public boolean containsKey(@CheckForNull Object key) {
      return backingSet().contains(key);
    }

    @Override
    @CheckForNull
    public V get(@CheckForNull Object key) {
      if (Collections2.safeContains(backingSet(), key)) {
        @SuppressWarnings("unchecked") // unsafe, but Javadoc warns about it
        K k = (K) key;
        return function.apply(k);
      } else {
        return null;
      }
    }

      return getOrDefault(key, null);
    }

    @Override
    @CheckForNull
    public V getOrDefault(@CheckForNull Object key, @CheckForNull V defaultValue) {
      if (Collections2.safeContains(backingSet(), key)) {
        @SuppressWarnings("unchecked") // unsafe, but Javadoc warns about it
        K k = (K) key;
        return function.apply(k);
      } else {
        return defaultValue;
      }