* headers.
   *
   * Official OkHttp releases follow [semantic versioning][semver]. Versions with the `-SNAPSHOT`
   * qualifier are not unique and should only be used in development environments. If you create
   * custom builds of OkHttp please include a qualifier your version name, like "4.7.0-mycompany.3".

This [sample factory](https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/PrintEvents.java) creates a unique ID for each call and uses that ID to differentiate calls in log messages.

```java
class PrintingEventListener extends EventListener {
  public static final Factory FACTORY = new Factory() {

## Version 4.1.1

_2019-09-05_

 *  Fix: Don't drop repeated headers when validating cached responses. In our Kotlin upgrade we
    introduced a regression where we iterated the number of unique header names rather than then
    number of unique headers. If you're using OkHttp's response cache this may impact you.


## Version 4.1.0

_2019-08-12_

 [brotli]: https://github.com/google/brotli

    assertThat(server.takeRequest().headers["If-None-Match"]).isNull()

    // Hit that stored response. It's different, but Vary says it doesn't matter.
    Thread.sleep(10) // Make sure the timestamps are unique.
    val cacheHit =
      executeSynchronously(
        "/",
        "Accept-Language",
        "en-US",
        "Accept-Charset",
        "UTF-8",
      )

     * outlined in RFC 2046. In forms, there are a series of fields to be supplied by the user who
     * fills out the form. Each field has a name. Within a given form, the names are unique.
     */
    @JvmField
    val FORM = "multipart/form-data".toMediaType()

    private val COLONSPACE = byteArrayOf(':'.code.toByte(), ' '.code.toByte())