*
   * @throws IllegalArgumentException if the stream consists of two or more elements.
   * @throws NullPointerException if any element in the stream is {@code null}.
   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {

                }));
    expectReplacement(entry(getKeyForNullValue(), v3()));
  }

  @MapFeature.Require({SUPPORTS_PUT, ALLOWS_NULL_KEYS})
  public void testMergeAbsentNullKey() {
    assertEquals(
        "Map.merge(null, value, function) should return value",
        v3(),
        getMap()
            .merge(
                null,
                v3(),

 *
 * <p>A converter always converts {@code null} to {@code null} and non-null references to non-null
 * references. It would not make sense to consider {@code null} and a non-null reference to be
 * "different representations of the same information", since one is distinguishable from
 * <i>missing</i> information and the other is not. The {@link #convert} method handles this null

public interface VersionParser extends Service {

    /**
     * Parses the specified version string, for example "1.0".
     *
     * @param version the version string to parse, must not be {@code null}
     * @return the parsed version, never {@code null}
     * @throws VersionParserException if the string violates the syntax rules of this scheme
     * @see org.apache.maven.api.Session#parseVersion(String)
     */
    @Nonnull

    } catch (RuntimeException tolerated) {
      // GWT's TreeMap.entrySet().removeAll(null) doesn't throws NPE.
    }
  }

  @Override
  public void testEntrySetRetainAllNullFromEmpty() {
    try {
      super.testEntrySetRetainAllNullFromEmpty();
    } catch (RuntimeException tolerated) {
      // GWT's TreeMap.entrySet().retainAll(null) doesn't throws NPE.
    }
  }

  @Override

  }

  public void testToString() {
    assertEquals("foo=1", entry("foo", 1).toString());
  }

  public void testToStringNull() {
    assertEquals("null=1", entry(NK, 1).toString());
    assertEquals("foo=null", entry("foo", NV).toString());
    assertEquals("null=null", entry(NK, NV).toString());
  }

  public void testEquals() {
    Entry<String, Integer> foo1 = entry("foo", 1);

        if (scopesToCollect == null) {
            scopesToCollect = Collections.emptySet();
        }
        if (scopesToResolve == null) {
            scopesToResolve = Collections.emptySet();
        }

        if (scopesToCollect.isEmpty() && scopesToResolve.isEmpty()) {
            return new SetWithResolutionResult(null, new LinkedHashSet<>());
        }

    /**
     * Gets the source from which the model was read.
     *
     * @return The source from which the model was read, never {@code null}.
     */
    @Nonnull
    ModelSource getSource();

    /**
     * Gets the file model.
     *
     * @return the file model, never {@code null}.
     */
    @Nonnull
    Model getFileModel();

    /**

     */
    private Repository repository;

    /**
     * Creates a new exception with specified detail message and cause for the given repository.
     *
     * @param message The detail message, may be {@code null}.
     * @param repository The repository that caused the error, may be {@code null}.
     * @param cause The cause, may be {@code null}.
     */

        this.delegate = requireNonNull(delegate, "delegate cannot be null");
        this.childrenTracking = parent != null ? parent::replace : null;
    }

    public BaseObject(Object delegate, ChildrenTracking parent) {
        this.delegate = requireNonNull(delegate, "delegate cannot be null");
        this.childrenTracking = parent;
    }

    public Object getDelegate() {