printEvent("callStart");
  }

  @Override public void callEnd(Call call) {
    printEvent("callEnd");
  }

  ...
}
```

We can use this listener to race a pair of concurrent HTTP requests:

```java
Request washingtonPostRequest = new Request.Builder()
    .url("https://www.washingtonpost.com/")
    .build();
client.newCall(washingtonPostRequest).enqueue(new Callback() {

 *
 * Instances of this class are immutable. Use [Builder] to create instances.
 */
@Suppress("NAME_SHADOWING")
class Headers internal constructor(
  internal val namesAndValues: Array<String>,
) : Iterable<Pair<String, String>> {
  /** Returns the last value corresponding to the specified field, or null. */
  operator fun get(name: String): String? = commonHeadersGet(namesAndValues, name)

  /**

      }

    /**
     * Sets the public/private key pair used for this certificate. If unset a key pair will be
     * generated.
     */
    fun keyPair(keyPair: KeyPair) =
      apply {
        this.keyPair = keyPair
      }

    /**
     * Sets the public/private key pair used for this certificate. If unset a key pair will be
     * generated.
     */
    fun keyPair(

   * }
   * ```
   *
   * The first property of the pair is the adapter that was used, the second property is the value.
   */
  internal val generalNameDnsName = Adapters.IA5_STRING.withTag(tag = 2L)
  internal val generalNameIpAddress = Adapters.OCTET_STRING.withTag(tag = 7L)
  internal val generalName: DerAdapter<Pair<DerAdapter<*>, Any?>> =
    Adapters.choice(
      generalNameDnsName,

  fun choice(vararg choices: DerAdapter<*>): DerAdapter<Pair<DerAdapter<*>, Any?>> {
    return object : DerAdapter<Pair<DerAdapter<*>, Any?>> {
      override fun matches(header: DerHeader): Boolean = true

      override fun fromDer(reader: DerReader): Pair<DerAdapter<*>, Any?> {
        val peekedHeader =
          reader.peekHeader()

    // punctuation space
    assertThat(parse("http://h/\u2008").encodedPath).isEqualTo("/%E2%80%88")
    // thin space
    assertThat(parse("http://h/\u2009").encodedPath).isEqualTo("/%E2%80%89")
    // hair space
    assertThat(parse("http://h/\u200a").encodedPath).isEqualTo("/%E2%80%8A")
    // zero-width space
    assertThat(parse("http://h/\u200b").encodedPath).isEqualTo("/%E2%80%8B")
    // zero-width non-joiner

  @ArgumentsSource(FileSystemParamProvider::class)
  fun emptyCache(parameters: Pair<FileSystem, Boolean>) {
    setUp(parameters.first, parameters.second)
    cache.close()
    assertJournalEquals()
  }

  @ParameterizedTest
  @ArgumentsSource(FileSystemParamProvider::class)
  fun recoverFromInitializationFailure(parameters: Pair<FileSystem, Boolean>) {
    setUp(parameters.first, parameters.second)

  fun testDnsRequest() {
    val (allAddresses, exception) = dnsQuery("localhost")

    assertThat(exception).isNull()
    assertThat(allAddresses).isNotEmpty()
  }

  private fun dnsQuery(hostname: String): Pair<List<InetAddress>, Exception?> {
    val allAddresses = mutableListOf<InetAddress>()
    var exception: Exception? = null
    val latch = CountDownLatch(1)

    // assumes an IPv4 address

## Connection Pool

A primary responsibility for any HTTP client is to efficiently manage network connections. Creating and establishing new connections require a fair amount of overhead and added latency. OkHttp will make every effort to reuse existing connections to avoid this overhead and added latency.

private val TOKEN_DELIMITERS = "\t ,=".encodeUtf8()

/**
 * Parse RFC 7235 challenges. This is awkward because we need to look ahead to know how to
 * interpret a token.
 *
 * For example, the first line has a parameter name/value pair and the second line has a single
 * token68:
 *
 * ```
 * WWW-Authenticate: Digest foo=bar
 * WWW-Authenticate: Digest foo=
 * ```
 *