java - 添加框架内容时出现空指针异常

Question

在 Swing 的 JFrame 中添加内容时出现空指针异常。

    public static void createAndShowGUI() {
    //Create and set up the window.
    frame = new JFrame("Data Entry Application");
    //Set up the content pane.
    // frame = new JFrame();
    addComponentsToPane(frame.getContentPane());  // null pointer exception in this line
     }

 public static void addComponentsToPane(Container pane) {
    if (RIGHT_TO_LEFT) {
        pane.setComponentOrientation(ComponentOrientation.RIGHT_TO_LEFT);
    }

    Border blackline, raisedetched = null, loweredetched,
            raisedbevel = null, loweredbevel, empty, orangeline, redline, greenline, blueline;
    blackline = BorderFactory.createLineBorder(Color.black);
    orangeline = BorderFactory.createLineBorder(Color.ORANGE);
    redline = BorderFactory.createLineBorder(Color.RED);
    greenline = BorderFactory.createLineBorder(Color.GREEN);
    blueline = BorderFactory.createLineBorder(Color.blue);

    JButton button;
    pane.setLayout(new GridBagLayout());
    GridBagConstraints c = new GridBagConstraints();
    if (shouldFill) {
        //natural height, maximum width
        c.fill = GridBagConstraints.HORIZONTAL;
    }

    /* Header Section start*/
    /* Username and designation starts */
    JPanel p_username = new JPanel();
    p_username.setMinimumSize(new Dimension(140, 30));
    l_username = new JLabel(Login.login_username);
    l_designation = new JLabel("Data Entry User");

    JPanel t9 = new JPanel(new GridLayout(0, 1));
    t9.add(l_username);
    t9.add(l_designation);
    t9.setPreferredSize(new Dimension(140, 30));
    p_username.add(t9);
   c.fill = GridBagConstraints.NORTH;
    c.gridx = 0;
    c.gridy = 0;
    c.ipady = 25;
    c.insets = new Insets(0, 10, 0, 0);
    pane.add(p_username, c);
    /* Username and designation end */
  }

提出一些想法。

Answer 1

我模拟了您的 addComponentsToPane() 方法来向您展示问题所在:

private static void addComponentsToPane(Container container)
{
    System.out.println("Is container null? " + container == null);
    JPanel panel = null;
    container.add(panel);
}

调用此方法会产生以下输出:

false
Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.addImpl(Container.java:1091)
    at java.awt.Container.add(Container.java:415)
    at FrameDemo.addComponentsToPane(FrameDemo.java:33)
    at FrameDemo.createAndShowGUI(FrameDemo.java:25)
    at FrameDemo.main(FrameDemo.java:14)

如果您阅读了 Javadoc，您会发现 getContentPane() 方法不会返回 null，正如您从结果输出中看到的那样。我的第二行声明了一个 JPanel 但没有实例化该对象，然后我尝试将其添加到内容 Pane 中。在我的例子中，这导致了 NullPointerException。

我的结论:您正在将一个组件添加到尚未正确实例化的容器中。事实上，如果您阅读 Container.add(comp) 方法的 Javadoc，它会指出，如果 comp 为无效的。检查您尝试添加到其中的所有组件，然后您就可以找出其余的组件。