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() {

    // 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

   * }
   * ```
   *
   * 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,

## 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.

  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()

  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

 */
package okhttp3

import okio.IOException
import okio.Timeout

/**
 * A call is a request that has been prepared for execution. A call can be canceled. As this object
 * represents a single request/response pair (stream), it cannot be executed twice.
 */
interface Call : Cloneable {
  /** Returns the original request that initiated this call. */
  fun request(): Request

  /**

      if (result == null) result = ArrayList(2)
      result.add(value(i))
    }
  }
  return result?.toList().orEmpty()
}

internal fun Headers.commonIterator(): Iterator<Pair<String, String>> {
  return Array(size) { name(it) to value(it) }.iterator()
}

internal fun Headers.commonNewBuilder(): Headers.Builder {
  val result = Headers.Builder()
  result.namesAndValues += namesAndValues

   2001; EM QUAD
   2002; EN SPACE
   2003; EM SPACE
   2004; THREE-PER-EM SPACE
   2005; FOUR-PER-EM SPACE
   2006; SIX-PER-EM SPACE
   2007; FIGURE SPACE
   2008; PUNCTUATION SPACE
   2009; THIN SPACE
   200A; HAIR SPACE
   200B; ZERO WIDTH SPACE
   202F; NARROW NO-BREAK SPACE
   205F; MEDIUM MATHEMATICAL SPACE

 *
 * 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)

  /**