*
     * - a `getFooOrDefault(@Nullable Foo defaultValue)` method that returns
     *   `map.getOrDefault(FOO_KEY, defaultValue)`
     *
     * - a call that consults a chain of maps, as in `mapA.getOrDefault(key, mapB.getOrDefault(key,
     *   ...))`
     *
     * So it makes sense for the parameter (and thus the return type) to be @Nullable.
     *
     * Two other points:

import org.jspecify.annotations.Nullable;

/**
 * Static utility methods pertaining to the {@link Future} interface.
 *
 * <p>Many of these methods use the {@link ListenableFuture} API; consult the Guava User Guide
 * article on <a href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
 * ListenableFuture}</a>.
 *

       * - Throw ClassCastException instead of NullPointerException, as documented in the current
       *   Javadoc. (Note that we can't necessarily "just" change our call to `getProperty` to `get`
       *   because `get` does not consult the default properties.)
       */
      builder.put(key, requireNonNull(properties.getProperty(key)));
    }

    return builder.buildOrThrow();
  }

  /**

       * - Throw ClassCastException instead of NullPointerException, as documented in the current
       *   Javadoc. (Note that we can't necessarily "just" change our call to `getProperty` to `get`
       *   because `get` does not consult the default properties.)
       */
      builder.put(key, requireNonNull(properties.getProperty(key)));
    }

    return builder.buildOrThrow();
  }

  /**

   * NavigableSet#subSet(Object, boolean, Object, boolean) subSet()}, {@link
   * NavigableSet#tailSet(Object, boolean) tailSet()}, and {@link NavigableSet#headSet(Object,
   * boolean) headSet()}) to actually construct the view. Consult these methods for a full
   * description of the returned view's behavior.
   *
   * <p><b>Warning:</b> {@code Range}s always represent a range of values using the values' natural

   * NavigableSet#subSet(Object, boolean, Object, boolean) subSet()}, {@link
   * NavigableSet#tailSet(Object, boolean) tailSet()}, and {@link NavigableSet#headSet(Object,
   * boolean) headSet()}) to actually construct the view. Consult these methods for a full
   * description of the returned view's behavior.
   *
   * <p><b>Warning:</b> {@code Range}s always represent a range of values using the values' natural

## External Dependency Version Information

Continuous integration builds have used the following versions of external dependencies, however, this is not a strong recommendation and users should consult an appropriate installation or upgrade guide before deciding what versions of etcd, docker or rkt to use.

* Docker versions 1.10.3 - 1.12.3
  * Docker version 1.11.2 known issues

* When determining the default external host of the kube apiserver, any configured cloud provider is now consulted ([#47038](https://github.com/kubernetes/kubernetes/pull/47038), [@yastij](https://github.com/yastij))
* Updated comments for functions. ([#47242](https://github.com/kubernetes/kubernetes/pull/47242), [@k82cn](https://github.com/k82cn))

## WARNING: etcd backup strongly recommended

Before updating to 1.6, you are strongly recommended to back up your etcd data.
Please consult the installation procedure you are using (kargo, kops, kube-up,
kube-aws, kubeadm etc) for specific advice.

1.6 encourages etcd3, and switching from etcd2 to etcd3 involves a full

- Dynamic...