Extend data class in Kotlin

Question

Data classes seem to be the replacement to the old-fashioned POJOs in Java. It is quite expectable that these classes would allow for inheritance, but I can see no convenien

Answer 1

You can inherit a data class from a non-data class.

Base class

open class BaseEntity (

@ColumnInfo(name = "name") var name: String? = null,
@ColumnInfo(name = "description") var description: String? = null,
// ...
)

child class

@Entity(tableName = "items", indices = [Index(value = ["item_id"])])
data class CustomEntity(

    @PrimaryKey
    @ColumnInfo(name = "id") var id: Long? = null,
    @ColumnInfo(name = "item_id") var itemId: Long = 0,
    @ColumnInfo(name = "item_color") var color: Int? = null

) : BaseEntity()

It worked.

Answer 2

Above solution using abstract class actually generates corresponding class and let the data class extends from it.

If you don't prefer abstract class, how about using an interface?

Interface in Kotlin can have properties as shown in this this article..

interface History {
    val date: LocalDateTime
    val name: String
    val value: Int
}

data class FixedHistory(override val date: LocalDateTime,
                        override val name: String,
                        override val value: Int,
                        val fixedEvent: String) : History

I was curious how Kotlin compile this. Here's equivalent Java code (generated using the Intellij [Kotlin bytecode] feature):

public interface History {
   @NotNull
   LocalDateTime getDate();

   @NotNull
   String getName();

   int getValue();
}

public final class FixedHistory implements History {
   @NotNull
   private final LocalDateTime date;
   @NotNull
   private final String name;
   private int value;
   @NotNull
   private final String fixedEvent;

   // Boring getters/setters as usual..
   // copy(), toString(), equals(), hashCode(), ...
}

As you can see, it works exactly like a normal data class!