Extended Data Class dangers

Language Design
1

I was trying to build data classes for Android Room. Coming from Java, I had my data classes always carry the fields “id” and “last_updated”, so I extended those classes.

In Kotlin data classes can not be extended, but you can extend a data class with an abstract class.

abstract class TestBase {
    var id: Long = 0 // given by database
    val last_update = System.currentTimeMillis() // given by database
}

data class PersonData(var personName: String = ""):TestBase() {
}

There is no compiler warning.

However, if I want to use the benefits of a data class I am flabbergasted:

    val person1 = PersonData("Smith")
    person1.id = 1
    val person2 = PersonData("Smith")
    person1.id = 2

    val isEqual = person1.equals(person2)
    val isEqual2 = person1 == person2
    println("Person1 equals Person2: $isEqual, $isEqual2")

The two persons are wrongly regarded as the same.
I noticed, because I needed the ID in toString(), which was missing.

Data Classes should not be allowed to be extended if these kind of results are the outcome.

2

At first in Kotlin 1.0 inheritance in data classes was forbidden, but it was too restrictive in practice. In that case it’s a developer’s responsibility to make sure that a hashCode()/equals() contract is correct.

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