Read and Write external storage permission isn't working

in android API >= 23 you need to request permission at runtime. Take a look here

Something like this

        // Here, thisActivity is the current activity
    if (ContextCompat.checkSelfPermission(thisActivity,
            Manifest.permission.WRITE_EXTERNAL_STORAGE)
            != PackageManager.PERMISSION_GRANTED) {

        // Permission is not granted
        // Should we show an explanation?
        if (ActivityCompat.shouldShowRequestPermissionRationale(thisActivity,
                Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
            // Show an explanation to the user *asynchronously* -- don't block
            // this thread waiting for the user's response! After the user
            // sees the explanation, try again to request the permission.
        } else {
            // No explanation needed; request the permission
            ActivityCompat.requestPermissions(thisActivity,
                    new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                    MY_PERMISSIONS_WRITE_EXTERNAL_STORAGE);

            // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
            // app-defined int constant. The callback method gets the
            // result of the request.
        }
    } else {
        // Permission has already been granted
    }

however, Ted permission library is a gread lib to avoid such boilerplate code.

//call back after permission granted
    PermissionListener permissionlistener = new PermissionListener() {
        @Override
        public void onPermissionGranted() {
            Toast.makeText(MainActivity.this, "Permission Granted", Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onPermissionDenied(List<String> deniedPermissions) {
            Toast.makeText(MainActivity.this, "Permission Denied\n" + deniedPermissions.toString(), Toast.LENGTH_SHORT).show();
        }


    };

    //check all needed permissions together
    TedPermission.with(this)
            .setPermissionListener(permissionlistener)
            .setDeniedMessage("If you reject permission,you can not use this service\n\nPlease turn on permissions at [Setting] > [Permission]")
            .setPermissions(Manifest.permission.READ_EXTERNAL_STORAGE, Manifest.permission.WRITE_EXTERNAL_STORAGE)
            .check();

Recommended solution: User a permission library like Dexter or KotlinPermissions or RxPermissions. I have used all of them and they are quite reliable.

Else:

Write these lines in your activity, just bellow super.onCreate(...) and before setContentView(...):

super.onCreate(savedInstanceState);

    // Here, thisActivity is the current activity

    if (ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, 1);
    }

    if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
    }

    if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, 1);
    }
    setContentView(R.layout.activity_main);

Now it should work, it fixed the problem for me!

Read and Write permission for storage and gallery usage for marshmallow or above:

  private void requestPermission() {
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
          requestPermissions(new String[] {
              android.Manifest.permission.WRITE_EXTERNAL_STORAGE}, REQUEST_WRITE_PERMISSION);
       } else {
           openFilePicker();
       }
  }

To check permission result use below method:

 @Override
 public void onRequestPermissionsResult(int requestCode, @NonNull String[] 
                                        permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);

    switch (requestCode) {
        case STORAGE_PERMISSION_REQUEST_CODE:
            if (grantResults.length > 0 && permissions[0].equals(Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
                // check whether storage permission granted or not.
                if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    // do what you want;
                }
            }
            break;
        default:
            break;
    }
}