code: public Set<K> getUnmatchedKeys() {
          stateLock.readLock().lock();
          try {
              return new HashSet<K>(this.actualIndices.keySet());
          } finally {
              stateLock.readLock().unlock();
          }
      }
docs: Get a point-in-time view of the unmatched keys.
  This may be immediately out of date unless additional
  synchronization is performed to prevent concurrent updates.