嘗試向JTextPane添加撤消時出現java.lang.NullPointerException

Question

我使用此網站嘗試將撤消和重做添加到我的JTextPane中，但出現此錯誤：
Exception in thread "main" java.lang.NullPointerException at guiWithSwing.TextEditor.main(TextEditor.java:137) 。
在第137行，它說的是menuEdit.add(menuItemUndo); ，這是將“撤消”按鈕添加到菜單的部分。 我認為該錯誤表示該按鈕沒有屬性，或者缺少某些內容，但是我不知道它是什么，因為它具有操作和文本。

package guiWithSwing;

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Event;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JFileChooser;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextPane;
import javax.swing.KeyStroke;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.UndoableEditEvent;
import javax.swing.event.UndoableEditListener;
import javax.swing.filechooser.FileFilter;
import javax.swing.filechooser.FileNameExtensionFilter;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
import javax.swing.text.StyledDocument;
import javax.swing.undo.CannotRedoException;
import javax.swing.undo.CannotUndoException;
import javax.swing.undo.UndoManager;

@SuppressWarnings("serial")
public class TextEditor extends JPanel {

static Container pane;
static Container paneText;
static BasicFrame frame;
static JTextPane textArea;
static JScrollPane areaScrollPane;
static FileFilter textFile;
static FileFilter htmlFile;
static FileFilter javaFile;
static JLabel label1;
static {
    try {
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (UnsupportedLookAndFeelException e) {
        e.printStackTrace();
    }
}
static JFileChooser save = new JFileChooser();
static JFileChooser open = new JFileChooser();
static JMenuBar menuBar;
static JMenu menuFile, menuEdit;
static JMenuItem menuItemNew, menuItemSave, menuItemOpen;
static JMenuItem menuItemUndo, menuItemRedo;
static UndoManager undoManager;
static String newLine = "\n";
static Document editorPaneDocument;
static UndoHandler undoHandler;
static UndoAction undoAction;
static RedoAction redoAction;

public static void main(String[] args) throws ClassNotFoundException,
        InstantiationException, IllegalAccessException,
        UnsupportedLookAndFeelException, IOException {
    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    frame = BasicFrame.getInstance();
    pane = frame.getContentPane();
    paneText = new JPanel();
    textArea = new JTextPane();
    label1 = new JLabel("          ");
    menuBar = new JMenuBar();
    menuFile = new JMenu("File");
    undoManager = new UndoManager();
    menuFile.setMnemonic(KeyEvent.VK_F);
    menuFile.getAccessibleContext().setAccessibleDescription("File");
    menuBar.add(menuFile);
    menuItemNew = new JMenuItem(
            "New",
            new ImageIcon("Images/new.png"));
    menuItemNew.setMnemonic(KeyEvent.VK_N);
    menuFile.add(menuItemNew);
    menuItemOpen = new JMenuItem(
            "Open",
            new ImageIcon("Images/folder.png"));
    menuItemOpen.setMnemonic(KeyEvent.VK_O);
    menuFile.add(menuItemOpen);
    menuItemSave = new JMenuItem(
            "Save",
            new ImageIcon("Images/save.png"));
    menuItemSave.setMnemonic(KeyEvent.VK_S);
    menuFile.add(menuItemSave);
    areaScrollPane = new JScrollPane(textArea);
    areaScrollPane
            .setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
    areaScrollPane.setPreferredSize(new Dimension(250, 250));
    pane.setLayout(new BorderLayout());
    textFile = new FileNameExtensionFilter("Text File (.txt)", "txt");
    htmlFile = new FileNameExtensionFilter(
            "HTML Document (.html, .htm, .shtml, .shtm, .xhtml, .hta)",
            "html", "htm", "shtml", "shtm", "xhtml", "hta");
    javaFile = new FileNameExtensionFilter("Java Source Code (.java)",
            "java");
    save.addChoosableFileFilter(textFile);
    save.addChoosableFileFilter(htmlFile);
    save.addChoosableFileFilter(javaFile);
    save.setAcceptAllFileFilterUsed(true);
    save.setFileFilter(textFile);
    open.addChoosableFileFilter(textFile);
    open.addChoosableFileFilter(htmlFile);
    open.addChoosableFileFilter(javaFile);
    open.setAcceptAllFileFilterUsed(true);
    open.setFileFilter(textFile);
    menuItemUndo = new JMenuItem(undoAction);
    menuItemRedo = new JMenuItem(redoAction);
    menuEdit.add(menuItemUndo);
    menuEdit.add(menuItemRedo);
    pane.add(areaScrollPane, BorderLayout.CENTER);
    pane.add(paneText, BorderLayout.SOUTH);
    paneText.setLayout(new BoxLayout(paneText, BoxLayout.Y_AXIS));
    paneText.add(label1);
    editorPaneDocument = textArea.getDocument();
    editorPaneDocument.addUndoableEditListener(undoHandler);
    KeyStroke undoKeystroke = KeyStroke.getKeyStroke(KeyEvent.VK_Z,
            Event.META_MASK);
    KeyStroke redoKeystroke = KeyStroke.getKeyStroke(KeyEvent.VK_Y,
            Event.META_MASK);
    undoAction = new UndoAction();
    textArea.getInputMap().put(undoKeystroke, "undoKeystroke");
    textArea.getActionMap().put("undoKeystroke", undoAction);
    redoAction = new RedoAction();
    textArea.getInputMap().put(redoKeystroke, "redoKeystroke");
    textArea.getActionMap().put("redoKeystroke", redoAction);

    menuItemNew.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            textArea.setText(null);
        }

    });

    menuItemSave.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent e) {
            save.setFileHidingEnabled(false);
            save.setSelectedFile(new File("new 1"));
            save.setMultiSelectionEnabled(true);
            save.setCurrentDirectory(new File(System
                    .getProperty("user.home") + "/Desktop"));
            save.setDialogTitle("Where Would You Like to Save This File?");
            save.setDragEnabled(true);
            int actionDialog = save.showSaveDialog(null);
            if (actionDialog != JFileChooser.APPROVE_OPTION) {
                return;
            } else {
                log("Done!", true);
                String name = save.getSelectedFile().getAbsolutePath();
                if (!name.endsWith(".txt")
                        && save.getFileFilter() == textFile) {
                    name += ".txt";
                } else if (!name.endsWith(".java")
                        && save.getFileFilter() == javaFile) {
                    name += ".java";
                } else if ((!name.endsWith(".html") && save.getFileFilter() == htmlFile)
                        || (!name.endsWith(".htm") && save.getFileFilter() == htmlFile)
                        || (!name.endsWith(".shtml") && save
                                .getFileFilter() == htmlFile)
                        || (!name.endsWith(".shtm") && save.getFileFilter() == htmlFile)
                        || (!name.endsWith(".xhtml") && save
                                .getFileFilter() == htmlFile)
                        || (!name.endsWith(".hta") && save.getFileFilter() == htmlFile)) {
                    name += ".html";
                }
                BufferedWriter outFile = null;
                try {
                    outFile = new BufferedWriter(new FileWriter(name));
                    textArea.write(outFile);

                } catch (IOException ioe) {
                    ioe.printStackTrace();
                } finally {
                    if (outFile != null) {
                        try {
                            outFile.close();
                        } catch (IOException ioee) {
                        }
                    }
                }
            }

        }
    });

    menuItemOpen.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            open.setFileHidingEnabled(false);
            open.setMultiSelectionEnabled(true);
            open.setCurrentDirectory(new File(System
                    .getProperty("user.home") + "/Desktop"));
            open.setDialogTitle("Which File Would You Like to Open?");
            open.setDragEnabled(true);
            int actionDialog = open.showOpenDialog(null);
            if (actionDialog != JFileChooser.APPROVE_OPTION) {
                return;
            } else {
                log("Done!", true);
                String name = open.getSelectedFile().getAbsolutePath();
                try {
                    BufferedReader in = new BufferedReader(new FileReader(
                            name));
                    String line;
                    textArea.setText(null);
                    while ((line = in.readLine()) != null) {
                        appendString(line);
                        appendString(newLine);
                    }
                    in.close();

                } catch (IOException ioe) {
                    ioe.fillInStackTrace();
                } catch (BadLocationException e1) {
                }
            }

        }
    });

    frame.setSize(500, 320);
    frame.setJMenuBar(menuBar);
    frame.setVisible(true);
}

static public void appendString(String str) throws BadLocationException {
    StyledDocument document = (StyledDocument) textArea.getDocument();
    document.insertString(document.getLength(), str, null);
}

static private void log(String msg, boolean remove) {
    label1 = new JLabel(msg);
    if (remove) {
        paneText.removeAll();
    }
    paneText.add(label1);
    paneText.validate();
    pane.validate();
    new Thread() {
        public void run() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ie) {
                ie.printStackTrace();
            }
            paneText.removeAll();
            label1 = new JLabel("          ");
            paneText.add(label1);
            paneText.validate();
            pane.validate();
        }
    }.start();
}

class UndoHandler implements UndoableEditListener {

    public void undoableEditHappened(UndoableEditEvent e) {
        undoManager.addEdit(e.getEdit());
        undoAction.update();
        redoAction.update();
    }
}

static class UndoAction extends AbstractAction {
    public UndoAction() {
        super("Undo");
        setEnabled(false);
    }

    public void actionPerformed(ActionEvent e) {
        try {
            undoManager.undo();
        } catch (CannotUndoException ex) {
            // TODO deal with this
            // ex.printStackTrace();
        }
        update();
        redoAction.update();
    }

    protected void update() {
        if (undoManager.canUndo()) {
            setEnabled(true);
            putValue(Action.NAME, undoManager.getUndoPresentationName());
        } else {
            setEnabled(false);
            putValue(Action.NAME, "Undo");
        }
    }
}

static class RedoAction extends AbstractAction {
    public RedoAction() {
        super("Redo");
        setEnabled(false);
    }

    public void actionPerformed(ActionEvent e) {
        try {
            undoManager.redo();
        } catch (CannotRedoException ex) {
            // TODO deal with this
            ex.printStackTrace();
        }
        update();
        undoAction.update();
    }

    protected void update() {
        if (undoManager.canRedo()) {
            setEnabled(true);
            putValue(Action.NAME, undoManager.getRedoPresentationName());
        } else {
            setEnabled(false);
                putValue(Action.NAME, "Redo");
            }
        }
    }
}

Answer 1

您尚未實例化menuEdit ，僅對其進行了聲明。 您需要在使用前添加此行：

menuEdit = new JMenu("Edit.");

當您收到NullPointerException ，很容易將方法/函數的參數引為問題的根源。 重要的是要記住檢查一下所引用的行上的所有對象是否實際上都有一個實例，否則您會發現自己陷入了瘋狂的追趕之中！

Answer 2

您永遠不會在整個代碼中實例化menuEdit 。 它為null，非常好，您正在獲取NullPointerException 。 像這樣初始化它：

menuEdit = new JMenu("Don't Dump Full Code on SO!");

Answer 3

當您收到java.lang.NullPointerException時，這意味着未初始化對象，在這種情況下，對象menuEdit未初始化，因此無法使用它。 您可以像這樣初始化它：

 menuEdit = new JMenu("Any Thing");

好運。