java - 使用 File.listfiles() 获取 NullPointer 异常

Question

所以我的 Android Gallery 中有 3 个文件夹。我正在尝试将这些文件夹的文件读入文件数组，然后从那里将图像源转换为实际的 ImageViews。

我的问题是在引用我的文件数组时我得到了 NullPointerException。这是一些代码:

// Get image files paths.
private File topsFolder = new File(Environment.getExternalStorageDirectory().getPath() + "/DressIt Tops/");
private File bottomsFolder = new File(Environment.getExternalStorageDirectory()
        .getPath() + "/DressIt Bottoms/");
private File shoesFolder = new File(Environment.getExternalStorageDirectory()
        .getPath() + "/DressIt Shoes/");

private File[] allTops = topsFolder.listFiles();
private File[] allBottoms = bottomsFolder.listFiles();
private File[] allShoes = shoesFolder.listFiles();

使用这些图像文件数组...

ImageAdapter topAdapter = new ImageAdapter(createList(allTops));
    topList.setAdapter(topAdapter);

    ImageAdapter midAdapter = new ImageAdapter(createList(allBottoms));
    midList.setAdapter(midAdapter);

    ImageAdapter botAdapter = new ImageAdapter(createList(allShoes));
    botList.setAdapter(botAdapter);

我的 CreateList 方法。错误是在我引用 list.length 时。

private List<ImageClass> createList(File[] list) {
    List<ImageClass> result = new ArrayList<ImageClass>();
    ImageClass img;
    Bitmap bmp;

    if (list.length > 0) {
        for (int i = 0; i < list.length; i++) {
            bmp = BitmapFactory.decodeFile(list[i].getAbsolutePath());
            img = new ImageClass(bmp);
            result.add(img);
        }
    } else
        result.add(new ImageClass(this.getResources().getIdentifier(
                "no_imgs_yet", "drawable", this.getPackageName())));
    return result;
}

正如我所说，错误是当我使用 list.length 来引用其中一个文件数组时，例如我在第一个代码段中创建的 allTops。

这是 LogCat 错误消息:

05-05 19:43:49.409: D/dalvikvm(537): Not late-enabling CheckJNI (already on)
05-05 19:43:51.069: D/dalvikvm(537): GC_FOR_ALLOC freed 70K, 3% free 10185K/10439K, paused 63ms
05-05 19:43:51.079: I/dalvikvm-heap(537): Grow heap (frag case) to 10.566MB for 562516-byte allocation
05-05 19:43:51.139: D/dalvikvm(537): GC_CONCURRENT freed 1K, 3% free 10733K/11015K, paused 6ms+7ms
05-05 19:43:51.278: D/com.example.dressit(537): /mnt/sdcard/DressIt Tops
05-05 19:43:51.289: D/AndroidRuntime(537): Shutting down VM
05-05 19:43:51.289: W/dalvikvm(537): threadid=1: thread exiting with uncaught exception (group=0x409961f8)
05-05 19:43:51.319: E/AndroidRuntime(537): FATAL EXCEPTION: main
05-05 19:43:51.319: E/AndroidRuntime(537): java.lang.RuntimeException: Unable to start activity 

ComponentInfo{com.example.dressit/com.example.dressit.MainActivity}: java.lang.NullPointerException
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.ActivityThread.access$600(ActivityThread.java:122)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.os.Handler.dispatchMessage(Handler.java:99)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.os.Looper.loop(Looper.java:137)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.ActivityThread.main(ActivityThread.java:4340)
05-05 19:43:51.319: E/AndroidRuntime(537):  at java.lang.reflect.Method.invokeNative(Native Method)
05-05 19:43:51.319: E/AndroidRuntime(537):  at java.lang.reflect.Method.invoke(Method.java:511)
05-05 19:43:51.319: E/AndroidRuntime(537):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
05-05 19:43:51.319: E/AndroidRuntime(537):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
05-05 19:43:51.319: E/AndroidRuntime(537):  at dalvik.system.NativeStart.main(Native Method)
05-05 19:43:51.319: E/AndroidRuntime(537): Caused by: java.lang.NullPointerException
05-05 19:43:51.319: E/AndroidRuntime(537):  at com.example.dressit.MainActivity.createList(MainActivity.java:125)
05-05 19:43:51.319: E/AndroidRuntime(537):  at com.example.dressit.MainActivity.onCreate(MainActivity.java:103)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.Activity.performCreate(Activity.java:4465)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
05-05 19:43:51.319: E/AndroidRuntime(537):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
05-05 19:43:51.319: E/AndroidRuntime(537):  ... 11 more
05-05 19:43:51.509: D/dalvikvm(537): GC_CONCURRENT freed 420K, 5% free 10747K/11271K, paused 6ms+5ms
05-05 19:44:25.629: I/Process(537): Sending signal. PID: 537 SIG: 9

我也得到了我的许可:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

很抱歉输入了这么多代码，但我觉得有人在这里解决问题是必要且足够的。

我现在已经花了几个小时在这上面，我想我应该问问社区!非常感谢大家。

编辑:感谢所有反馈!这是我保存拍摄图像的代码。这应该会向您显示已保存图像的所有路径。另外，我的 LogCat 说文件路径是/mnt/sdcard/DressIt Tops。

/** Create a File for saving an image or video */
private static File getOutputMediaFile(int type) {
    // To be safe, you should check that the SDCard is mounted
    // using Environment.getExternalStorageState() before doing this.

    String dirName;

    if (imageType == 1) {
        dirName = "DressIt Tops";
    } else if (imageType == 2) {
        dirName = "DressIt Bottoms";
    } else
        dirName = "DressIt Shoes";

    File mediaStorageDir = new File(
            Environment
                    .getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),
            dirName);
    // This location works best if you want the created images to be shared
    // between applications and persist after your app has been uninstalled.

    // Create the storage directory if it does not exist
    if (!mediaStorageDir.exists()) {
        if (!mediaStorageDir.mkdirs()) {
            Log.d(dirName, "failed to create directory");
            return null;
        }
    }

    // Create a media file name
    String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss")
            .format(new Date());
    File mediaFile;
    if (type == MEDIA_TYPE_IMAGE) {
        mediaFile = new File(mediaStorageDir.getPath() + File.separator
                + "IMG_" + timeStamp + ".jpg");
    } else if (type == MEDIA_TYPE_VIDEO) {
        mediaFile = new File(mediaStorageDir.getPath() + File.separator
                + "VID_" + timeStamp + ".mp4");
    } else {
        return null;
    }
    return mediaFile;
}

Answer 1

根据 File.listFiles() 的文档，如果 File 不是目录，它将返回 null。

关于为什么我会这样......有三种(非常相似的)选择

1. 返回的 String(例如)... Environment.getExternalStorageDirectory().getPath() + "/DressIt Tops/" 不代表有效目录(同样适用于 ..."/DressIt Bottoms/" 和/或 ..."/DressIt Shoes/")。
2. 返回的字符串 是 有效路径但是其中一个是文件不是目录.
3. 目录DO 存在但是您的目录名称有错字——记住文件系统是区分大小写的。如果您将“/DressIt Tops/”目录创建为“”/dressIt tops/”(例如)，它将不会被发现为有效路径。

还有一件事 - 永远不要假设任何路径到 Android 文件系统上的任何地方 - 记录 Environment.getExternalStorageDirectory().getPath() 实际返回的内容，检查它是你创建目录的地方，并检查你将所有内容都放在正确的大小写中。