ArrayList<Class> sharedpreferences

Question

Is it any way to save ArrayList to sharedpreferences? Thank you

ArrayList<Class> activityList = new ArrayList<>();
activityList.add(Level1Activity.class);
activityList.add(Level2Activity.class);
activityList.add(Level3Activity.class);
activityList.add(Level4Activity.class);
activityList.add(Level5Activity.class);

Answer 1

I already answered this to your other question but just in case, I'll re-write it here and explain it more a bit.

You can use Gson to convert your list into a Json String so that you can save it in SharedPreferences.

You will need to add implementation 'com.google.code.gson:gson:2.8.6' inside your app gradle dependencies to be able to use Gson.

But, you cannot simply parse the list using Gson to Json or viceversa when you use the Class class. In order to do so, you will need to create your own serializer and deserializer for it. Or you'll face this exception:

java.lang.UnsupportedOperationException: Attempted to serialize java.lang.Class: com.etc.etc.Level1Activity. Forgot to register a type adapter?

So let's create a custom adapter that implements JsonSerializer and JsonDeserializer. Don't forget to put inside the angle brackets the type we're working with, which is Class.

ClassAdapter class

public class ClassAdapter implements JsonSerializer<Class>, JsonDeserializer<Class> {

    @Override
    public JsonElement serialize(Class src, Type typeOfSrc, JsonSerializationContext context) {
        // Get our class 'src' name
        return new JsonPrimitive(src.getName());
    }

    @Override
    public Class deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        try {
            // Get class
            return Class.forName(json.getAsString());
        } catch (ClassNotFoundException e) {
            // If class could not be found or did not exists, handle error here...
            e.printStackTrace();
        }
        return null;
    }

}

To convert our list to Json String and save it inside SharedPreferences:

// Create new GsonBuilder and register our adapter for Class objects
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Class.class, new ClassAdapter());

// Initialize our list of levels (ie. classes)
List<Class> classes = new ArrayList<>();
classes.add(Level1Activity.class);
classes.add(Level2Activity.class);
classes.add(Level3Activity.class);
classes.add(Level4Activity.class);
classes.add(Level5Activity.class);

// Create Gson from GsonBuilder and convert list to json
Gson gson = gsonBuilder.create();
String json = gson.toJson(classes);

// Save json to SharedPreferences
SharedPreferences sharedPreferences = getSharedPreferences("app_name", MODE_PRIVATE);
sharedPreferences.edit().putString("levels", json).apply();

And to retrieve the list back:

// Retrieve json from SharedPreferences
SharedPreferences sharedPreferences = getSharedPreferences("app_name", MODE_PRIVATE);
String json = sharedPreferences.getString("levels", null);

// Handle here if json doesn't exist yet
if (json == null) {
    // ...
}

// Create new GsonBuilder and register our adapter for Class objects
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Class.class, new ClassAdapter());

// Create Gson from GsonBuilder and specify type of list
Gson gson = gsonBuilder.create();
Type type = new TypeToken<ArrayList<Class>>(){}.getType();

// Convert json to list
List<Class> classes = gson.fromJson(json, type);

Hope this helps, happy coding!