[JAVA] Save ArrayList using GSON on Android

When saving an ArrayList on Android, if it's as simple as using SQLite, It is convenient to convert it to JSON and save it in Preference.

Also, I think it will be easier to use GSON (Google library) when converting to JSON. https://github.com/google/gson

This time, convert the ʻArrayList `type and save it.

Introduction of GSON

Introduced gson library with gradle in Android Studio.

dependencies{
      ...
      ...
      compile 'com.google.code.gson:gson:2.7' //May be a little old
}

Read

private ArrayList<String> arrayList;
SharedPreferences pref = getSharedPreferences("pref", MODE_PRIVATE);
Gson gson = new Gson();
arrayList = gson.fromJson(pref.getString(SAVE_KEY, ""), new TypeToken<ArrayList<String>>(){}.getType());

If you want to branch the processing of the unsaved state to preference, like this

private ArrayList<String> arrayList;
SharedPreferences pref = getSharedPreferences("pref", MODE_PRIVATE);
Gson gson = new Gson();
String json = pref.getString(SAVE_KEY, "");
if(json.equals("[]"))
{
    arrayList = new ArrayList<>();
}
else
{
    arrayList = gson.fromJson(json, new TypeToken<ArrayList<String>>(){}.getType());
}

writing

SharedPreferences pref = getSharedPreferences("pref", MODE_PRIVATE);
Gson gson = new Gson();
pref.edit().putString(SAVE_KEY, gson.toJson(arrayList)).apply();