How to change a Bipmap variable to String Base64 with Android Studio

img = (ImageView) findViewById (R.id.image);

public void onActivityResult (int requestCode, int resultCode, Intent data) {         super.onActivityResult (requestCode, resultCode, data);

    if (resultCode == Activity.RESULT_OK) {
        Bundle ext = data.getExtras();
        Bmp = (Bitmap)ext.get("data");
        img.setImageBitmap(Bmp);

    }
}