Kotlin `object` 初始化顺序导致意外的 null 实例

Question

Consider the following code:

sealed class DataType<T : Any> {
    abstract fun inputToType(input: String): T
    abstract fun typeToSql(value: T): String

    companion object {
        val all = listOf(StringDt, LongDt)
    }
}

object StringDt : DataType<String>() {
    override fun inputToType(input: String) = input
    override fun typeToSql(value: String) = "\"${value}\""
}

object LongDt : DataType<Long>() {
    override fun inputToType(input: String) = input.toLong()
    override fun typeToSql(value: Long) = value.toString()
}

val dataTypeList = listOfNotNull(StringDt, LongDt)

println(dataTypeList)
println(DataType.all)

Things to consider:

• object as per documentation (and my understanding as well) is singleton and always instantiated
• the two objects ( StringDt and LongDt ) are quite similar

The result of println(DataType.all) shows that one of the objects are not initialized. How is that possible? I would expect all the list elements to be initialized.

IntelliJ version: CE 2020.2 Kotlin plugin version: 1.4.0-release-IJ2020.2-1

Here's a running example which shows that the static list has a null element, while the non-static one contains both objects initialized.

Answer 1

It happens due to cyclical static initializations. It's pretty hard to explain this problem in two words but you can read about it here .

To fix this behavior you can change all initialization like this:

val all by lazy { listOf(StringDt, LongDt) }