checkNotNull(elements); // for GWT
      while (elements.hasNext()) {
        add(checkNotNull(elements.next()));
      }
      return this;
    }

    public abstract ImmutableCollection<E> build();
  }

     *
     * <p>Note that each builder class covariantly returns the appropriate type of {@code
     * ImmutableCollection} from this method.
     */
    public abstract ImmutableCollection<E> build();
  }

  private static final long serialVersionUID = 0xcafebabe;

     *
     * <p>Note that each builder class covariantly returns the appropriate type of {@code
     * ImmutableCollection} from this method.
     */
    public abstract ImmutableCollection<E> build();
  }

  abstract static class ArrayBasedBuilder<E> extends ImmutableCollection.Builder<E> {
    // The first `size` elements are non-null.
    @Nullable Object[] contents;
    int size;
    boolean forceCopy;