Oreo (WRITE EXTERNAL STORAGE) Permission

You are correct in adding the permissions to the manifest:

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

For versions of Lollipop and higher, you need to also request the permissions at runtime. To solve this problem, I created a new method requestAppPermission that I call when the main activity is created. This method runs only for Lollipop and higher, and returns early otherwise:

private void requestAppPermissions() {
    if (android.os.Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
        return;
    }

    if (hasReadPermissions() && hasWritePermissions()) {
        return;
    }

    ActivityCompat.requestPermissions(this,
            new String[] {
                    Manifest.permission.READ_EXTERNAL_STORAGE,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE
            }, REQUEST_WRITE_STORAGE_REQUEST_CODE); // your request code
}

private boolean hasReadPermissions() {
    return (ContextCompat.checkSelfPermission(getBaseContext(), Manifest.permission.READ_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED);
}

private boolean hasWritePermissions() {
    return (ContextCompat.checkSelfPermission(getBaseContext(), Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED);
}

I call this method in the activity's onCreate:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Other things
    requestAppPermissions();
}

On first load, this will prompt the user to accept permissions.

Then, before you run any code that needs to read and write to storage, you can check these permissions, either by storing the values or running those checks again using methods hasReadPermissions() and hasWritePermissions() defined above.

UPDATE: See this answer for a better solution

---

EDIT: This is not a solution, just a quick workaround.

If you get permission denied error even when the permissions are granted and you already implemented permission checks,

make sure you're not targetting api level 29:

Change targetSdkVersion and compilesdkversion from 29 to 28 or any other lower level.

Actually, It was an external problem. One of App Libs I'm using request the WRITE_EXTERNAL_PERMISSION with android:maxSdkVersion. So when merging with my Manifest it will remove the permission for the whole application.

The Solution is to add:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" tools:node="replace"/>