java - 如何修复 NullPointerException 错误？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 8 年前。

当我运行简单的 Java 浏览器时，我尝试访问诸如 http://google.com 之类的网页。它从我的 try catch 代码中返回 NullPointerException 错误，我该如何解决这个问题？

框架类:

public class Frame extends JFrame {

public EditorPane pane;
public URLBar urlbar;

public static void main(String[] args) throws Exception {

    Frame frame = new Frame();

}

public Frame() throws Exception {
    super("Java Browser v1.0");

    JPanel mainPanel = new JPanel(new BorderLayout());
    URLBar addressBar = new URLBar("Enter URL here!", pane);
    EditorPane contentDisplay = new EditorPane(urlbar);

    mainPanel.add(contentDisplay, BorderLayout.CENTER);
    mainPanel.add(addressBar, BorderLayout.NORTH);

    add(mainPanel);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setSize(700, 400);
    add(new JScrollPane(mainPanel));
    setVisible(true);
}

}

URLBar 类:

public class URLBar extends JTextField {

public EditorPane pane;

public URLBar(String text, EditorPane pane) {

    super(text);

     addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
                loadContent(event.getActionCommand());
            }
        }
     );
}

public void loadContent(String userInput) {
    try
    {
        pane.setPage(userInput);
        setText(userInput);
    }
    catch (Exception e)
    {
        System.out.println("A wild exception appeared! Type: " + e);
    }
}

}

EditorPane 类:

public class EditorPane extends JEditorPane {

public URLBar urlbar;

public EditorPane(URLBar urlbar) {

    setEditable(false);
    setVisible(true);
    addHyperlinkListener(
            new HyperlinkListener() {
                public void hyperlinkUpdate(HyperlinkEvent event) {
                        if(event.getEventType()==HyperlinkEvent.EventType.ACTIVATED) {
                        urlbar.loadContent(event.getURL().toString());
                    }
                }
            }
    );

}

}

Answer 1

您似乎忘记在 URLBar 构造函数中初始化 pane 成员，这意味着当您调用 loadContent 时它为 null。

修复方法如下:

public URLBar(String text, EditorPane pane) {

    super(text);

    this.pane = pane; // add this
     addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
                loadContent(event.getActionCommand());
            }
        }
     );
}