我用了this website尝试将撤消和重做添加到我的 JTextPane，但出现此错误:
guiWithSwing.TextEditor.main(TextEditor.java:137) 线程“main”中出现 java.lang.NullPointerException 异常。
在第 137 行，它显示了 menuEdit.add(menuItemUndo);，这是将“撤消”按钮添加到菜单的部分。我认为该错误意味着该按钮没有属性，或者缺少某些内容，但我不知道它是什么，因为它有一个操作和文本。
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");
}
}
}
}
最佳答案
您尚未实例化 menuEdit，只是声明了它。在使用之前，您需要在某处添加此行:
menuEdit = new JMenu("Edit.");
当您收到 NullPointerException 时，很容易将方法/函数的参数视为问题的根源。重要的是要记住检查所引用的行上的所有对象实际上都有一个实例，否则您会发现自己陷入了白费力气!
关于尝试向 JTextPane 添加撤消时出现 java.lang.NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24578741/