}
          else -> null
        }
      }

    // If the call's connection wasn't released, reuse it. We don't call connectionAcquired() here
    // because we already acquired it.
    if (connectionUser.candidateConnection() != null) {
      check(toClose == null)
      return ReusePlan(candidate)
    }

    // The call's connection was released.

The process that happens when your API app calls the *external API* is named a "callback". Because the software that the external developer wrote sends a request to your API and then your API *calls back*, sending a request to an *external API* (that was probably created by the same developer).

    }
    return false;
  }

  @Override
  @ParametricNullness
  public final T next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    state = State.NOT_READY;
    // Safe because hasNext() ensures that tryToComputeNext() has put a T into `next`.
    T result = uncheckedCastNullableTToT(next);
    next = null;
    return result;
  }

  @Override
  public final void remove() {

    * Follow me to read when I publish something new.

## Tweet about **FastAPI** { #tweet-about-fastapi }

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INFO: Received response for https://publicobject.com/helloworld.txt in 1179.7ms
Server: nginx/1.4.6 (Ubuntu)
Content-Type: text/plain
Content-Length: 1759
Connection: keep-alive
```

We can see that we were redirected because `response.request().url()` is different from `request.url()`. The two log statements log two different URLs.

### Network Interceptors

   * <pre>
   * collection.add(existingElement);
   * expectAdded(existingElement);</pre>
   *
   * <p>In this case, {@code collection} was not modified as a result of the {@code add()} call, and
   * the test will fail because the number of occurrences of {@code existingElement} is unchanged.
   *
   * @param elements expected additional contents of {@link #container}
   */
  protected final void expectAdded(E... elements) {

    // Enqueueing the red task starts a thread because the head of the queue changed.
    redQueue.execute("red task") {
      log += "red:starting@${taskFaker.nanoTime}"
      taskFaker.sleep(100.µs)
      log += "red:finishing@${taskFaker.nanoTime}"
    }
    assertThat(taskFaker.executeCallCount).isEqualTo(1)

    // Enqueueing the blue task doesn't start a thread because the red one is still starting.

   */
  protected final void notifyFailed(Throwable cause) {
    checkNotNull(cause);

    monitor.enter();
    try {
      State previous = state();
      switch (previous) {
        case NEW:
        case TERMINATED:
          throw new IllegalStateException("Failed while in state:" + previous, cause);
        case RUNNING:
        case STARTING:
        case STOPPING:

    public <T> Iterable<? super CharSequence> wildCardsDoNotMatchByLowerBound(
        List<? super String> list) {
      return notSubtype(list);
    }

    // Can't test getSupertype() or getSubtype() because JDK reflection doesn't consider
    // Foo<?> and Foo<? extends Bar> equal for class Foo<T extends Bar>
    @TestSubtype(suppressGetSupertype = true, suppressGetSubtype = true)

 */
package okhttp3

import okio.IOException

interface Callback {
  /**
   * Called when the request could not be executed due to cancellation, a connectivity problem or
   * timeout. Because networks can fail during an exchange, it is possible that the remote server
   * accepted the request before the failure.
   */
  fun onFailure(
    call: Call,
    e: IOException,
  )

  /**