Java 为什么不是'；我的JMenu不工作吗？

所有的东西都显示出来并且工作正常，但是每当我加载它时，菜单就不会显示出来。我做错了什么？为了让它工作，我试着用其他的东西做实验，但是它从来没有工作过。有什么建议吗

Java代码

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.text.*;

public class DVD extends JFrame implements ActionListener
{
//construct components
JLabel sortPrompt = new JLabel("Sort by:");
JComboBox fieldCombo = new JComboBox();
JTextPane textPane = new JTextPane();

//initialize data in arrays
String title[] = {"Casablanca", "Citizen Kane", "Singin in the Rain", "The Wizard of Oz"};
String studio[] = {"Warner Bros.", "RKO Pictures", "MGM", "MGM"};
String year[] = {"1942", "1941", "1952", "1939"};

public DVD()
{
    super("Classics on DVD");
}

// Create the menu system
public JMenuBar createMenuBar()
{
    // create an instance of the menu
    JMenuBar mnuBar = new JMenuBar();
    setJMenuBar(mnuBar);

    //construct and populate the File menu
    JMenu mnuFile = new JMenu("File", true);
    mnuFile.setMnemonic(KeyEvent.VK_F);
    mnuFile.setDisplayedMnemonicIndex(1);
    mnuFile.add(mnuFile);

    // construct and populate the Exit menu
    JMenuItem mnuFileExit = new JMenuItem("Exit");
    mnuFileExit.setMnemonic(KeyEvent.VK_X);
    mnuFileExit.setDisplayedMnemonicIndex(1);
    mnuFile.add(mnuFileExit);
    mnuFileExit.setActionCommand("Exit");
    mnuFileExit.addActionListener(this);

    // construct and populate the Edit menu
    JMenu mnuEdit = new JMenu("Edit", true);
    mnuEdit.setMnemonic(KeyEvent.VK_E);
    mnuEdit.setDisplayedMnemonicIndex(0);
    mnuEdit.add(mnuEdit);

    // Insert
    JMenuItem mnuEditInsert = new JMenuItem("Insert new DVD");
    mnuEditInsert.setMnemonic(KeyEvent.VK_I);
    mnuEditInsert.setDisplayedMnemonicIndex(0);
    mnuEdit.add(mnuEditInsert);
    mnuEditInsert.setActionCommand("Insert");
    mnuEditInsert.addActionListener(this);

    // Search
    JMenu mnuEditSearch = new JMenu("Search", true);
    mnuEditSearch.setMnemonic(KeyEvent.VK_R);
    mnuEditSearch.setDisplayedMnemonicIndex(3);
    mnuEdit.add(mnuEditSearch);

    JMenuItem mnuEditSearchByTitle = new JMenuItem("by Title");
    mnuEditSearchByTitle.setMnemonic(KeyEvent.VK_T);
    mnuEditSearchByTitle.setDisplayedMnemonicIndex(3);
    mnuEditSearch.add(mnuEditSearchByTitle);
    mnuEditSearchByTitle.setActionCommand("title");
    mnuEditSearchByTitle.addActionListener(this);

    JMenuItem mnuEditSearchByStudio = new JMenuItem("by Studio");
    mnuEditSearchByStudio.setMnemonic(KeyEvent.VK_S);
    mnuEditSearchByStudio.setDisplayedMnemonicIndex(3);
    mnuEditSearch.add(mnuEditSearchByStudio);
    mnuEditSearchByStudio.setActionCommand("studio");
    mnuEditSearchByStudio.addActionListener(this);

    JMenuItem mnuEditSearchByYear = new JMenuItem("by Year");
    mnuEditSearchByYear.setMnemonic(KeyEvent.VK_Y);
    mnuEditSearchByYear.setDisplayedMnemonicIndex(3);
    mnuEditSearch.add(mnuEditSearchByYear);
    mnuEditSearchByYear.setActionCommand("year");
    mnuEditSearchByYear.addActionListener(this);

    return mnuBar;
}

// create the content pane
public Container createContentPane()
{
    fieldCombo.addItem("Title");
    fieldCombo.addItem("Studio");
    fieldCombo.addItem("Year");
    fieldCombo.addActionListener(this);
    fieldCombo.setToolTipText("Click the drop down arrow to display sort fields.");

    //construct and populate the north panel
    JPanel northPanel = new JPanel();
    northPanel.setLayout(new FlowLayout());
    northPanel.add(sortPrompt);
    northPanel.add(fieldCombo);

    // create the JTextPane and center panel
    JPanel centerPanel = new JPanel();
    setTabsAndStyles(textPane);
    textPane = addTextToTextPane();
    JScrollPane scrollPane = new JScrollPane(textPane);
    textPane.setEditable(false);

    scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
    scrollPane.setPreferredSize(new Dimension(500, 200));
    centerPanel.add(scrollPane);

    // create Container and set attributes
    Container c = getContentPane();
    c.setLayout(new BorderLayout(10,10));
    c.add(northPanel, BorderLayout.NORTH);
    c.add(centerPanel, BorderLayout.CENTER);

    return c;
}


// method to create tab stops and set font styles
protected void setTabsAndStyles(JTextPane textPane)
{
    //create Tab stops
    TabStop[] tabs = new TabStop[2];
    tabs[0] = new TabStop(200, TabStop.ALIGN_LEFT, TabStop.LEAD_NONE);
    tabs[1] = new TabStop(350, TabStop.ALIGN_LEFT, TabStop.LEAD_NONE);
    TabSet tabset = new TabSet(tabs);

    // set tab style
    StyleContext tabStyle = StyleContext.getDefaultStyleContext();
    AttributeSet aset = tabStyle.addAttribute(SimpleAttributeSet.EMPTY,  StyleConstants.TabSet,  tabset);
    textPane.setParagraphAttributes(aset, false);

    // set font style
    Style fontStyle = StyleContext.getDefaultStyleContext().getStyle(StyleContext.DEFAULT_STYLE);

    Style regular = textPane.addStyle("regular", fontStyle);
    StyleConstants.setFontFamily(fontStyle, "SansSerif");

    Style s = textPane.addStyle("italic", regular);
    StyleConstants.setItalic(s, true);

    s = textPane.addStyle("bold", regular);
    StyleConstants.setBold(s, true);

    s = textPane.addStyle("large", regular);
    StyleConstants.setFontSize(s, 16);
}

// method to add new text to the JTextPane
public JTextPane addTextToTextPane()
{
    Document doc = textPane.getDocument();
    try
    {
        // clear previous text
        doc.remove(0, doc.getLength());

        // insert title
        doc.insertString(0, "TITLE\tSTUDIO\tYEAR\n", textPane.getStyle("large"));

        // insert detail
        for(int j = 0; j < title.length; j++)
        {
            doc.insertString(doc.getLength(), title[j] + "\t", textPane.getStyle("bold"));
            doc.insertString(doc.getLength(), studio[j] + "\t", textPane.getStyle("italic"));
            doc.insertString(doc.getLength(), year[j] + "\n", textPane.getStyle("regular"));
        }
    }

    catch(BadLocationException ble)
    {
        System.err.println("Couldn't insert text");
    }

    return textPane;
}

// event to process user clicks
public void actionPerformed(ActionEvent e)
{
    String arg = e.getActionCommand(); // execute the getActionCommand() (java method) store in arg
    //user clicks the sort by Combo box // combo field is one of the selections in an array format
    if(e.getSource() == fieldCombo)
    {
        switch(fieldCombo.getSelectedIndex())
        {
        case 0:
            sort(title);
            break;
        case 1:
            sort(studio);
            break;
        case 2:
            sort(year);
            break;
        }
    }

    // user clicks Exit on the File menu

    if(arg == "Exit")
        System.exit(0);

    // user clicks Insert new DVD on the Edit menu
    if(arg == "Insert")
    {
        // accept new data
        String newTitle = JOptionPane.showInputDialog(null, "Please enter the new movie's title:");
        String newStudio = JOptionPane.showInputDialog(null, "Please enter the studio for " + newTitle);
        String newYear = JOptionPane.showInputDialog(null, "Please enter the year for "+ newTitle);

        // enlarge arrays
        title = enlargeArray(title);
        studio = enlargeArray(studio);
        year = enlargeArray(year);

        //add new data to arrays
        title[title.length-1] = newTitle;
        studio[studio.length-1] = newStudio;
        year[year.length-1] = newYear;

        // call sort method necessary for search
        sort(title);
        fieldCombo.setSelectedIndex(0);
    }

    // user clicks Title on the Search submenu
    if(arg == "title")
        search(arg, title);

    // user clicks Studio on the Search submenu
    if(arg == "studio")
        search(arg, studio);

    // user clicks Year on the Search submenu
            if(arg == "year")
                search(arg, year);
}

// method to enlarge and array by 1
public String[] enlargeArray(String[] currentArray)
{
    String[] newArray = new String[currentArray.length + 1];
    for(int i = 0; i < currentArray.length; i++)
        newArray[i] = currentArray[i];
    return newArray;
}

// method to sort arrays
public void sort(String tempArray[])
{
    // loop to control number of passes
    for(int pass = 1; pass < tempArray.length; pass++)
    {
        for(int element = 0; element < tempArray.length - 1; element++)
            if(tempArray[element].compareTo(tempArray[element+1]) > 0)
            {
                swap(title, element, element + 1);
                swap(studio, element, element + 1);
                swap(year, element, element + 1);
            }
    }
    addTextToTextPane();
}

// method to swap two elements of an array
public void swap(String swapArray[], int first, int second)
{
    String hold; // temporary holding area for swap
    hold = swapArray[first];
    swapArray[first] = swapArray[second];
    swapArray[second] = hold;
}

public void search(String searchField, String searchArray[])
{
    try
    {
        Document doc = textPane.getDocument(); // assign text to document object
        doc.remove(0,  doc.getLength()); // clear previous text

        //display column titles
        doc.insertString(0, "TITLE\tSTUDIO\tYEAR\n", textPane.getStyle("large"));

        // prompt user for search data
        String search = JOptionPane.showInputDialog(null, "Please enter the " + searchField);
        boolean found = false;

        // search arrays
        for(int i = 0; i < title.length; i++)
        {
            if(search.compareTo(searchArray[i]) == 0)
            {
                doc.insertString(doc.getLength(), title[i]+ "\t", textPane.getStyle("bold"));
                doc.insertString(doc.getLength(), studio[i]+ "\t", textPane.getStyle("italic"));
                doc.insertString(doc.getLength(), year[i] + "\n", textPane.getStyle("regular"));
                found = true;

            }
        }
        if(found == false)
        {
            JOptionPane.showMessageDialog(null, "Your Search produced no results", "No results found", JOptionPane.INFORMATION_MESSAGE);
            sort(title);
        }
    }

    catch(BadLocationException ble)
    {
        System.err.println("Couldn't insert text");
    }
}

// main method executes at run time
public static void main(String args[])
{
    JFrame.setDefaultLookAndFeelDecorated(true);
    DVD f = new DVD();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setJMenuBar(f.createMenuBar());
    f.setContentPane(f.createContentPane());
    f.setSize(600,375);
    f.setVisible(true);

}

}

import java.awt.*；
导入java.awt.event.*；
导入javax.swing.*；
导入javax.swing.text.*；
公共类DVD扩展JFrame实现ActionListener
{
//构造组件
JLabel sortPrompt=新的JLabel（“排序依据：”）；
JComboBox fieldCombo=新的JComboBox（）；
JTextPane textPane=新的JTextPane（）；
//初始化数组中的数据
字符串标题[]={“卡萨布兰卡”、“公民凯恩”、“雨中歌唱”、“绿野仙踪”}；
弦乐工作室[]={“华纳兄弟”、“RKO电影”、“米高梅”、“米高梅”}；
字符串年份[]={“1942”、“1941”、“1952”、“1939”}；
公共DVD（）
{
超级（“DVD上的经典作品”）；
}
//创建菜单系统
公共JMenuBar createMenuBar（）
{
//创建菜单的一个实例
JMenuBar mnuBar=新的JMenuBar（）；
setJMenuBar（mnuBar）；
//构造并填充“文件”菜单
JMenu mnuFile=新JMenu（“文件”，true）；
mnuFile.setMnemonic（KeyEvent.VK_F）；
mnuFile.setDisplayedMononicIndex（1）；
mnuFile.add（mnuFile）；
//构造并填充退出菜单
JMenuItem mnuFileExit=新的JMenuItem（“退出”）；
mnuFileExit.setMnemonic（KeyEvent.VK_X）；
mnuFileExit.setDisplayedMonicIndex（1）；
mnuFile.add（mnuFileExit）；
mnuFileExit.setActionCommand（“退出”）；
mnuFileExit.addActionListener（此）；
//构造并填充“编辑”菜单
JMenu mnuEdit=新JMenu（“编辑”，真）；
mnuEdit.setMnemonic（KeyEvent.VK_E）；
mnuEdit.setDisplayedMonicIndex（0）；
mnuEdit.add（mnuEdit）；
//插入
JMenuItem mnuEditInsert=新JMenuItem（“插入新DVD”）；
mnuEditInsert.setMnemonic（KeyEvent.VK_I）；
mnuEditInsert.setDisplayedMonicIndex（0）；
mnuEdit.add（mnuEditInsert）；
mnuEditInsert.setActionCommand（“插入”）；
mnuEditInsert.addActionListener（此）；
//搜寻
JMenu mnuEditSearch=新JMenu（“搜索”，true）；
mnuEditSearch.setMnemonic（KeyEvent.VK_R）；
mnuEditSearch.setDisplayedMonicIndex（3）；
mnuEdit.add（mnuEditSearch）；
JMenuItem mnuEditSearchByTitle=新的JMenuItem（“按标题”）；
mnuEditSearchByTitle.set助记符（KeyEvent.VK_T）；
mnuEditSearchByTitle.setDisplayedMonicIndex（3）；
mnuEditSearch.add（mnuEditSearchByTitle）；
mnuEditSearchByTitle.setActionCommand（“标题”）；
mnuEditSearchByTitle.addActionListener（此）；
JMenuItem mnuEditSearchByStudio=新JMenuItem（“由工作室”）；
mnuEditSearchByStudio.set助记符（KeyEvent.VK_S）；
mnuEditSearchByStudio.setdisplayednemonicindex（3）；
mnuEditSearch.add（mnuEditSearchByStudio）；
mnuEditSearchByStudio.setActionCommand（“studio”）；
mnuEditSearchByStudio.addActionListener（此）；
JMenuItem mnuEditSearchByYear=新的JMenuItem（“按年”）；
mnuEditSearchByYear.setMemonic（KeyEvent.VK_Y）；
mnuEditSearchByYear.SetDisplayedMonicIndex（3）；
mnuEditSearch.add（mnuEditSearchByYear）；
mnuEditSearchByYear.setActionCommand（“年”）；
mnuEditSearchByYear.addActionListener（此）；
返回mnuBar；
}
//创建内容窗格
公共容器createContentPane（）
{
fieldCombo.addItem（“标题”）；
fieldCombo.addItem（“工作室”）；
fieldCombo.addItem（“年度”）；
fieldCombo.addActionListener（此）；
setToolTipText（“单击下拉箭头以显示排序字段”）；
//构造并填充北面板
JPanel northPanel=新的JPanel（）；
setLayout（新的FlowLayout（））；
添加（sortPrompt）；
添加（字段组合）；
//创建JTextPane和中心面板
JPanel centerPanel=新的JPanel（）；
设置选项卡和样式（文本窗格）；
textPane=addTextToTextPane（）；
JScrollPane scrollPane=新的JScrollPane（textPane）；
textPane.setEditable（false）；
scrollPane.setVerticalScrollBarPolicy（JScrollPane.VERTICAL\u SCROLLBAR\u ALWAYS）；
scrollPane.setPreferredSize（新维度（500200））；
添加（滚动窗格）；
//创建容器并设置属性
容器c=getContentPane（）；
c、 setLayout（新边界布局（10,10））；
c、 添加（northPanel，BorderLayout.NORTH）；
c、 添加（中心面板，BorderLayout.CENTER）；
返回c；
}
//方法创建制表位并设置字体样式
受保护的无效设置选项卡和样式（JTextPane textPane）
{
//创建制表位
TabStop[]tabs=新TabStop[2]；
制表符[0]=新制表符（200，制表符.ALIGN\u LEFT，制表符.LEAD\u NONE）；
制表符[1]=新制表符（350，TabStop.ALIGN\u LEFT，TabStop.LEAD\u NONE）；
TabSet TabSet=新的TabSet（tabs）；
//设置选项卡样式
StyleContext tabStyle=StyleContext.getDefaultStyleContext（）；
AttributeSet aset=tabStyle.addAttribute（SimpleAttributeSet.EMPTY，StyleConstants.TabSet，TabSet）；
setParagraphAttributes（aset，false）；
//设置字体样式
Style fontStyle=StyleContext.getDefaultStyleContext（）.getStyle（StyleContext.DEFAULT_Style）；
Style regular=textPane.addStyle（“常规”，fontStyle）；
setFontFamily（fontStyle，“SansSerif”）；
Style s=textPane.addStyle（“斜体”，常规）；
StyleConstants.setItalic（s，true）；
s=textPane.addStyle（“粗体”，常规）；
StyleConstants.setBold（s，true）；
s=textPane.addStyle（“大”，常规）；
setFontSize（s，16）；
}
//方法将新文本添加到JTextPane
公共JTextPane addTextToTextPane（）
{
Document doc=textPane.getDocument（）；
尝试
{
//清除以前的文本
doc.remove（0，doc.getLength（））；
//插入标题
doc.insertString（0，“TITLE\tSTUDIO\tYEAR\n”，textPane.getStyle（“大”）；
//插入详细信息
对于（int j=0；jmnuBar.add(mnuFile);

mnuFile.add(mnuFile);