Distinguish nulls and non-present values with Jackson, Optional<T>, and Kotlin

TestJacksonWithOptional.kt

package testing

import com.fasterxml.jackson.databind.ObjectMapper
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test
import java.util.*

class TestJacksonWithOptional {

    val defaultObjectMapper = ObjectMapper().apply {
        findAndRegisterModules()
        // jackson-datatype-jdk8 is present
        // jackson-module-kotlin is present
    }

    data class Example(
        val greeting: Optional<String>? = null
    )

    @Test
    fun `handles optional type with non-null value`() {
        val json = """{ "greeting": "hello" }"""
        val obj = defaultObjectMapper.readValue(json, Example::class.java)
        assertEquals("hello", obj.greeting?.get())
    }

    @Test
    fun `handles optional type with null value`() {
        val json = """{ "greeting": null }"""
        val obj = defaultObjectMapper.readValue(json, Example::class.java)
        assertEquals(false, obj.greeting?.isPresent)
    }

    @Test
    fun `handles optional type with non-present value`() {
        val json = """{  }"""
        val obj = defaultObjectMapper.readValue(json, Example::class.java)
        assertEquals(null, obj.greeting)
    }

}

This demo test code is for this stackoverflow question: https://stackoverflow.com/a/64683356/516910