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我有一个 GUI 程序，它根据按下的按钮附加一个文本字段。我在实现我的动作监听器时遇到问题。我在 ActionPerformed 方法中放入什么代码并不重要，我总是得到相同的错误。有很多错误，我无法列出它们，但它们似乎表明 actionListener 没有触发。我不知道我是否没有为按钮正确添加 ActionListener 或什么。我目前在 ActionPerformed 方法中的代码只是一个测试，看看它是否适用于我按下的任何按钮，但它仍然不起作用。提前致谢。

*编辑：好的，我更改了 ActionPerformed 方法中的代码，因此它会根据按下的按钮附加文本区域，并且我再次遇到相同的错误。

**编辑：没有更多错误，只是我的输出有问题。我的 ENTER 和 SPACE 按钮的空白区域越来越大。更改了 ActionPerformed 方法，因此它现在适当地附加到每个按钮。

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.text.*;

/**
 * GUI contains a grid of buttons and a text area. As buttons are pressed,
 * corresponding text is displayed in the text area.
 * 
 * This class serves as the program driver, the GUI frame, and the
 * ActionListener for JButton-generated events.
 * 
 **/
public class TextButtonsHW extends JFrame implements ActionListener {
    private JButton[] buttons;// Do not change
    private String[] labels;
    private JTextArea textArea; // Do not change
    // 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
        buttons = new JButton[12];
        String[] labels = { "A", "B", "C", "1", "2", "3", "X", "Y", "Z",
                "ENTER", "SPACE", "CLEAR" };
        // sets the value of the ENTER SPACE and CLEAR ints to indicate their
        // index in the array
        ENTER = 9;
        SPACE = 10;
        CLEAR = 11;
        // TODO: instantiate all JButtons, add them to the buttons array,
        // and register "this" as the ActionListener for each button.
        for (int i = 0; i < buttons.length; i++) {
            buttons[i] = new JButton(labels[i]);
            buttons[i].addActionListener(this);

        }

        // TODO: create the JTextArea textArea
        textArea = new JTextArea();
        textArea.setEditable(false);
        textArea.setText("");
        // 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.
        for (int i = 0; i < buttons.length; i++) {
            if (i < ENTER && e.getSource() == buttons[i])
                textArea.append(buttons[i].getText());
            else if (e.getSource() == buttons[ENTER])
                textArea.append("\n");
            else if (e.getSource() == buttons[SPACE])
                textArea.append(" ");
            else if (e.getSource() == buttons[CLEAR])
                textArea.setText("");
        }

    }

    /**
     * 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);
    }
}

TextButtonsHWPanel 代码：

import java.awt.Dimension;
import java.awt.GridLayout;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;

/**
 * Used by TextButtonsHW to display a grid of JButtons
 * and a JTextArea.
 * 
 */
public class TextButtonsHWPanel extends JPanel {

    /**
     * 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) {
        //Creates a sub-panel with a 4 row, 3 column GridLayout and fills it with buttons
        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(4, 3));
        for(int i=0; i<buttons.length; i++)
            buttonPanel.add(buttons[i]);
        //adds the buttonPanel
        add(buttonPanel);
        //TODO: Create a JScrollPane containing textArea
        JScrollPane scrollPane = new JScrollPane(textArea);
        scrollPane.setPreferredSize(new Dimension(80, 120));
        //TODO: Set the preferred size of the scroll pane to 80x120
        //TODO: Add the scroll pane to this panel
        add(scrollPane);
    }
}

堆栈跟踪：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at TextButtonsHW.actionPerformed(TextButtonsHW.java:74)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
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1 回答 1

1

您正在隐藏变量textArea。代替

JTextArea textArea = new JTextArea();

和

textArea = new JTextArea();

这同样适用于buttons数组：

JButton[] buttons = new JButton[12];

用。。。来代替

buttons = new JButton[12];
于 2013-04-27T00:15:27.783 回答