NullPointerException - Attempt to get length of null array (readDirectory())

All you need to do is go into your manifest file and under the backup line just paste it

android:requestLegacyExternalStorage="true"

Done :)

f.listFiles()

will return null if the path does not exist.

Please check your path.

Update

Android will also require a permission to read certain files. Maybe you need to add this to your manifest:

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

More information:

http://developer.android.com/reference/android/Manifest.permission.html