Question

我已经将ActionListener实现为我的主类的内部类，并且我试图在该类的实例上调用其中一个主类方法。但是它继续在ActionListener中的方法调用上返回NullPointerException。

提供的代码。

尝试点击＆＃34; RefreshButton＆＃34;按预期将文本附加到TextArea，但随后返回＆＃34;线程中的异常＆＃34; AWT-EventQueue-0＆＃34;显示java.lang.NullPointerException 在frontend.MainGUI $ RefreshListener.actionPerformed（MainGUI.java:104）＆＃34;

指向gui.testing（）;在RefreshListener中。我可能错过了一些简单的东西，但它给了我一些麻烦。

public class MainGUI {
    JFrame MainFrame;
    JLabel MainLabel;
    JLabel FooterLabel;
    JTextArea ListText;
    MainGUI gui;

public static void main (String[] args) {
   MainGUI staticGui = new MainGUI();
   staticGui.begin();
} // Close Main

public void begin() {
    gui = new MainGUI();
    gui.testing();
    gui.go();
}

public void go() {
    MainFrame = new JFrame();
    MainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // Setup Buttons, and their Listeners
    JButton RefreshButton = new JButton("Refresh");
    RefreshButton.addActionListener(new RefreshListener());

    // GUI stuff, working and unrelated, I think //      
    // Setup Labels
    MainLabel = new JLabel("Observations");
    Font Header = new Font("serif", Font.BOLD, 32);
    MainLabel.setFont(Header);

    FooterLabel = new JLabel("FooterText", SwingConstants.CENTER);
    Font Footer = new Font("serif", Font.PLAIN, 12);
    FooterLabel.setFont(Footer);
    FooterLabel.setForeground(Color.DARK_GRAY);

    // Setup Panel that holds the buttons
    MyDrawPanel buttonPanel = new MyDrawPanel();
    Dimension d = new Dimension(150,100);
    buttonPanel.setPreferredSize(d);
    buttonPanel.setBackground(Color.darkGray);
    //drawPanel.setLayout(new BoxLayout(buttonPanel, BoxLayout.Y_AXIS)); // Old Layout, Grid works better.
    buttonPanel.setLayout(new GridLayout(9, 1, 0, 1));

    // Setup Text area for displaying Observations        
    ListText = new JTextArea(24,48);
    ListText.setLineWrap(true);
    ListText.setMargin(new Insets(2,4,0,0)); // Text Padding, Top/Left

    JScrollPane scroller = new JScrollPane(ListText);
    scroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED);
    scroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        // Default Values
    ListText.setText("Nothing currently loaded...\n");
    ListText.append("Please import a HTML file.\n");
    ListText.setEditable(false);

    // Put Everything into JFrames
    MainFrame.getContentPane().add(BorderLayout.NORTH, MainLabel);
    MainFrame.getContentPane().add(BorderLayout.EAST, buttonPanel);
    MainFrame.getContentPane().add(BorderLayout.CENTER, scroller);
    MainFrame.getContentPane().add(BorderLayout.SOUTH, FooterLabel);
    buttonPanel.add(ImportButton);
    buttonPanel.add(RefreshButton);
    buttonPanel.add(ExitButton);

    //MainFrame Settings
    MainFrame.setSize(720, 480);
    MainFrame.setVisible(true);    
    MainFrame.setResizable(false);   

} // Close Go

public void testing() {
    System.out.print("TestMethodSuccess");
}

class ImportListener implements ActionListener { // Inner Class
    public void actionPerformed(ActionEvent event) {
    ListText.append("Import Button!\n");
    }
}

    class RefreshListener implements ActionListener { // Inner Class
     public void actionPerformed(ActionEvent event) {
      ListText.append("Refresh Button!\n");
      gui.testing();       // Error traces to here //
    }
}

    class CloseListener implements ActionListener { // Inner Class
    public void actionPerformed(ActionEvent event) {
    System.exit(0);
    }
} 

}  // Close Class

我还尝试了以下主方法，而不是使用begin（）方法，在同一个地方发布。我认为这与变量阴影有关吗？无论如何，解决方案或解释任何一个都会很棒。

public static void main (String[] args) {
    MainGUI gui = new MainGUI();
    gui.go();
}

从ActionListener调用类方法

0 个答案: