Android Room - error: Cannot figure out how to save this field into database

Question

Detailed log

error: Cannot figure out how to save this field into database. You can 
consider adding a type converter for it.
private final java.util.Date mTime = null;

I have an entity with a field as

var mStartTime : Date = Date() // java.util.Date

Why cant Room persist Date objects? What can be best converter for Date?

Answer 1

Date is exactly the example given in https://developer.android.com/training/data-storage/room/referencing-data.

For example, if we want to persist instances of Date, we can write the following TypeConverter to store the equivalent Unix timestamp in the database:

public class Converters {
    @TypeConverter
    public static Date fromTimestamp(Long value) {
        return value == null ? null : new Date(value);
    }
    @TypeConverter
    public static Long dateToTimestamp(Date date) {
        return date == null ? null : date.getTime();
    }
}

The preceding example defines 2 functions, one that converts a Date object to a Long object and another that performs the inverse conversion, from Long to Date. Since Room already knows how to persist Long objects, it can use this converter to persist values of type Date.

Next, you add the @TypeConverters annotation to the AppDatabase class so that Room can use the converter that you've defined for each entity and DAO in that AppDatabase:

AppDatabase.java

@Database(entities = {User.class}, version = 1)
@TypeConverters({Converters.class})
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

A side note: java.util.Date is considered to be badly designed (and java.util.Calendar is much worse). If you have any non-trivial date-time logic and can get away with API level 26 (Java 8 on desktop), it's generally better to use java.time package. And if you can't, see https://github.com/JakeWharton/ThreeTenABP for a backport.

Answer 2

    // Java code will not convert to Kotlin very 
    // well so here is the Kotlin: Converter 
    // class

    public class Converters {
        @TypeConverter
        fun fromTimestamp( value: Long?) : 
                       java.sql.Date {
            return java.sql.Date(value ?: 0)
        }
        @TypeConverter
        fun dateToTimestamp(date :java.sql.Date?) 
                                 :Long {
            return date?.getTime() ?: 0
       }

    // Here is the type converters example in 
    // Kotlin
    @Database(entities = [DbNasaPictures::class], 
              version = 2)
    @TypeConverters(Converters::class)
    abstract class PicturesDatabase: 
                     RoomDatabase() {