code: public void makeImmutable() {
      if (!isImmutable) {
        // Note: There's no need to wrap the entryList in an unmodifiableList
        // because none of the list's accessors are exposed. The iterator() of
        // overflowEntries, on the other hand, is exposed so it must be made
        // unmodifiable.
        overflowEntries = overflowEntries.isEmpty() ?
            Collections.<K, V>emptyMap() :
            Collections.unmodifiableMap(overflowEntries);
        isImmutable = true;
      }
    }
docs: Make this map immutable from this point forward.