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() {

## 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.

   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

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=
 * ```
 *

  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

    assertThat(headers["c"]).isEqualTo("d")
    assertThat(headers["e"]).isNull()
  }

  @Test fun iteratorOperator() {
    val headers = headersOf("a", "b", "c", "d")

    val pairs = mutableListOf<Pair<String, String>>()
    for ((name, value) in headers) {
      pairs += name to value
    }

    assertThat(pairs).containsExactly("a" to "b", "c" to "d")
  }

  @Test fun builderGetOperator() {

    cipherSuitesAsString.intersect(socketEnabledCipherSuites, CipherSuite.ORDER_BY_NAME)
  } else {
    socketEnabledCipherSuites
  }
}

internal fun MediaType?.chooseCharset(): Pair<Charset, MediaType?> {
  var charset: Charset = Charsets.UTF_8
  var finalContentType: MediaType? = this
  if (this != null) {
    val resolvedCharset = this.charset()
    if (resolvedCharset == null) {

import okio.Buffer
import okio.ForwardingSink
import okio.ForwardingSource
import okio.Sink
import okio.Source
import okio.buffer

/**
 * Transmits a single HTTP request and a response pair. This layers connection management and events
 * on [ExchangeCodec], which handles the actual I/O.
 */
class Exchange(
  internal val call: RealCall,
  internal val eventListener: EventListener,