protected Map<Class<? extends B>, B> delegate() {
    return delegate;
  }

  @Override
  @SuppressWarnings("unchecked") // value could not get in if not a T
  @CheckForNull
  public <T extends B> T getInstance(Class<T> type) {
    return (T) delegate.get(checkNotNull(type));
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always

  protected Map<Class<? extends B>, B> delegate() {
    return delegate;
  }

  @Override
  @SuppressWarnings("unchecked") // value could not get in if not a T
  @CheckForNull
  public <T extends B> T getInstance(Class<T> type) {
    return (T) delegate.get(checkNotNull(type));
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always