我的程序抛出一个空指针异常,我无法弄清楚为什么.有谁给我一双新鲜的眼睛？

Question

你好互联网的人.我正在用Java编写一个程序来介绍cs类,它创建了一个带有12个按钮和一个文本字段的gui.当用户按下其中一个按钮时,相应的字符将显示在文本字段中.基本上,我创建了一个方法,该方法使用12个按钮对象的数组来使用这些按钮填充按钮面板.此方法用于创建数组并使用按钮对象填充的另一个类.当我运行该程序时,我收到此错误:

Exception in thread "main" java.lang.NullPointerException
    at TextButtonsHWPanel.<init>(TextButtonsHWPanel.java:26)
    at TextButtonsHW.<init>(TextButtonsHW.java:56)
    at TextButtonsHW.main(TextButtonsHW.java:77)

这是我的两个班级:

public class TextButtonsHWPanel extends JPanel {
    private JPanel buttonPanel;
    private JScrollPane scrollPane;

/**
 * Constructor
 * @param buttons array of JButtons to appear in a grid for text input
 * @param textArea JTextArea for display of text in response to pressed buttons
 */
public TextButtonsHWPanel(JButton[] buttons, JTextArea textArea) {
    //TODO: Create a sub-panel with a 4 row, 3 column GridLayout
    buttonPanel.setLayout(new GridLayout(4,3));
    //TODO: Populate the grid with buttons
    for (int i = 0; i < 12; i++) {
        buttonPanel.add(buttons[i]);
    }
    //TODO: Add the grid panel to this panel
    this.add(buttonPanel);
    //TODO: Create a JScrollPane containing textArea
    scrollPane = new JScrollPane(textArea);
    //TODO: Set the preferred size of the scroll pane to 80x120
    scrollPane.setPreferredSize(new Dimension(80, 120));
    //TODO: Add the scroll pane to this panel
    this.add(scrollPane);
}



public class TextButtonsHW extends JFrame implements ActionListener {
    private JButton[] buttons; //Do not change
    private JTextArea textArea; //Do not change
    private String[] buttonNames = {"a", "b", "c", "1", "2", "3", "x", "y", "z", "Enter", "Space", "Clear"};

//Assign values for these constants in the constructor
private final int ENTER;    //Index of Enter button in buttons
private final int SPACE;    //Index of Space button in buttons
private final int CLEAR;    //Index of Clear button in buttons

/**
 * Set up this frame and its contents.
 * @param title Title to appear in JFrame title bar
 */
public TextButtonsHW(String title) {
    super(title); //call parent JFrame constructor to set the title

    //TODO: instantiate all JButtons, add them to the buttons array,
    //  and register "this" as the ActionListener for each button.
    buttons = new JButton[12];
    for (int j = 0; j < buttons.length; j++) {
        buttons[j] = new JButton(buttonNames[j]);
        buttons[j].addActionListener(this);
    }

    //TODO: assign values to ENTER, SPACE, and CLEAR constants to
    //  indicate the indexes of those buttons in the buttons array
    ENTER = 9;
    SPACE = 10;
    CLEAR = 11;

    //TODO: create the JTextArea textArea
    textArea = new JTextArea();

    //TODO: set its "editable" property to false
    textArea.setEditable(false);

    //Create a TextButtonsHWPanel to display the buttons and textArea
    TextButtonsHWPanel mainPanel = new TextButtonsHWPanel(buttons, textArea);

    this.getContentPane().add(mainPanel);
    this.pack();
}

/* (non-Javadoc)
 * @see java.awt.event.ActionListener#actionPerformed(java.awt.event.ActionEvent)
 */
@Override
public void actionPerformed(ActionEvent e) {
    //TODO: update the text of textArea according to which
    //  button generated the ActionEvent.

}

/**
 * Create this JFrame 
 * @param args not used
 */
public static void main(String[] args) {
    final TextButtonsHW f = new TextButtonsHW("Text Buttons");
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setLocationRelativeTo(null); //centers frame on screen
    f.setVisible(true);
}

}

在这一点上,我觉得我应该能够运行程序并显示一堆什么都不做的按钮和一个空白文本字段.谁能看到我出错的地方？

编辑:已解决

Answer 1

是的,看这里:

private JPanel buttonPanel;

这将保留buttonPanel其默认值null.

现在是构造函数的第一行:

buttonPanel.setLayout(new GridLayout(4,3));

...你正在取消引用buttonPanel,它仍然是空的.砰.你需要指定一个非空的参考buttonPanel,如

private JPanel buttonPanel = new JPanel();

您似乎正在分配所有其他变量值 - 虽然在构造函数体中这样做,这也很好 - 它只是缺少的那个.

堆栈跟踪应该已经显示了哪一行引发了异常 - 应该已经为您提供了足够的信息来解决问题.