The API docs use **Swagger UI** and **ReDoc**, and each of those need some JavaScript and CSS files.

By default, those files are served from a <abbr title="Content Delivery Network: A service, normally composed of several servers, that provides static files, like JavaScript and CSS. It's commonly used to serve those files from the server closer to the client, improving performance.">CDN</abbr>.

///

You can declare additional responses, with additional status codes, media types, descriptions, etc.

Those additional responses will be included in the OpenAPI schema, so they will also appear in the API docs.

But for those additional responses you have to make sure you return a `Response` like `JSONResponse` directly, with your status code and content.

You also define in some way at which **moments** your app will send those requests or events.

And **your users** define in some way (for example in a web dashboard somewhere) the **URL** where your app should send those requests.

All the **logic** about how to register the URLs for webhooks and the code to actually send those requests is up to you. You write it however you want to in **your own code**.

You could still use most of the features in **FastAPI** with the knowledge from the main [Tutorial - User Guide](../tutorial/index.md){.internal-link target=_blank}.

    int expectedHashCode = 0;
    for (E element : getSampleElements()) {
      expectedHashCode += (element == null) ? 0 : element.hashCode();
    }
    assertEquals(
        "A Set's hashCode() should be the sum of those of its elements.",
        expectedHashCode,
        getSet().hashCode());
  }

  @CollectionSize.Require(absent = CollectionSize.ZERO)
  @CollectionFeature.Require(ALLOWS_NULL_VALUES)

    int expectedHashCode = 0;
    for (Entry<K, V> entry : getSampleEntries()) {
      expectedHashCode += hash(entry);
    }
    assertEquals(
        "A Map's hashCode() should be the sum of those of its entries.",
        expectedHashCode,
        getMap().hashCode());
  }

  @CollectionSize.Require(absent = CollectionSize.ZERO)
  @MapFeature.Require(ALLOWS_NULL_KEYS)

/// note | Technical Details

`Header` is a "sister" class of `Path`, `Query` and `Cookie`. It also inherits from the same common `Param` class.

But remember that when you import `Query`, `Path`, `Header`, and others from `fastapi`, those are actually functions that return special classes.

///

/// info

To declare headers, you need to use `Header`, because otherwise the parameters would be interpreted as query parameters.

///

 * provided {@code standardAddAll} method.
 *
 * <p><b>{@code default} method warning:</b> This class does <i>not</i> forward calls to {@code
 * default} methods. Instead, it inherits their default implementations. When those implementations
 * invoke methods, they invoke methods on the {@code ForwardingSet}.
 *
 * <p>The {@code standard} methods are not guaranteed to be thread-safe, even when all of the
 * methods that they depend on are thread-safe.

 * {@link ValueGraph}, and {@link Network}):
 *
 * {@snippet :
 * someGraphAlgorithm(startNode, graph);
 * }
 *
 * This works because those types each implement {@code PredecessorsFunction}. It will also work
 * with any other implementation of this interface.
 *
 * <p>If you have your own graph implementation based around a custom node type {@code MyNode},

 * {@link ValueGraph}, and {@link Network}):
 *
 * {@snippet :
 * someGraphAlgorithm(startNode, graph);
 * }
 *
 * This works because those types each implement {@code SuccessorsFunction}. It will also work with
 * any other implementation of this interface.
 *
 * <p>If you have your own graph implementation based around a custom node type {@code MyNode},