if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }

  /**
   * Ensures that an object reference passed as a parameter to the calling method is not null.
   *
   * @param reference an object reference
   * @param errorMessage the exception message to use if the check fails; will be converted to a

      }
    }
  }

  /** A weakly referenced value that also has a reference to its containing entry. */
  interface WeakValueReference<K, V, E extends InternalEntry<K, V, E>> {
    /**
     * Returns the current value being referenced, or {@code null} if there is none (e.g. because

   * accessed directly after this method returns. These conditions are ensured if the map is created
   * empty, passed directly to this method, and no reference to the map is retained, as illustrated
   * in the following code fragment:
   *
   * <pre>{@code
   * Set<Object> identityHashSet = Sets.newSetFromMap(
   *     new IdentityHashMap<Object, Boolean>());

                // This means a module which is build has a dependency which has the same
                // groupId, artifactId, version and classifier coordinates. This is in consequence
                // a self reference or in other words a circular reference which can not being resolved.
                addViolation(
                        problems,
                        Severity.FATAL,
                        Version.V31,

                        .orElse(null);
                if (cycle != null) {
                    throw new RuntimeException(new ProjectCycleException(
                            "The projects in the reactor contain a cyclic reference: " + cycle.getMessage(),
                            (CycleDetectedException) cycle.getException()));
                }
                throw new ProjectBuildingException(results);
            }

       * don't overwrite an element in the array until after reading it.
       */
      ListenableFuture<? extends T> inputFuture = requireNonNull(inputFutures[inputFutureIndex]);
      // Null out our reference to this future, so it can be GCed
      inputFutures[inputFutureIndex] = null;
      for (int i = delegateIndex; i < delegates.size(); i++) {
        if (delegates.get(i).setFuture(inputFuture)) {

   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and the collections
   * returned by {@code factory}. Those objects should not be manually updated and they should not
   * use soft, weak, or phantom references.
   *
   * @param map place to store the mapping from each key to its corresponding values
   * @param factory supplier of new, empty collections that will each hold all values for a given
   *     key

    return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
  }

  /**
   * This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
   * referenced by emptyModifiableIterator().
   */
  private enum EmptyModifiableIterator implements Iterator<Object> {
    INSTANCE;

    @Override
    public boolean hasNext() {
      return false;
    }