code: public static File writeBitmap(Context context, Bitmap bitmap, String dir, String filename) {
          if ((bitmap == null) || TextUtils.isEmpty(dir) || TextUtils.isEmpty(filename)) {
              return null;
          }
          final File dirFile = new File(context.getFilesDir(), dir);
          return writeBitmapToDirectory(bitmap, filename, dirFile);
      }
docs: Writes a {@link Bitmap} object to a file in the current context's files directory.
  
  @param context The current context.
  @param bitmap The bitmap object to output.
  @param dir The output directory name within the files directory.
  @param filename The name of the file to output.
  @return A file where the Bitmap was stored, or {@code null} if the write
  operation failed.