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

java

room-database

android-development

data-storage

byAlex Kataev·Dec 17, 2024

To use Room Database handling an ArrayList, we need to implement TypeConverter to serialize the list into a JSON String with the aid of Gson and also deserialize it back. The converter is then integrated by annotating your Room Database class with @TypeConverters.

public class Converters {
    @TypeConverter
    public static String arrayListToString(ArrayList<String> list) {
        // Ain't no party like a JSON party
        return new Gson().toJson(list);
    }

    @TypeConverter
    public static ArrayList<String> stringToArrayList(String json) {
        // Gson to the rescue, turning JSON back to ArrayList
        return new Gson().fromJson(json, new TypeToken<ArrayList<String>>() {}.getType());
    }
}

@TypeConverters({Converters.class})
public abstract class AppDatabase extends RoomDatabase {
    // Boom! Converters are in the room!
}

This ensures that ArrayList fields have correct persistence in your Room entities.

Detailed insights & best practices

Storing complex data types such as ArrayList in Room Database needs careful steps to ensure efficiency and reliability. Below, you'll find some of the advanced techniques and potential issues that you should bear in mind:

Custom converters for complex item types

In cases where your ArrayList encompasses custom objects, e.g., ArrayList<MyListItems>, you'll need a special custom converter. Here's how you can modify your converter to accommodate such:

public class Converters {
    private static final Gson gson = new Gson();

    @TypeConverter
    public static String fromSpecialArrayList(ArrayList<MyListItems> list) {
        // Stringify all things!
        return gson.toJson(list);
    }

    @TypeConverter
    public static ArrayList<MyListItems> toSpecialArrayList(String json) {
        // Turning String into MyListItems, feel the magic!
        return gson.fromJson(json, new TypeToken<ArrayList<MyListItems>>() {}.getType());
    }
}

Using relationships and foreign keys

As an alternative to TypeConverters, consider modeling your entities to contain natural relationships thanks to @Entity and @ForeignKey:

@Entity
public class MainActivityData {
    @PrimaryKey
    public int id;
    // ...
}

@Entity(foreignKeys = @ForeignKey(entity = MainActivityData.class,
        parentColumns = "id",
        childColumns = "mainActivityId",
        onDelete = ForeignKey.CASCADE))
public class MyListItems {
    @PrimaryKey
    public int id;
    public int mainActivityId; // Linking back to MainActivityData's id
    // ...
}

In such cases, don't forget to use a ViewModel to handle interactions between entities and the corresponding ArrayLists.

Serialization in Kotlin

Given Gson is widely accepted, Kotlin developers may prefer the kotlinx.serialization library offering a more streamlined solution:

@Serializer(forClass = ArrayList::class)
object MyListItemsListSerializer: KSerializer<ArrayList<MyListItems>>

Potential pitfalls and trade-offs

If items of your list could be stored as a single JSON String, try avoiding setting up separate tables using a TypeConverter.
Keep serialization and deserialization performance in mind when dealing with large lists.
Think about normalizing data if there are dupes in your ArrayLists. This can save space and optimize queries.