## Um aplicativo com callbacks

Vamos ver tudo isso com um exemplo.

Imagine que você tem um aplicativo que permite criar faturas.

Essas faturas terão um `id`, `title` (opcional), `customer` e `total`.

O usuário da sua API (um desenvolvedor externo) criará uma fatura em sua API com uma solicitação POST.

Então sua API irá (vamos imaginar):

 * limitations under the License.
 */

package com.google.common.collect;

import com.google.caliper.BeforeExperiment;
import com.google.caliper.Benchmark;
import com.google.caliper.Param;
import com.google.common.collect.BenchmarkHelpers.SetImpl;
import com.google.common.collect.CollectionBenchmarkSampleData.Element;
import java.util.Set;

 * limitations under the License.
 */

package com.google.common.collect.testing.testers;

import static com.google.common.collect.testing.features.CollectionFeature.ALLOWS_NULL_VALUES;
import static com.google.common.collect.testing.features.CollectionSize.ZERO;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.WrongType;
import com.google.common.collect.testing.features.CollectionFeature;

 * limitations under the License.
 */

package com.google.common.collect.testing;

import static com.google.common.collect.testing.Helpers.copyToList;
import static com.google.common.collect.testing.features.CollectionFeature.DESCENDING_VIEW;
import static com.google.common.collect.testing.features.CollectionFeature.SUBSET_VIEW;

import com.google.common.annotations.GwtIncompatible;

{* ../../docs_src/security/tutorial002_an_py310.py...

   <url>
      <loc>http://www.example.com/</loc>
      <lastmod>2005-01-01</lastmod>
      <changefreq>monthly</changefreq>
      <priority>0.8</priority>
   </url>
   <url>
      <loc>http://www.example.com/catalog?item=12&amp;desc=vacation_hawaii</loc>
      <changefreq>weekly</changefreq>
   </url>
   <url>
      <loc>http://www.example.com/catalog?item=73&amp;desc=vacation_new_zealand</loc>

 * limitations under the License.
 */

package com.google.common.collect.testing.testers;

import static com.google.common.collect.testing.features.CollectionFeature.SERIALIZABLE;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractCollectionTester;
import com.google.common.collect.testing.features.CollectionFeature;
import com.google.common.testing.SerializableTester;

-->
<super-source path="super"/>

<inherits name="com.google.common.annotations.Annotations" />
<inherits name="com.google.common.base.Base" />
<inherits name="com.google.common.collect.Collect" />
<inherits name="com.google.gwt.core.Core" />
<inherits name="com.google.gwt.user.User" />

-->
<super-source path="super"/>

<inherits name="com.google.common.annotations.Annotations" />
<inherits name="com.google.common.base.Base" />
<inherits name="com.google.common.escape.Escape" />
<inherits name="com.google.gwt.core.Core" />
<inherits name="com.google.gwt.user.User" />

        .Builder()
        .url("https://sni.cloudflaressl.com/cdn-cgi/trace")
        .header("Host", "cloudflare-dns.com")
        .build()
    client.newCall(request).execute().use { response ->
      assertThat(response.code).isEqualTo(200)
      assertThat(response.protocol).isEqualTo(Protocol.HTTP_2)

      assertThat(response.body.string()).contains("h=cloudflare-dns.com")
    }
  }

  @Test
  fun getWithDns() {