Android Room Database: How to handle Arraylist in an Entity?

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Type Converter are made specifically for that. In your case, you can use code snippet given below to store data in DB.

public class Converters {
    @TypeConverter
    public static ArrayList<String> fromString(String value) {
        Type listType = new TypeToken<ArrayList<String>>() {}.getType();
        return new Gson().fromJson(value, listType);
    }

    @TypeConverter
    public static String fromArrayList(ArrayList<String> list) {
        Gson gson = new Gson();
        String json = gson.toJson(list);
        return json;
    }
}

And mention this class in your Room DB like this

@Database (entities = {MainActivityData.class},version = 1)
@TypeConverters({Converters.class})

More info here

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