@Override
  public boolean put(@ParametricNullness K key, @ParametricNullness V value) {
    return super.put(key, value);
  }

  /**
   * {@inheritDoc}
   *
   * <p>Though the method signature doesn't say so explicitly, the returned map has {@link List}
   * values.
   */
  @Override
  public Map<K, Collection<V>> asMap() {
    return super.asMap();
  }

  /**

     * because we know that newClassMap() is implemented using ConstrainedMap which is itself
     * well-tested. A purist would object to this, but what can I say, we're dirty cheaters.
     */
    map.put(Integer.class, new Integer(5));
    assertThrows(ClassCastException.class, () -> map.put(Double.class, new Long(42)));
    // Won't compile: map.put(String.class, "x");
  }

    * As connecting to an email server and sending an email tends to be "slow" (several seconds), you can return the response right away and send the email notification in the background.
* Processing data:
    * For example, let's say you receive a file that must go through a slow process, you can return a response of "Accepted" (HTTP 202) and process the file in the background.

## Using `BackgroundTasks` { #using-backgroundtasks }

## Custom response class { #custom-response-class }

You can create your own custom response class, inheriting from `Response` and using it.

For example, let's say that you want to use [`orjson`](https://github.com/ijl/orjson) with some settings.

Let's say you want it to return indented and formatted JSON, so you want to use the orjson option `orjson.OPT_INDENT_2`.

  }

  public void testNulls() {
    /*
     * Don't bother testing. All non-primitive parameters are used only to construct error messages.
     * We never want to pass null for them, so we haven't annotated them to say that null is
     * allowed. But at the same time, it seems wasteful to bother inserting the checkNotNull calls
     * that NullPointerTester wants.
     *

  @Override
  public boolean put(@ParametricNullness K key, @ParametricNullness V value) {
    return super.put(key, value);
  }

  /**
   * {@inheritDoc}
   *
   * <p>Though the method signature doesn't say so explicitly, the returned map has {@link List}
   * values.
   */
  @Override
  public Map<K, Collection<V>> asMap() {
    return super.asMap();
  }

  /**

But it's signed. So, when you receive a token that you emitted, you can verify that you actually emitted it.

That way, you can create a token with an expiration of, let's say, 1 week. And then when the user comes back the next day with the token, you know that user is still logged in to your system.

Now, let's see how you could use that to return a custom response.

Let's say that you want to return an [XML](https://en.wikipedia.org/wiki/XML) response.

You could put your XML content in a string, put that in a `Response`, and return it:

    `@Deprecated` APIs will remain (again, unless they are `@Beta`). We have no
    plans to start removing things again, but officially, we're leaving our
    options open in case of surprises (like, say, a serious security problem).

3.  Guava has one dependency that is needed for linkage at runtime:
    `com.google.guava:failureaccess:1.0.3`. It also has

    return (Set<V>) super.replaceValues(key, values);
  }

  /**
   * {@inheritDoc}
   *
   * <p>Though the method signature doesn't say so explicitly, the returned map has {@link Set}
   * values.
   */
  @Override
  public Map<K, Collection<V>> asMap() {
    return super.asMap();
  }

  /**
   * Stores a key-value pair in the multimap.