downloads"));

为什么这段代码会出现NPE错误？

我知道它返回null,但目录在那里......