# Background Tasks

You can define background tasks to be run *after* returning a response.

This is useful for operations that need to happen after a request, but that the client doesn't really have to be waiting for the operation to complete before receiving the response.

This includes, for example:

* Email notifications sent after performing an action:

// There are many different S3 client implementations. Most of them
// access the ETag by looking for the HTTP response header key "Etag".
// However, some of them assume that the header key has to be "ETag"
// (case-sensitive) and will fail otherwise.
// Further, some clients require that the ETag value is a double-quoted
// string. Therefore, this package provides dedicated functions for
// adding and extracting the ETag to/from HTTP headers.

    .sslSocketFactory(clientCertificates.sslSocketFactory(), clientCertificates.trustManager())
    .build();
```

With a server that holds a certificate and a client that trusts it we have enough for an HTTPS
handshake. The best part of this example is that we don't need to make our test code insecure with a
a fake `HostnameVerifier` or `X509TrustManager`.

Certificate Authorities
-----------------------

  /**
   * The largest power of two that can be represented as a signed {@code byte}.
   *
   * @since 10.0
   */
  public static final byte MAX_POWER_OF_TWO = 1 << 6;

  /**
   * Returns the {@code byte} value that is equal to {@code value}, if possible.
   *
   * @param value any value in the range of the {@code byte} type
   * @return the {@code byte} value that equals {@code value}

name is a type or an ordinary identifier, so there will be no syntax
errors that might stop parsing early.

An error on not-declared:1 indicates that foo is undeclared.
An error on not-type:1 indicates that foo is not a type (if declared at all, it is an identifier).
An error on not-int-const:1 indicates that foo is not an integer constant.
An error on not-num-const:1 indicates that foo is not a number constant.

        assertWithMessage("population covariance of " + values).that(populationCovariance).isNaN();
        assertWithMessage("population covariance by addAll(PairedStats) of " + values)
            .that(populationCovarianceByAddAllPartitionedPairedStats)
            .isNaN();
      } else {
        assertWithMessage("population covariance of " + values)
            .that(populationCovariance)
            .isWithin(ALLOWED_ERROR)

 *
 * <p>Checks proper flushing behavior, and ensures that IOExceptions on Flushable.flush() are not
 * propagated out from the {@link Flushables#flush} method if {@code swallowException} is true.
 *
 * @author Michael Lancaster
 */
public class FlushablesTest extends TestCase {
  private Flushable mockFlushable;

  public void testFlush_clean() throws IOException {
    // make sure that no exception is thrown regardless of value of

     * frequently than that, and they may crawl pages marked "yearly" more
     * frequently than that. Crawlers may periodically crawl pages marked
     * "never" so that they can handle unexpected changes to those pages.
     */
    private String changefreq;

    /**
     * The priority of this URL relative to other URLs on your site. Valid

 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that
 * might be absent" as two distinct types in your program, which can aid clarity.
 *
 * <p>Some uses of this class include
 *
 * <ul>
 *   <li>As a method return type, as an alternative to returning {@code null} to indicate that no
 *       value was available

   * values by applying the remapping function. The remapping function accepts the map's existing
   * value for that part of the range and the given value. It returns the value to be associated
   * with that part of the map, or it returns {@code null} to clear that part of the map.
   *
   * <p>Any parts of the range not already present in this map are mapped to the specified value,