backingMap.put(element, occurrences);
      size += occurrences;
      return 0;
    }
    int oldCount = backingMap.getValue(entryIndex);
    long newCount = (long) oldCount + (long) occurrences;
    checkArgument(newCount <= Integer.MAX_VALUE, "too many occurrences: %s", newCount);
    backingMap.setValue(entryIndex, (int) newCount);
    size += occurrences;
    return oldCount;
  }

  @CanIgnoreReturnValue
  @Override