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

Question

I just implemented Room for offline data saving. But in an Entity class, I am getting the following error:

Error:(27, 30) error: Cannot figure out how to save this field into database. You can consider adding a type converter for it.

And the class is as following:

@Entity(tableName = "firstPageData")
public class MainActivityData {

    @PrimaryKey
    private String userId;

    @ColumnInfo(name = "item1_id")
    private String itemOneId;

    @ColumnInfo(name = "item2_id")
    private String itemTwoId;

    // THIS IS CAUSING THE ERROR... BASICALLY IT ISN'T READING ARRAYS
    @ColumnInfo(name = "mylist_array")
    private ArrayList<MyListItems> myListItems;

    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    public ArrayList<MyListItems> getMyListItems() {
        return myListItems;
    }

    public void setCheckListItems(ArrayList<MyListItems> myListItems) {
        this.myListItems = myListItems;
    }

}

So basically I want to save the ArrayList in the database but I was not able to find anything relevant to it. Can you guide me regarding how to save an Array using Room?

NOTE: MyListItems Pojo class contains 2 Strings (as of now)

Thanks in advance.

Answer 1

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