Question

点击一个按钮后，我得到一个NullPointerException，我希望在屏幕上打印ArrayList中的值。例如，如果我点击指向“A”的按钮，屏幕上将显示“A”，如果我点击指向“B”的按钮，屏幕上将显示“B”。

public class Media extends JPanel {

    //Declares our media player component
    private JPanel video_pnl, control_pnl;
    private JButton play_btn;
    private JLabel loc_lbl;
    private int increment;
    ArrayList<String> file_location;

    public Media(ArrayList<String> file_location) {
        this.file_location = file_location;
        increment = 0;
        while (increment < file_location.size()) {
            video_pnl = new JPanel();
            video_pnl.setLayout(new BoxLayout(video_pnl, BoxLayout.Y_AXIS));
            loc_lbl = new JLabel();
            loc_lbl.setText(file_location.get(increment));
            play_btn = new JButton("Play");
            control_pnl = new JPanel();
            control_pnl.setLayout(new FlowLayout(FlowLayout.CENTER));
            play_btn.setActionCommand("play");
            video_pnl.add(loc_lbl);
            control_pnl.add(play_btn);
            video_pnl.add(control_pnl, BorderLayout.SOUTH);

            Handler handler = new Handler();
            play_btn.addActionListener(handler);

            video_pnl.revalidate();
            add(video_pnl);
            increment++;
        }
    }

    private class Handler implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getActionCommand().equals("play")) {
                play();
            }
    //            if (e.getSource() == play_btn){
    //                play();
    //            }
        }
    }

    public void play() {
        for (int i = 0; i < file_location.size(); i++) {
            System.out.println(file_location.get(i));
        }
    }

    public static void main(String[] args) {
        //Declare and initialize local variables
        ArrayList<String> file_location = new ArrayList<>();
        file_location.add("A");
        file_location.add("B");
        file_location.add("C");
        file_location.add("D");
        file_location.add("E");

        //creates instances of the VlcPlayer object, pass the mediaPath and invokes the method "run"
        Media mediaplayer = new Media(file_location);
        JFrame ourframe = new JFrame();
        ourframe.setContentPane(mediaplayer);
        ourframe.setLayout(new GridLayout(5, 1));
        ourframe.setSize(300, 560);
        ourframe.setVisible(true);
        ourframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

Answer 1

添加：

this.file_location = file_location;

进入构造函数（在此行下：public Media（ArrayList file_location）{）

Answer 2

在构造函数中，您引用参数file_location来构建面板，但是您从不将数组存储在类变量中。由于默认情况下它已初始化为null，因此当您在play ()方法中引用NPE时，会获得NPE。

使用参数的值（this.file_location = file_location）在构造函数中初始化类成员，它将解决您的问题。

另外，我建议您使用尚未用于类成员的参数名称。如果您指的是参数或成员，那么在阅读代码时会更容易注意，并且可以防止您刚刚观察到的问题。

我希望一旦按下按钮，就会显示ArrayList中的值

2 个答案: