Trying to list all folders/files from internal sdcard android but get a null

问题

I am trying to make a list of all the files in my external storage but always comes back null.

if (Environment.getExternalStorageDirectory().exists()) {
        Log.i(FILE, "is not empty");

        File[] files = Environment.getExternalStorageDirectory().listFiles();

        for (File file : files) {
            Log.i("File ", file.getAbsolutePath());
        }
    }

How do I make it list all the files from the external directory

I get an exception

Caused by: java.lang.NullPointerException
                                                                                  at com.example.sam.read.fragments.MyFragment.onCreate(MyFragment.java:138)
                                                                                  at android.support.v4.app.Fragment.performCreate(Fragment.java:1763)
                                                                                  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:913)
                                                                                  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
                                                                                  at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
                                                                                  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
                                                                                  at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:548)
                                                                                  at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1164)
                                                                                  at android.app.Activity.performStart(Activity.java:5114)
                                                                                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2153)
                                                                                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
                                                                                  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
                                                                                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:99) 
                                                                                  at android.os.Looper.loop(Looper.java:137) 
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5041) 
                                                                                  at java.lang.reflect.Method.invokeNative(Native Method) 
                                                                                  at java.lang.reflect.Method.invoke(Method.java:511) 
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
                                                                                  at dalvik.system.NativeStart.main(Native Method) 

回答1:

You are missing a permission in your manifest. Add

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

to your manifest.

回答2:

The NullPointer is probably coming from for (File file : files), reason being that files is null.

From the documentation, listFiles() will return null if the specified path is not a directory; what it does not tell you is that you will most likely also get this value if you have a FileSystem permissions issue.

if ( Environment.getExternalStorageDirectory().exists() )

Do you have the required permissions defined in your manifest?

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

If this is not the issue, then my only thought is that FILE is null, or line 138 is not in this snippet.

来源：https://stackoverflow.com/questions/34661450/trying-to-list-all-folders-files-from-internal-sdcard-android-but-get-a-null