Full code example from https://jakewharton.com/r8-optimization-method-outlining/

Json.kt

import java.io.StringReader
import java.time.OffsetDateTime
import android.util.JsonReader
import android.util.JsonToken

fun main() {
  println(TweetParser.fromJson(JsonReader(StringReader(""))))
  println(UserParser.fromJson(JsonReader(StringReader(""))))
}

data class User(
  val id: String,
  val username: String,
  val displayName: String,
  val email: String,
  val created: OffsetDateTime,
  val isPublic: Boolean
)

data class Tweet(
  val id: String,
  val userId: String,
  val content: String,
  val created: OffsetDateTime
)

object UserParser {
  fun fromJson(reader: JsonReader): User {
    var id: String? = null
    var username: String? = null
    var displayName: String? = null
    var email: String? = null
    var created: OffsetDateTime? = null
    var isPublic: Boolean? = null

    reader.beginObject()
    while (reader.peek() != JsonToken.END_OBJECT) {
      when (reader.nextName()) {
        "id" -> id = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("id")
                    .append("' was null at ").append(reader).toString())
        "username" -> username = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("username")
                    .append("' was null at ").append(reader).toString())
        "displayName" -> displayName = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("displayName")
                    .append("' was null at ").append(reader).toString())
        "email" -> email = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("email")
                    .append("' was null at ").append(reader).toString())
        "created" -> created = reader.nextString()?.let(OffsetDateTime::parse) ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("created")
                    .append("' was null at ").append(reader).toString())
        "isPublic" -> isPublic = reader.nextString()?.toBoolean() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("isPublic")
                .append("' was null at ").append(reader).toString())
        else -> reader.skipValue()
      }
    }
    reader.endObject()

    return User(
      id = id ?: throw IllegalStateException(
          StringBuilder("Required property '").append("id")
             .append("' missing at ").append(reader).toString()),
      username = username ?: throw IllegalStateException(
          StringBuilder("Required property '").append("username")
             .append("' missing at ").append(reader).toString()),
      displayName = displayName ?: throw IllegalStateException(
          StringBuilder("Required property '").append("displayName")
             .append("' missing at ").append(reader).toString()),
      email = email ?: throw IllegalStateException(
          StringBuilder("Required property '").append("email")
             .append("' missing at ").append(reader).toString()),
      created = created ?: throw IllegalStateException(
          StringBuilder("Required property '").append("created")
             .append("' missing at ").append(reader).toString()),
      isPublic = isPublic ?: throw IllegalStateException(
          StringBuilder("Required property '").append("isPublic")
             .append("' missing at ").append(reader).toString())
    )
  }
}


object TweetParser {
  fun fromJson(reader: JsonReader): Tweet {
    var id: String? = null
    var userId: String? = null
    var content: String? = null
    var created: OffsetDateTime? = null

    reader.beginObject()
    while (reader.peek() != JsonToken.END_OBJECT) {
      when (reader.nextName()) {
        "id" -> id = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("id")
                    .append("' was null at ").append(reader).toString())
        "userId" -> userId = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("userId")
                    .append("' was null at ").append(reader).toString())
        "content" -> content = reader.nextString() ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("content")
                    .append("' was null at ").append(reader).toString())
        "created" -> created = reader.nextString()?.let(OffsetDateTime::parse) ?:
            throw IllegalStateException(
                StringBuilder("Non-null value '").append("created")
                    .append("' was null at ").append(reader).toString())
        else -> reader.skipValue()
      }
    }
    reader.endObject()

    return Tweet(
      id = id ?: throw IllegalStateException(
          StringBuilder("Required property '").append("id")
             .append("' missing at ").append(reader).toString()),
      userId = userId ?: throw IllegalStateException(
          StringBuilder("Required property '").append("userId")
             .append("' missing at ").append(reader).toString()),
      content = content ?: throw IllegalStateException(
          StringBuilder("Required property '").append("content")
             .append("' missing at ").append(reader).toString()),
      created = created ?: throw IllegalStateException(
          StringBuilder("Required property '").append("created")
             .append("' missing at ").append(reader).toString())
    )
  }
}