//    把JSON写到文件中
  public void saveCrimes(ArrayList<Crime> crimes) throws JSONException, IOException {
    // build an array in JSON
    JSONArray array = new JSONArray();
    for (Crime c : crimes) array.put(c.toJSON());

    // write the file to disk
    Writer writer = null;
    try {
      OutputStream out = mContext.openFileOutput(mFilename, Context.MODE_PRIVATE);
      writer = new OutputStreamWriter(out);
      writer.write(array.toString());
    } finally {
      if (writer != null) writer.close();
    }
  }