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File数组，然后从那里将图像源转换为real
ImageViews。

我的问题是NullPointerException引用文件数组时遇到问题。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。错误是当我引用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;
}

就像我说的那样，错误是当我使用list.length引用一个File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

我也已经获得我的许可：

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

抱歉，我投入了太多代码，但是我觉得有人在这里找出问题是必要和充分的。

我现在已经花了几个小时，以为我会问社区！非常感谢大家。

编辑：感谢您的所有反馈！这是我的代码，用于保存拍摄的图像。这将显示所有已保存图像的路径。另外，我的LogCat说文件路径为/ 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，null如果File不是目录，它将返回。

关于为什么这是我的三种选择（非常相似）

1. 在String通过（例如）…返回Environment.getExternalStorageDirectory().getPath() + "/DressIt Tops/"并不代表一个有效的目录（相同的，可以申请..."/DressIt Bottoms/"和/或..."/DressIt Shoes/"）。
2. 在Strings返回 ARE 有效路径， 但 一种或另一种人是一个文件， 不是 一个目录。
3. 该目录 DO 存在 但 你必须与目录名称的错字-记住文件系统是区分大小写的。例如，如果您将“ / DressIt Tops /”目录创建为“” / dressIt tops /”，则找不到该目录作为有效路径。

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，检查它是您创建目录的位置，并检查所有内容是否正确。