code: public long size() throws FileLockException, IOException {
          long size = 0L;
          for (int bucketId = 0; bucketId < hashFunction.getNumberOfBuckets(); bucketId++) {
              HeaderIndexFile<Data> headerIndexFile = new HeaderIndexFile<Data>(gp.DATABASE_DIRECTORY + "/"
                      + hashFunction.getFilename(bucketId), gp.HEADER_FILE_LOCK_RETRY, gp);
              size += headerIndexFile.getFilledUpFromContentStart() / gp.getElementSize();
              headerIndexFile.close();
          }
          return size;
      }
docs: Determines the number of elements in each buckets by opening all files.
  
  @return the number of elements in the database.
  @throws IOException
  @throws FileLockException