基本上你的问题的答案是否定的。你的行动方针是
1.将文件读入存储器，
1.修改内存中的内容
1.用修改的内容覆盖旧文件。
下面是关于如何修改文本文件的问题，并给出了很好的答案：Modifying existing file content in Java

我已经找到答案了。基本上我需要读取现有的JSON文件，然后替换现有的JSON对象值

private void writeFile(String age) {
    try {
        FileInputStream fileInputStream = openFileInput("credentials.json");
        InputStreamReader isr = new InputStreamReader(fileInputStream);
        BufferedReader br = new BufferedReader(isr);

        StringBuilder jsonStringBuilder = new StringBuilder();
        String line;
        while ((line = br.readLine()) != null) {
            jsonStringBuilder.append(line);
        }
        br.close();

        String jsonString = jsonStringBuilder.toString();
        JsonParser jsonParser = new JsonParser();
        JsonObject jsonObject = jsonParser.parse(jsonString).getAsJsonObject();

        jsonObject.addProperty("age", age);

        FileOutputStream fileOutputStream = openFileOutput("credentials.json", MODE_PRIVATE);
        OutputStreamWriter osw = new OutputStreamWriter(fileOutputStream);
        BufferedWriter bw = new BufferedWriter(osw);

        Gson gson = new GsonBuilder().setPrettyPrinting().create();
        String updatedJsonString = gson.toJson(jsonObject);

        bw.write(updatedJsonString);
        bw.flush();
        bw.close();

    } catch (FileNotFoundException e) {
        intentActivity(Login.class);
    } catch (IOException e) {
        intentActivity(Login.class);
    }
}