Kotlin lazy property in data class results in NullPointerException

Question

I have a Kotlin interface that looks like this:

interface IGeneralData {
    val id: Int
    val name: String
}

and a data class implementing IGeneralData which is created by either GSON or Jetpack Room:

@Entity
data class MyData(
    @PrimaryKey override val id: Int,
    override val name: String
) : IGeneralData {
    @delegate:Ignore
    val last: String by lazy {
        name.substring(name.lastIndexOf('.') + 1)
    }
}

Unfortunately, I get a NullPointerException that I don't understand when accessing the last property:

java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object kotlin.Lazy.getValue()' on a null object reference

When I look at the object in the debugger, at the time of accessing last , both the id field and the name field are populated. So where does this NPE come from?

The following version with a getter works just fine:

@Entity
data class MyData(
    @PrimaryKey override val id: Int,
    override val name: String
) : IGeneralData {
    val last: String
        @Ignore
        get() {
            return name.substring(name.lastIndexOf('.') + 1)
        }
}

Thank you!

Answer 1

This is just a guess, but I think Ignore fixes the issue for Room, but for GSON you need to use Transient . So you need both annotations:

@Entity
data class MyData(
    @PrimaryKey override val id: Int,
    override val name: String
) : IGeneralData {
    @delegate:Ignore 
    @delegate:Transient
    val last: String by lazy {
        name.substring(name.lastIndexOf('.') + 1)
    }
}