存储自定义对象的数组列表

我已经在共享首选项中保存了自定义对象的ArrayList，如下所示:

I have saved ArrayList of custom objects in the shared preferences like this:

SharedPreferences prefs = context.getSharedPreferences("prefName", Context.MODE_PRIVATE);
Editor editor = prefs.edit();
editor.putStringSet("myList", new Gson().toJson(arraylist).toString());
editor.apply();  

这样做，一旦保存了值，但是当我退出应用程序并重新启动并尝试保存新值时，旧值就消失了.
知道如何保存旧值并附加新值吗?
我想将所有值保存在同一数组中，并保存数组以在每次加载应用程序时显示值.

By doing this, once values are saved but when i exit from the app and relaunch and try to save new values then the old values are gone.
Any idea how can i save old values and append new ones?
I want to keep all values in the same array and keep array saved to show values every time when the app loads.