this.iterator = iterator;
  }

  @Override
  public Iterator<E> iterator() {
    if (iterator == null) {
      // TODO: throw something else? Do we worry that people's code and tests
      // might be relying on this particular type of exception?
      throw new IllegalStateException();
    }
    try {
      return iterator;
    } finally {
      iterator = null;
    }
  }

/// tip

Some editors check for unused function parameters, and show them as errors.

Using these `dependencies` in the *path operation decorator* you can make sure they are executed while avoiding editor/tooling errors.

It might also help avoid confusion for new developers that see an unused parameter in your code and could think it's unnecessary.

///

/// info

In this example we use invented custom headers `X-Key` and `X-Token`.

      Runnable runnable, @ParametricNullness V result) {
    return new TrustedListenableFutureTask<>(callable(runnable, result));
  }

  /*
   * In certain circumstances, this field might theoretically not be visible to an afterDone() call
   * triggered by cancel(). For details, see the comments on the fields of TimeoutFuture.
   *
   * <p>{@code volatile} is required for j2objc transpiling:

import jcifs.smb.NtlmPasswordAuthentication;

/**
 * This class is used internally by {@code NtlmHttpFilter},
 * {@code NtlmServlet}, and {@code NetworkExplorer} to negotiate password
 * hashes via NTLM SSP with MSIE. It might also be used directly by servlet
 * containers to incorporate similar functionality.
 * <p>
 * How NTLMSSP is used in conjunction with HTTP and MSIE clients is

Just functions that look the same as the *path operation functions*.

But still, it is very powerful, and allows you to declare arbitrarily deeply nested dependency "graphs" (trees).

/// tip

All this might not seem as useful with these simple examples.

But you will see how useful it is in the chapters about **security**.

And you will also see the amounts of code it will save you.

<img src="/img/tutorial/path-operation-configuration/image01.png">

### Tags with Enums { #tags-with-enums }

If you have a big application, you might end up accumulating **several tags**, and you would want to make sure you always use the **same tag** for related *path operations*.

In these cases, it could make sense to store the tags in an `Enum`.

     */
    @Test
    void testToString() {
        // The actual toString() method uses super.toString(), so we mimic that for the check.
        // A direct string comparison might be brittle, so we check for the class name and brackets.
        String actualString = response.toString();
        assertTrue(actualString.startsWith("Trans2SetFileInformationResponse["),

    Function<Float, Boolean> c1 = Functions.compose(Functions.compose(h, g), f);
    Function<Float, Boolean> c2 = Functions.compose(h, Functions.compose(g, f));

    // Might be nice (eventually) to have:
    //     assertEquals(c1, c2);

    // But for now, settle for this:
    assertEquals(c1.hashCode(), c2.hashCode());

    assertEquals(c1.apply(1.0f), c2.apply(1.0f));

 *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treats it
 *       identically to {@code Nullable}</a> as of version 0.9.9.
 *   <li><a href="https://developers.google.com/j2objc">J2ObjC</a>, maybe: It might no longer be
 *       necessary there, since we have stopped using the {@code @ParametersAreNonnullByDefault}
 *       annotations that {@code ParametricNullness} was counteracting.
 * </ul>
 *

 *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treats it
 *       identically to {@code Nullable}</a> as of version 0.9.9.
 *   <li><a href="https://developers.google.com/j2objc">J2ObjC</a>, maybe: It might no longer be
 *       necessary there, since we have stopped using the {@code @ParametersAreNonnullByDefault}
 *       annotations that {@code ParametricNullness} was counteracting.
 * </ul>
 *