Question

我尝试使用此代码示例使用模拟器（WTK2.5）浏览一些图片，但我收到此异常消息（在showdir（）函数上）：

 java.lang.NullPointerException
 at javax.microedition.lcdui.Form.append(Form.java:641)
 at GUI.FileBrowser.showCurrDir(+151)
 at GUI.FileBrowser.<init>(FileBrowser.java:115)
 at GUI.Menu.run(Menu.java:109)

这是我在Netbeans IDE中输入的代码： `

 /* special string that denotes upper directory accessible by this browser.
 * this virtual directory contains all roots.
 */

 public class FileBrowser extends Form implements CommandListener, Runnable         {
 private static final String[] attrList = {"Read", "Write", "Hidden"};
 private static final String[] typeList = {"Regular File", "Directory"};
 private static final String[] monthList = {"Jan", "Feb", "Mar", "Apr"};
 Displayable d;
 /* special string denotes upper directory */
 private static final String UP_DIRECTORY = "..";
 private static final String MEGA_ROOT = "/";

 /* separator string as defined by FC specification */
 private static final String SEP_STR = "/";

 /* separator character as defined by FC specification */
 private static final char SEP = '/';
 private String currDirName;
 private Command view;
 private Command creat;


 private Command delete;
 private Command creatOK;
 private Command prop;
 private Command back;
 private Command exit;
 private TextField nameInput; // Input field for new file name
 private ChoiceGroup typeInput; // Input field for file type (regular/dir)
 private Image dirIcon;
 private Image fileIcon;
 private Image[] iconList;
 private TextField viewer2;
 String login;
 String image;
 HttpConnection hc;
 DataInputStream dis;
 StringBuffer sb = new StringBuffer();
 int ch;

 public FileBrowser() {
    super("Choisir Image");
    view = new Command("View", Command.ITEM, 1);
    creat = new Command("New", Command.ITEM, 2);
    delete = new Command("Delete", Command.ITEM, 3);
    creatOK = new Command("OK", Command.OK, 1);
    prop = new Command("Properties", Command.ITEM, 2);
    back = new Command("Back", Command.BACK, 2);
    exit = new Command("Exit", Command.EXIT, 3);
    currDirName = MEGA_ROOT;
    try {
        dirIcon = Image.createImage("/dir.png");
    } catch (IOException e) {
        dirIcon = null;
    }

    try {
        fileIcon = Image.createImage("/file.png");
    } catch (IOException e) {
        fileIcon = null;
    }

    iconList = new Image[]{fileIcon, dirIcon};

    try {
        showCurrDir();
    } catch (SecurityException e) {


        addCommand(exit);
        setCommandListener(this);

    } catch (Exception e) {
        e.printStackTrace();
    }
   }
    public void commandAction(Command c, Displayable d) {
    if (c == view) {
        Thread thread = new Thread();
        thread.start();

    } else if (c == prop) {
        List curr = (List) d;
        String currFile = curr.getString(curr.getSelectedIndex());
        showProperties(currFile);
    } else if (c == creat) {
        createFile();
    } else if (c == creatOK) {
        String newName = nameInput.getString();
        image = nameInput.getString().trim();
        if ((newName == null) || newName.equals("")) {
        } else 
            executeCreateFile(newName, typeInput.getSelectedIndex() != 0);
            removeCommand(creatOK);
            removeCommand(back);
        }
    } else if (c == back) {
        showCurrDir();
    } else if (c == exit) {
        this.deleteAll();
    } else if (c == delete) {
        List curr = (List) d;
        String currFile = curr.getString(curr.getSelectedIndex());
        executeDelete(currFile);
    }
  }
  void delete(String currFile) {
    if (!currFile.equals(UP_DIRECTORY)) {
        if (currFile.endsWith(SEP_STR)) {
            checkDeleteFolder(currFile);
        } else {
            deleteFile(currFile);
            showCurrDir();
        }
    } else {

    }
 }

 private void executeDelete(String currFile) {
    Thread thread = new Thread();
    thread.start();
 }
 private void checkDeleteFolder(String folderName) {
    try {

        Enumeration content = fcdir.list("*", true);
        if (!content.hasMoreElements()) {
            fcdir.delete();
            showCurrDir();
        } else {

        }
    } catch (IOException ioe) {
        System.out.println(currDirName + folderName);

        ioe.printStackTrace();
    }
 }
 private void executeCreateFile(final String name, final boolean val) {
    Thread thread = new Thread();
    thread.start();
 }


 /**
 * Show file list in the current directory .
 */
 void showCurrDir() {
    Enumeration e;
    FileConnection currDir = null;
    List browser;

    try {
        if (MEGA_ROOT.equals(currDirName)) {
            e = FileSystemRegistry.listRoots();
            browser = new List(currDirName, List.IMPLICIT);
        } else {

            e = currDir.list();
            browser = new List(currDirName, List.IMPLICIT);
            append(UP_DIRECTORY);
            append(dirIcon);
        }

        while (e.hasMoreElements()) {
            String fileName = (String) e.nextElement();

            if (fileName.charAt(fileName.length() - 1) == SEP) {
                // This is directory
                //browser.append(fileName, dirIcon);
                append(fileName);
                append(dirIcon);

            } else {
                // this is regular file
                //browser.append(fileName, fileIcon);
                append(fileName);
                append(fileIcon);
            }
        }

       // browser.setSelectCommand(view);
        //Do not allow creating files/directories beside root
        if (!MEGA_ROOT.equals(currDirName)) {
            addCommand(delete);
            addCommand(prop);
            addCommand(creat);
            addCommand(delete);
        }

        //browser.addCommand(exit);
        addCommand(exit);

        //browser.setCommandListener(this);
        this.setCommandListener(this);

        if (currDir != null) {
            currDir.close();
        }

        // utils.StaticMidlet.disp.setCurrent(this);
    } catch (IOException ioe) {
        ioe.printStackTrace();
    }
  }

 void traverseDirectory(String fileName) {
    /* In case of directory just change the current directory
     * and show it
     */
    if (currDirName.equals(MEGA_ROOT)) {
        if (fileName.equals(UP_DIRECTORY)) {
            // can not go up from MEGA_ROOT
            return;
        }

        currDirName = fileName;
    } else if (fileName.equals(UP_DIRECTORY)) {
        // Go up one directory
        int i = currDirName.lastIndexOf(SEP, currDirName.length() - 2);

        if (i != -1) {
            currDirName = currDirName.substring(0, i + 1);
        } else {
            currDirName = MEGA_ROOT;
        }
    } else {
        currDirName = currDirName + fileName;
    }

    showCurrDir();
 }

 void showFile(String fileName) {
    try {


        if (!fc.exists()) {
            throw new IOException("File does not exists");
        }

        InputStream fis = fc.openInputStream();
        byte[] b = new byte[1024];

        int length = fis.read(b, 0, 1024);

        fis.close();
        fc.close();


        append(viewer2);
        addCommand(back);
        addCommand(exit);

        this.setCommandListener(this);

        if (length > 0) {
            viewer2.setString(new String(b, 0, length));
        }

        utils.StaticMidlet.disp.setCurrent(this);
    } catch (Exception e) {

        alert.setTimeout(Alert.FOREVER);
        utils.StaticMidlet.disp.setCurrent(alert);
    }
 }

 void deleteFile(String fileName) {
    try {

        fc.delete();
    } catch (Exception e) {

        alert.setTimeout(Alert.FOREVER);
        utils.StaticMidlet.disp.setCurrent(alert);
    }
 }

 void showProperties(String fileName) {
    try {
        if (fileName.equals(UP_DIRECTORY)) {
            return;
        }



        if (!fc.exists()) {
            throw new IOException("File does not exists");
        }

        //Form props = new Form("Properties: " + fileName);
        ChoiceGroup attrs = new ChoiceGroup("Attributes:", Choice.MULTIPLE,     attrList, null);

        addCommand(back);
        addCommand(exit);
        setCommandListener(this);
        append(new StringItem("Location:", currDirName));


        addCommand(back);
        addCommand(exit);
        this.setCommandListener(this);

        fc.close();

        utils.StaticMidlet.disp.setCurrent(this);
    } catch (Exception e) {

        alert.setTimeout(Alert.FOREVER);
        utils.StaticMidlet.disp.setCurrent(alert);
    }
}

void createFile() {
    //Form creator = new Form("New File");
    nameInput = new TextField("Enter Name", null, 256, TextField.ANY);

    append(nameInput);
    append(typeInput);
    addCommand(creatOK);
    addCommand(back);
    addCommand(exit);
    this.setCommandListener(this);

 }

 void createFile(String newName, boolean isDirectory) {
    try {


        if (isDirectory) {
            fc.mkdir();
        } else {
            fc.create();
        }

        showCurrDir();
    } catch (Exception e) {
        String s = "Can not create file '" + newName + "'";

        if ((e.getMessage() != null) && (e.getMessage().length() > 0)) {
            s += ("\n" + e);
        }

        Alert alert = new Alert("Error!", s, null, AlertType.ERROR);
        alert.setTimeout(Alert.FOREVER);
        utils.StaticMidlet.disp.setCurrent(alert);
        // Restore the commands that were removed in commandAction()
        addCommand(creatOK);
        addCommand(back);
        this.setCommandListener(this);
    }
 }

 private String myDate(long time) {
    Calendar cal = Calendar.getInstance();

    cal.setTime(new Date(time));

    StringBuffer sb = new StringBuffer();

    sb.append(cal.get(Calendar.HOUR_OF_DAY));
    sb.append(':');
    sb.append(cal.get(Calendar.MINUTE));
    sb.append(':');
    sb.append(cal.get(Calendar.SECOND));
    sb.append(',');
    sb.append(' ');
    sb.append(cal.get(Calendar.DAY_OF_MONTH));
    sb.append(' ');
    sb.append(monthList[cal.get(Calendar.MONTH)]);
    sb.append(' ');
    sb.append(cal.get(Calendar.YEAR));

    return sb.toString();
  }

  public void inserer(String image) {
    try {

        dis = new DataInputStream(hc.openDataInputStream());
        while ((ch = dis.read()) != -1) {
            sb.append((char) ch);
        }
        if ("OK".equals(sb.toString().trim())) {

        } else {

        }
        sb = new StringBuffer();
    } catch (IOException ex) {
        ex.printStackTrace();
    }
 }

 public void run() {
    List curr = (List) d;
    final String currFile = curr.getString(curr.getSelectedIndex());
    if (currFile.endsWith(SEP_STR) || currFile.equals(UP_DIRECTORY)) {
        traverseDirectory(currFile);
    } else {
        // Show file contents
        showFile(currFile);
    }
    final String name = "";
    final boolean val = false;
    createFile(name, val);
    final String file = currFile;
    delete(file);
  }
}  `

此代码仅显示文件内容（例如：file1.txt），我想显示图片。

Answer 1

<强>摘要

我完全不了解Java，但是您的异常消息

java.lang.NullPointerException
at javax.microedition.lcdui.Form.append(Form.java:641)
at GUI.FileBrowser.showCurrDir(+151)
at GUI.FileBrowser.<init>(FileBrowser.java:115)
at GUI.Menu.run(Menu.java:109)

表示在NullPointerException中呼叫append时您有showCurrDir。

我强烈建议您阅读类似“What is a Null Pointer Exception, and how do I fix it?”的内容，如果您不知道如何通过查看堆栈跟踪来解决您的问题......

我对您的问题的分析

您使用四个唯一对象调用append六次：

append(UP_DIRECTORY);
append(dirIcon);
append(fileIcon);
append(fileName);

意味着一个是空的。

我们知道UP_DIRECTORY不是null，因为它是一个初始化的字段：

private static final String UP_DIRECTORY = "..";

这会留下dirIcon，fileIcon和fileName。

在构造函数中，您通过尝试使用dirIcon加载它们来初始化fileIcon和Image.createImage，但是如果找不到文件，则通过将异常设置为null来吞咽异常，即< / p>

try {
    dirIcon = Image.createImage("/dir.png");
} catch (IOException e) {
    dirIcon = null;
}

try {
    fileIcon = Image.createImage("/file.png");
} catch (IOException e) {
    fileIcon = null;
}

因此dirIcon或fileIcon 中的一个可能为空。

fileName来自以下块：

Enumeration e;
FileConnection currDir = null;

if (MEGA_ROOT.equals(currDirName)) {
    e = FileSystemRegistry.listRoots();
} else {
    e = currDir.list();
}

while (e.hasMoreElements()) {
    String fileName = (String) e.nextElement();
    // elided
}

所以大概fileName不是空的（正如我所说的，我不太了解Java，所以我可能错了 - 我想没有理由阻止null偷偷进入{{ 1}}？）。

此处有另一个问题，因为您永远不会将e设置为任何内容的实例，它将是currDir，导致另一个null if { {1}}是假的。由于问题出在构造函数中（在调用NullPointerException之前），它们相等，所以这不会导致问题。

解决方案

MEGA_ROOT.equals(currDirName)的结果似乎是traverseDirectory的使用似乎是安全的，例如（1）或（2），所以我认为我们可以排除hasMoreElements为FileSystemRegistry.listRoots()。

我会在你加载图片文件的部分添加一个断点，看看它们是否真的存在 - 如果没有，这就是你fileName的原因。

Jme浏览文件例外

1 个答案: