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Biju Kunjummen

Kotlin – Reified type parameters sample

This post walks through a sample that demonstrates Kotlin’s ability to cleverly reify generic type parameters.

So consider first a world where Kotlin does not support this feature, if we were using the Jackson library to convert a JSON to a Map with String based keys and Integer based values, I would use a code along these lines:

@Test
fun `sample parameterized retrieval raw object mapper`() {
    val objectMapper = ObjectMapper()
    val map: Map<String, Int> = objectMapper.readValue("""
        | {
        |   "key1": 1,
        |   "key2": 2,
        |   "key3": 3
        | }
        """.trimMargin(), object : TypeReference<Map<String, Int>>() {})

    assertThat(map).isEqualTo(mapOf("key1" to 1, "key2" to 2, "key3" to 3))
}

TypeReference used above implements a pattern called Super type token which allows the type of a parameterized type to be captured by sub-classing. Note the ugly way to creating an anonymous sub-class in Kotlin.

object : TypeReference<Map<String, Int>>() {}

What I would like to do is to invoke the ObjectMapper the following way instead:

@Test
fun `sample parameterized retrieval`() {
    val om = ObjectMapper()
    val map: Map<String, Int> = om.readValue("""
        | {
        |   "key1": 1,
        |   "key2": 2,
        |   "key3": 3
        | }
        """.trimMargin())

    assertThat(map).isEqualTo(mapOf("key1" to 1, "key2" to 2, "key3" to 3))
}

The generic type parameter is being inferred based on the type of what is to be returned (the left-hand side).

This can be achieved using an extension function on ObjectMapper which looks like this:

inline fun <reified T> ObjectMapper.readValue(s: String): T = 
        this.readValue(s, object : TypeReference<T>() {})

The inline function is the heart of the support for being able to reify generic type parameter here – after compilation, the function would be expanded out into any place this function is called and thus the second version is exactly same as the first version of the test but reads far better than before.

Note that Jackson already implements these Kotlin extension functions in the excellent jackson-module-kotlin library.

Published on Java Code Geeks with permission by Biju Kunjummen, partner at our JCG program. See the original article here: Kotlin – Reified type parameters sample

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