override fun configureTlsExtensions(
    sslSocket: SSLSocket,
    hostname: String?,
    protocols: List<Protocol>,
  ) {
    // No TLS extensions if the socket class is custom.
    socketAdapters.find { it.matchesSocket(sslSocket) }
      ?.configureTlsExtensions(sslSocket, hostname, protocols)
  }

  override fun getSelectedProtocol(sslSocket: SSLSocket): String? =

  override fun configureTlsExtensions(
    sslSocket: SSLSocket,
    hostname: String?,
    protocols: List<@JvmSuppressWildcards Protocol>,
  ) {
    // No TLS extensions if the socket class is custom.
    socketAdapters.find { it.matchesSocket(sslSocket) }
      ?.configureTlsExtensions(sslSocket, hostname, protocols)
  }

  override fun getSelectedProtocol(sslSocket: SSLSocket): String? =

  private val cacheFs = FakeFileSystem()
  private val bootstrapClient =
    OkHttpClient.Builder()
      .protocols(listOf(Protocol.HTTP_2, Protocol.HTTP_1_1))
      .build()

  @BeforeEach
  fun setUp(server: MockWebServer) {
    this.server = server
    server.protocols = bootstrapClient.protocols
    dns = buildLocalhost(bootstrapClient, false)
  }

  @Test
  fun getOne() {
    server.enqueue(

 */
package okhttp3.internal.http

import okhttp3.OkHttpClient
import okhttp3.Protocol
import okhttp3.Request

object ExternalHttp2Example {
  @JvmStatic
  fun main(args: Array<String>) {
    val client =
      OkHttpClient.Builder()
        .protocols(listOf(Protocol.HTTP_2, Protocol.HTTP_1_1))
        .build()
    val call =
      client.newCall(
        Request.Builder()

   *
   * For cipher suites, at least one of the [required cipher suites][cipherSuites] must match the
   * socket's enabled cipher suites. If there are no required cipher suites the socket must have at
   * least one cipher suite enabled.
   *
   * For protocols, at least one of the [required protocols][tlsVersions] must match the socket's
   * enabled protocols.

        connectTls(sslSocket, connectionSpec)
        user.secureConnectEnd(handshake)
      } else {
        socket = rawSocket
        protocol =
          when {
            Protocol.H2_PRIOR_KNOWLEDGE in route.address.protocols -> Protocol.H2_PRIOR_KNOWLEDGE
            else -> Protocol.HTTP_1_1
          }
      }

      val connection =
        RealConnection(
          taskRunner = taskRunner,

            sslSocketFactory(
              sslSocketFactory,
              handshakeCertificates.trustManager,
            )

            when (socketMode.protocol) {
              HTTP_2 -> protocols(listOf(HTTP_2, HTTP_1_1))
              HTTP_1_1 -> protocols(listOf(HTTP_1_1))
              else -> TODO()
            }

            val serverSslSocketFactory =

  fun configureTlsExtensions(
    sslSocket: SSLSocket,
    hostname: String?,
    protocols: List<Protocol>,
  )

  fun getSelectedProtocol(sslSocket: SSLSocket): String?

Kotlin does have properties and we take advantage of them in OkHttp.

 * **Address**: certificatePinner, connectionSpecs, dns, hostnameVerifier, protocols, proxy,
   proxyAuthenticator, proxySelector, socketFactory, sslSocketFactory, url
 * **Cache**: directory
 * **CacheControl**: immutable, maxAgeSeconds, maxStaleSeconds, minFreshSeconds, mustRevalidate,

        HTTP_1_0.protocol -> HTTP_1_0
        HTTP_1_1.protocol -> HTTP_1_1
        H2_PRIOR_KNOWLEDGE.protocol -> H2_PRIOR_KNOWLEDGE
        HTTP_2.protocol -> HTTP_2
        SPDY_3.protocol -> SPDY_3
        QUIC.protocol -> QUIC
        else -> {
          // Support HTTP3 draft like h3-29
          if (protocol.startsWith(HTTP_3.protocol)) HTTP_3 else throw IOException("Unexpected protocol: $protocol")
        }