ImmutableArrayMap(int size) {
      this.size = size;
    }

    abstract ImmutableMap<K, Integer> keyToIndex();

    // True if getValue never returns null.
    private boolean isFull() {
      return size == keyToIndex().size();
    }

    K getKey(int index) {
      return keyToIndex().keySet().asList().get(index);
    }

    abstract @Nullable V getValue(int keyIndex);

    @Override